rare earth reactor final report

7/22/2019 Rare Earth Reactor Final Report

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Tufts UniversitySchool of Engineering

Department of Mechanical Engineering

ME43Senior DesignFall 2012

Ultra-High Temperature Reactor

Design and Construction of an Induction Heated Furnacefor Molten Oxide Electrolysis of Rare Earth Elements

Tyler Andrews Jack Carter Nathaniel Eckman Bradley Nakanishi


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Executive Summary

This report discusses progress towards a novel means of refining rare earth elements (REEs).These elements are crucial to a wide range of modern technologies, but current processing techniques are

expensive and environmentally destructive.The Sadoway Group at MIT has been exploring an alternative means of rare earth metal

production called molten oxide electrolysis (MOE). This method involves heating REE-containing metaloxide electrolytes to their melting points in order to perform electrolysis, separating oxygen from the puremetals. However, they currently have no means of melting electrolytes of high REE concentration andperforming electrolysis. They have ordered a resistance-heated furnace to do so, but it will take hours toreach maximum temperature, limiting their capacity to rapidly perform experiments.

Our group was asked to develop an induction-heated reactor that would be capable of performingMOE experiments more rapidly. We worked with Rachel DeLucas of the Sadoway Group and ProfessorMatson of Tufts University to develop engineering specifications. Temperature and rigidity were the twomost critical specifications. The reactor had to be capable of reaching at least 2300C, and its standneeded to be sturdy enough to precisely lower an anode into the molten REE electrolyte.

A reactor that achieves both of these goals, as well as other secondary goals, has been built andtested. Four tests have been completed, demonstrating that the reactor is capable of reaching 2300C intwenty minutes. Room-temperature testing indicates that the linear actuator can successfully lower ananode into the electrolyte.

Our reactor consists of a quartz tube, purged of oxygen, with vacuum-sealed instrumentationports in aluminum caps. Paul Sander developed a furnace for the purpose of performing MOE to extractiron, a process that takes place at 1500C. Sanders furnace was the basis for our design. We reconfiguredthis basic design to meet the more rigorous demands of electrowinning rare earth elements.

We increased the diameter of the quartz tube to allow for additional radiation shielding, added asecond aluminum cap, and built a stand to increase experimental repeatability. We also added a linearactuator to insert an anode into the slag, eliminating operator error and safety concerns associated withmanually positioning the anode.

Despite our best efforts, we did not have the time to run a final test demonstrating electrolysis.However, two of our team members, Brad Nakanishi and Tyler Andrews, will continue working on thisproject in the Spring 2013 semester. Our team has made significant strides towards a fully-functionalMOE reactor with this design project.


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Contents

Executive Summary ............................................................................................................................... 2

Introduction ............................................................................................................................................ 5Problem Statement and Goals ............................................................................................................ 7

Design Overview ................................................................................................................................ 8

Crusceptode ................................................................................................................................... 8

Engineering Specifications................................................................................................................. 9

Engineering Analysis ........................................................................................................................... 12

Linear Actuator ................................................................................................................................ 12

Reactor Stand/Holder ....................................................................................................................... 14

Assembly Procedure .................................................................................................................... 14

Stand Design ............................................................................................................................... 16

Holder Design ............................................................................................................................. 18

Reactor/Stand Interface ............................................................................................................... 21

Material Considerations .............................................................................................................. 22

Stress Calculations ....................................................................................................................... 23

Tipping Calculations .................................................................................................................... 24

Reactor Caps .................................................................................................................................... 24

Overview of Analysis .................................................................................................................. 24

Incident Radiation ........................................................................................................................ 24

Channel Flow and Cooling .......................................................................................................... 26

COMSOL Modeling .................................................................................................................... 27

Discussion .................................................................................................................................... 28

Inner Reactor Analysis ..................................................................................................................... 29

Temperature Measurement............................................................................................................... 35

Engineering Solution ............................................................................................................................ 36

Construction Process ........................................................................................................................ 36

Stand Construction ....................................................................................................................... 36Reactor Construction ................................................................................................................... 37

Reactor Tests .................................................................................................................................... 37

Team Responsibilities ...................................................................................................................... 38

Conclusion ............................................................................................................................................ 39

Future Work ..................................................................................................................................... 41


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Appendix #1: Engineering Drawings ...................................................................................................... i

Parts List ............................................................................................................................................. i

Vendor Catalog Information ............................................................................................................. iii

Appendix #2: Calculations .................................................................................................................. xxv

Appendix #3: Sources ......................................................................................................................... xxx

Appendix #4: Project Proposal and Progress Report............................... Error! Bookmark not defined.

Project Proposal .................................................................................. Error! Bookmark not defined.

Progress Report ................................................................................... Error! Bookmark not defined.


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Introduction

Rare earth elements (REEs) are increasingly important for modern technology. Color televisions,lasers, and semiconductors all rely on small quantities of these 17 elements. There currently exist no

known substitutes for these elements thanks to their specific elemental properties.1Contrary to their name, REEs are actually not particularly rare elements. They are, however,

generally found in very low concentrations. REEs have been found in ores of alkaline rock, carbonatites,and other various igneous rocks. However, the concentrations of REE found in these ores can be between1-10%.5This makes extraction and difficult and expensive process. Moreover, current extraction methodsfor these REEs are dangerous, expensive, and environmentally hazardous. For example, the extraction ofthe REE mineral monazite can produce highly radioactive radium and thorium. This has been a seriousenough concern that the extraction process from monazite has largely stopped in the United States.1

Figure 1: US decline and Chinas rise in rare earth oxide production since 1990 1

This concern over the safety and efficiency of the REE extraction process has caused a dramaticshift in the production of pure REEs over the past several decades. Between 2002 and 2010, the RareEarth Price Index increased by an order of magnitude. This enormous spike in price can be significantly

drawn to the United States essential withdrawal from the REE-extraction market. With the US no longerin the market, China became the next largest exporter of REEs and capitalized on the opportunity tocontrol the market. Huge export tariffs were imposed on REEs coming from China.6

Thus, there exist enormous economic, geopolitical, and environmental incentives to develop anew method of extraction of REEs. One of the most promising new methods is molten oxide electrolysis(MOE). In MOE, slag or ore containing a desired element is melted. Electrolysis is then performed on theslurry which converts the oxides to metal and oxygen by applying a voltage difference across the slurry,leaving the pure metal and oxygen separated. MOE has been demonstrated as a method for extractingaluminum (see figure 2 below), and recent studies have demonstrated the feasibility of iron MOE for use


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in the steelmaking industry3. Research into MOE is currently being performed by the Sadoway ResearchGroup at the Massachusetts Institute of Technology, under the guidance of which this project wasconducted. Nonetheless, a significant lack of examples of rare earth MOE exists at the present; in fact,many properties of REEs are not well known. Thus, a new design for an ultra-high temperature MOEreactor is presented in this report.

Figure 2: A cross-section of a modern, industrial-scaled aluminum electrolytic celli


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Problem Statement and Goals

Currently, there exists no way to rapidly melt rare earth oxides (REOs) and perform MOE. Aswas outlined in the introduction, there exist clear economic, geopolitical, and environmental motivationsfor developing this technology.

The fundamental goal of this project was to develop an induction-heated reactor capable of

heating a sample of REOs in less than one hour above its melting temperature and maintainingtemperature at that melt plateau for at least 20 minutes in a safe and repeatable experimental apparatus.To achieve these goals, a reactor was created to perform the following functions:

Safely melt REOs Maintain atmospheric and structural integrity long enough to perform experiments Accurately measure and control temperature Support at least three electrodes (anode, cathode, reference) of various geometries and material

compositions

Raise/lower electrodes remotely Extract pure REE from oxide via electrolysis


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Design Overview

Design efforts were focused on these most critical features: the reactor stand, linear actuator,reactor caps, inner reactor structure and radiation shielding, and temperature measurement (seeFigure 3

and Error! Not a valid bookmark self-reference.). Engineering analyses drew upon many principal topics

in mechanical engineering, including heat transfer, machine design, fluid dynamics, mechanics, data

acquisition and instrumentation, FEA, robotics, and material selection. It was the aim of this project todesign a robust and safe MOE reactor testing facility for laboratory use that would optimize repeatabilityand minimize turnaround time betweenexperimentation.

Crusceptode

This report introduces a neologism: Crusceptode. This is a portmanteau of the words crucible,susceptor, andcathode. It refers to the molybdenum component at the center of the reactor, which hasthree distinct but related functions. First, it operates as a standard crucible, holding the molten rare earthswhile remaining solid. Second, it couples with the inductive field, generating the heat required to melt theoxides. Finally, it acts as the cathode for electrolysis. When current is passed through an anode into theelectrolyte, it returns via a lead attached to the crusceptode.

Figure 4: A diagram of the reactor including the caps

and inner reactor structure.

Figure 3: An overview of the reactor facility with keyfeatures labeled.


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Engineering Specifications

The complexities of this project mandated numerous specifications. Rachel DeLucas, of theSadoway Group, and Professor Matson, of Tufts University, were consulted to develop the projectspecifications,Table 1.

Number Specification UnitsMarginal

Value

Optimal

Value

Importan

Factor

1 Sustainable maximum temperature C 2300 2400 5

2 Maximum anode eccentricity in. 0.05 0.01 5

3 Partial pressure O2in reactor psi Minimal Minimal 4

4Maximum uncertainty in crusceptode

temperatureC 50 30 4

5 Number of electrode ports # 2 3 4

6Distance between metallic frame and

coils

in. 3 4 4

7 Anode actuator force lbs. 10 20 4

8 Time to maintain max temperature Minutes 20 60+ 3

9 Resolution of linear actuator in. 0.06 0.02 3

10 Minimum water cooling mass flow kg/s 0.25 0.5 3

11 Anode actuator stroke length in. 0.5 3 3

12 Time to achieve max temperature Minutes 30 10 2

13 Number of thermocouple ports # 1 4 2

14 Maximum pressure loss from reactor psi/min Minimal Minimal 2

Table 1: Summary of engineering specifications

The fundamental goal of this project was to develop a reactor which could heat to the desiredtemperature. The goal temperature for this reactor was 2300oC. This value was decided upon because it isthe temperature which could sustain a melt of many rare earth oxides, specifically lanthanum oxide anddysprosium oxide. An induction furnace had not yet been developed and tested to reach and sustain thistemperature, so this was the most fundamental goal of the project. An optimal result, if this initialtemperature was achieved, would be heating to a temperature 2400oC. With a reactor capable of reachingthis temperature, almost any rare earth oxide could be melted.

A second critical engineering specification was the maximum anode eccentricity. One of the keydesign elements of this reactor was the ability to perform molten oxide electrolysis in a safe andrepeatable experimental environment. A linearly actuated anode was not a part of a previous iteration ofinduction reactor design. Instead, a lab technician lowered an anode into the melt by hand. This posed a

safety issue as the technician had to lean over the coils and reactor at high temperature with no protection.Second, the accuracy of a human hand lowering the anode was not found to be exact enough tosuccessfully insert the anode into the crusceptode slag. In an attempt in the previous design iteration, theanode was lowered by hand into the slag but brushed against the crusceptode wall. A short circuit wascaused which ended the experiment and damaged a significant amount of equipment. For this design, alinear actuator was to be used to lower the anode. From the geometry of the tube and crusceptode, it wasdetermined that the maximum allowable anode eccentricity was 0.050 inches. If this 0.050 incheccentricity was achieved, an optimal value was 0.010 inches. This would become be the basis for thedesign of the linear actuator and reactor stand discussed later in the report.


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With an importance factor of 4, slightly less than the first two specifications, was the partialpressure of oxygen in the reactor. The fundamental goal of this reactor was to be able to yield pure metalsfrom metal oxides. Thus, it would be necessary to purge the reactor of oxygen prior to a test. This is animportant engineering specification, though it is difficult to measure. Thus, the marginal and optimalvalues are qualitative. The partial pressure of oxygen is simply to be minimized to as low a value aspossible.

Another important engineering specification was the maximum uncertainty in temperaturemeasurement. In past experiments, accurate temperature measurement has been difficult because of theextremely high temperatures involved. Thermocouples were found to be faulty and inconsistent and onlyworked up to their maximum temperature (for Type C thermocouples, 2300 oC). Moreover, thethermocouples are affected by the magnetic field used for induction. This creates a systematic but non-constant error in the temperature reading, which can be recognized by quickly turning the current off ofthe induction coils and tracking the subsequent spike in the thermocouple reading. Temperature is alsomeasured via the non-contact pyrometer. The pyrometer is attached via a fiber-optic cable to a sight at thebottom cap of the reactor. These data from the pyrometer are not affected by the induction field, but theymust be scaled relative to the thermocouple data. From these two sources, the thermocouple data and thepyrometer data, a temperature measurement can be estimated. Due to the extremely high temperaturesneeded to melt the rare earth oxides, it is important for the temperature measurement to be as accurate as

possible. The marginal value for this uncertainty was determined to be 50oC, with an ideal value of 10oC.Another engineering specification with an importance factor of 4 was the number of

instrumentation ports available for electrode insertion into the reactor. The previous design iterationfeatured a single cap, which made instrumentation ports much more cramped. It was a goal of this designto feature a cap or caps which allowed for easier and more plentiful instrumentation. Room for twoelectrodes was considered to be the marginal value. This would allow for an anode and a referenceelectrode while performing electrolysis. Room for three or more ports would be an optimal value, whichwould allow room for multiple reference electrodes.

The last engineering specification with an importance factor of 4 was the distance between themetallic frame of the reactor stand and the copper induction coils. An extremely powerful magnetic fieldis generated by the alternating current flowing through the copper coils. Any metal too close to this fieldwould be inductively heated. Inductive heating is valuable since it is responsible for heating the

crusceptode, but also must be controlled such that only the crusceptode is heated. Thus, it is importantthat the reactor stand, constructed primarily from aluminum, be kept at least 3 inches away from thecopper coils to avoid any coupling with the magnetic field. An optimal value would be 4 inches toeliminate any risk of coupling between the induction field and the reactor stand.

Three engineering specifications refer to the actuators ability to insert an anode into the reactor.The first is the actuators stroke length, or maximum displacement. Because the process of electrolysiswill erode the anode, it must be lowered constantly at a slow rate. This will be important for an industrialprocess, but for our purposes it will be sufficient to insert the anode for a brief period of electrolysis. Forthis reason, the marginal value is 0.5 inches, and the ideal value is 3 inches. This specification has animportance factor of 3.

The second actuator specification is the amount of force that the linear actuator can supply. Theanode tube must be able to push through the Ultra-Torr vacuum seal, which utilizes a relatively high-

friction O-ring. Through experiment, it was determined that the force required to push an alumina tubethrough this fitting is 5 lbs. For a factor of safety of 2, a 10 lb. linear actuator force was selected as themarginal value. However, the ceramic tubes being used as anode holders are not made to tight tolerances,and larger tubes may result in more friction. For this reason, an ideal value of 20 lbs. was chosen. Thisspecification has an importance factor of 4.

With an importance factor of 3, the linear actuator resolution was considered a slightly less vitalengineering specification. The linear actuator needed to lower the anode into the molten slag held insidethe crusceptode. Because the crusceptode is small (only 0.75 inches tall), it was determined that the linearactuator have a resolution of at most 0.060 inches. This would allow for the experimental user to be sure


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that the anode was lowered into the slag without a possibility for the anode making contact with thebottom of the crusceptode. An optimal value would be a resolution of 0.010 inches. This resolution wouldneed to be deliverable via a control system programmed in LabView or other software.

Another engineering specification at this level was the mass flow rate in the system providingwater-cooling to the top cap of the reactor. This system was important in that it provided a heat flux to thetop cap, removing heat and keeping the cap at a cool and sustainable temperature. An engineering

analysis was computed using COMSOL Multiphysics software. This analysis determined that theminimum water cooling mass flow rate was 0.25 kg/sec. An optimal value of 0.5 kg/s was determined toprovide an even higher factor of safety for the top.

The final three engineering specifications were given an importance rating of 3. First, the time toachieve maximum reactor temperature was considered. Since induction heating has been shown to bemuch faster than heating with a resistance furnace, it was clear that the new design ought to be faster thancurrent technology. A desired time to achieve maximum reactor temperature was determined to be 30minutes. If this was easily achievable, an optimal time to max temperature would be 10 minutes ofheating.

Similar to the number of electrode ports, it was decided that the number of thermocouple portswould be an important factor. Thermocouples have been an important part of the experimental procedurewith past iterations of the reactor. This is because the thermocouples can provide direct data against

which the pyrometer data can be calibrated. Thermocouples can be inserted into the reactor to measurethe temperature of internal components, such as radiation shields or the inner tube, or they can be used tomeasure the temperature of components on the outside of the reactor, like the caps or reactor stand. Forinternal measurements, it is important to have ports on the cap to allow the thermocouples into the sealedreactor. The marginal number of thermocouple ports was determined to be one; however, an optimaldesign would have up to four thermocouple ports to allow for monitoring of the crusceptode and each ofthe three heat shields.

Finally, the last engineering specification considered was the maximum pressure loss from thereactor. It was deemed important to keep the partial pressure of oxygen as low as possible, in order toprevent oxidation of the crusceptode and other vulnerable components. Because the pressure loss wasdifficult to measure in this instance, it was simply decided to minimize pressure loss as much as possibleby creating as tight a seal as possible in the junction between the reactor caps and tube.


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Engineering Analysis

Linear Actuator

In order to raise and lower the anode, a linear actuator was added to the design. To keepefficiency high, the anode must be carefully positioned in the electrolyte. Therefore, the actuator must beprecise in the vertical direction. Ideally, the anode's vertical position should be tightly controlled: .035"minimum resolution.

The stroke length of the actuator need not be enough to completely extract the anode from thereactor. It should be enough to insert and extricate the anode from the molten slag. This corresponds to adistance of 0.5-3".

The actuator must be powerful enough to push the anode tube through a vacuum seal. Testsperformed on a Swagelok Ultra-Torr vacuum fitting indicated that this requires up to 5 pounds of force.

The precision and force requirements for the actuator limited the available commercial options.Of the actuators that met both requirements, most cost more than $1000. MicroMo produces a servomotor actuator for $500, but the stroke length is insufficient. The team contacted Vincent Miraglia, Tufts'

Mechanical Engineering Coordinator, asking if he knew of any available linear actuators from priorprojects. Mr. Miraglia did discover an actuator in pristine condition and allowed the team to utilize it.

This actuator is built around a Nippon Pulse PJ42C1 stepper motor. This is a unipolar-type motorwith 6 leads. The motor drives a 3.5" lead screw with a 3/8" major diameter and a pitch of 12 threads perinch. The lead screw is constrained by brass bushings. Three 1/4" aluminum rods hold the aluminum platethat constrains the screw at the endaway from the motor. Two of theserods also constrain the "nut," whichis actually an adjustable componentwith threaded holes for attachments.The nut has a spring-loaded interfacewith the lead screw, which may be

manually pulled to allow freemovement.

Finding the linear actuatorsaved the project team time andmoney, but modifications had to bemade before it could be used to holdan anode. First, the stepper motorneeded a logic board andprogramming. Second, the actuatorassembly had to be mounted to thestand. Finally, the actuator had to beinterfaced with the tube holding the

anode.Nippon Pulse was contacted

for help understanding the old anddiscontinued stepper motor. Theyrecommended purchasing aproprietary logic board, whichwould allow control of the steppermotor but would not facilitateLabView integration. Instead, an

Figure 5: An overview of the linear actuator assembly, attached to the stand and holdinan anode tube.


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EasyDriver stepper motor driver was purchased and connected to an Arduino Uno. The driver runs on a9V, 1300mA power supply. An onboard potentiometer allows current adjustment, limiting it to theNippon-recommended 300mA.

The LabView Interface for Arduino (LIFA) library was used to interface the Arduino withLabView, allowing the motor control to be integrated with the existing interface. To control the steppermotor's direction, a HIGH or LOW output is sent to digital pin 2 on the Arduino. To control the speed, a

square wave is sent to digital pin 3. The frequency of this wave determines the motor's rotation rate. Atfaster rotation rates, the motor loses torque and is liable to move fewer steps than expected. Becauseprecision is far more important to this application than speed, the step frequency was reduced to 325Hz,or 20% of the motor's maximum.

To mount the actuator to the stand, a new base plate was designed and machined. This plateattaches directly to the stand via screws, securely holding the actuator upright. The stepper motor is heldmore than 6" above the inductive coils, ensuring that there is no chance of interference from the field.

Finally, an attachment had to be created that would allow the actuator to securely hold the anodetube. A plate is screwed to the linear actuator nut. This plate is braced by a bushing that slides on thelinear actuator's rod. At the opposite end of the plate, four threaded holes allow a smaller plate to beattached. This smaller plate is threaded to accept a male NPT Swagelok fitting. The smaller plate may beadjusted by loosening its screws and moving it. The loose tolerances in the screw joint may thus be taken

advantage of to allow small adjustment of the anode location. This ensures that the anode may be properlycentered over the reactor.

The bushing that braces the large plate is essential to prevent binding. Initially, a 3/8" lengthbushing was used. However, this allowed the plate to rotate under the reaction force of the anode enteringthe reactor's Ultra-Torr fitting. The plate's rotation caused the lead screw to bind against the nut, stoppingthe linear actuator. By replacing the 3/8" bushing with a longer 3/4" size, the plate was more tightlyconstrained from rotating. The binding ceased, allowing the linear actuator to use its full range of motion,inserting and extracting the anode.


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Reactor Stand/Holder

As previously discussed, a major goal of this project was to build a system to remotely lower theanode into the crusceptode. It was decided early on that a motorized linear actuator would perform thisfunction. In order to lower the anode precisely, the linear actuator must be constrained at a set positionabove the reactor. A stand was designed to provide this constraint, as well as to hold the reactor at a setposition within the induction coils. Both of these functions increase the reactors experimentalrepeatability. The stand also increases the ease with which the reactor can be assembled anddisassembled, thus decreasing the turnaround time between experiments.

Assembly Procedure

In order to design a stand that would meet these needs, it was first necessary to decide upon aprocedure for assembling the reactor. This was not a simple task due to the number of concentriccomponents within the reactor. However, establishing this procedure before beginning the design processprevented costly mistakes from being made down the line. Figure 6 displays the final reactor designperforming the five steps required to assemble the reactor. First, the entire holder is slid horizontally

towards the edge of the lab bench, providing easy access to the reactor. The bottom cap is placed on thestand with the pyrometer tube protruding into the center of the reactor and the quartz tube sealed in place(A). With the cap resting in the stand, the internal reactor components are assembled (B). The reactor isthen slid horizontally away from the edge of the workbench until it lines up with the Lepel inductionpower supply. The induction coils are then slid over the quartz tube and attached to the Lepel (C). Thereactors vertical height is then adjusted to align the crusceptode with the induction coils and the top capis sealed in place (D). Finally, the anode is lowered into the top cap and attached to the linear actuator.Originally it was planned that the linear actuator would not be removed from the stand during thisprocess. However, due to interference issues, the procedure was modified slightly to involve removingand replacing the linear actuator as shown in steps A-E. This change required a slight post-productiondesign change as well as some extra machining.


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C

A B

D

Figure 6: Assembly procedure for the reactor. A) Place bottom cap on stand with pyrometer tube protruding from the bottom and quartz tube abov

B) Assembly nested heat shields. C) Slide reactor back toward Lepel and attach induction coils. D) Place top cap onto quartz tube. E) Insert anode intoreactor, assemble linear actuator to stand.

This assembly process clearly requires the stand to provide two separate degrees of freedom forthe reactor: one horizontal and one vertical. The horizontal axis of motion allows the reactor to be slidtoward the edge of the lab bench, allowing easy access for researchers assembling or altering the reactorsinternal components. It can then be slid back to the middle of the lab bench where it is aligned with theinduction coils and the Lepel induction power supply. The vertical axis of motion allows the crusceptodeheight to be adjusted so that it is centered within the induction coils. This is necessary to accommodatedifferent crusceptode heights or heat shield layouts that may be used in this reactor.


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Power Screw

Custom Design (1)Pulley System

Custom Design (2)

Small Stage

Counterweight (3)

Large Stage

Off-the-shelf (4)

Adjustment Screw

Custom Design (5)

Figure 7: Designs considered in an effort to reduce the cost of providingvertical motion for the reactor.

Stand Design

Multiple stand designs were createdand analyzed in an attempt to provide bothdegrees of freedom in a simple and low-costmanner. The largest challenge was

designing a stand that would be sufficientlyrigid and allow for at least 5 of motion ineach axis. Initially, a stand was designedusing 80/20 T-slotted framing due to thelarge amount of free material available inthe lab. In this design, both horizontal andvertical motion were provided by 80/20linear bearings. However, it was uncertainwhether the 80/20 linear bearingresponsible for providing vertical motionwould be sufficiently strong to support thecombined weight of the reactor, linearactuator, etc. Wishing to act conservatively,this initial design was abandoned for a morerobust design.


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Shortly after abandoning the initial design, a sturdy, 3-axis stage was found in the lab. Permission

to use the stage was obtained and a design was created around its use. However, upon submission of thedesign, it was discovered that the stage had been taken for another project. Therefore, it was necessary tofind yet another means to provide vertical motion for the reactor. A review was made of commercially-available single-axis stages and it was found that those able to support the necessary loading and providethe necessary length of motion generally cost around $800. This was significantly over budget for a singlecomponent of the stand, so alternative means of providing vertical motion were considered. The optionsthat were considered are shown inFigure 7.Option 1 involves making a custom linear stage using powerscrews; option 2 involves making a custom linear stage using a pulley system; option 3 uses a stage that isincapable of supporting the full cantilevered load with a counterweight that helps to reduce the momenton the stage; option 4 is the original idea of buying a large linear stage at a high price; option 5 is to makea custom stage using an adjustment screw. The options were weighed against each other using thedecision matrix shown in Table 2.Ultimately, despite pushing the stand over budget, the large linear

stage proved to be the most effective option. The customer quickly agreed to provide the necessaryfunding, agreeing with the teams recommendation that it was desirable to create an effective product,even if it exceeded the original budget. An appropriate stage was found on sale for roughly $400 and thepurchase was approved. However, before the purchase was completed an unused linear stage of similarspecifications was found at Tufts University. This stage was donated to the group, preventing the standfrom exceeding budget and allowing the stand design to be finalized, Figure 9. Vertical motion isachieved by turning a power screw that raises the carriage of the linear stage.

Table 2: Decision matrix used to determine which method for providing vertical motion was most desirable.Ultimately the large, off-the-shelf linear stage (4) was used.

Scores

1(worst)-

5(best)

Weight

(1-5)

Power

Screw

Custom

(1)

Pulley

System

Custom

(2)

Small Stage

Counterweight

(3)

Large Stage

Off-the-

shelf (4)

Adjustment

Screw

Custom

Design (5)

Cost 3 4 5 2 1 4

Design

Time4 2 2 4 5 3

Lead

Time5 4 4 4 3 5

Risk of

Failure3 3 2 4 5 4

Precisi

on1 3 2 5 5 3

TravelDistance

3 5 5 2 5 2

Ease

of Use3 4 2 4 4 3

Reliabi

lity4 4 2 4 5 3

Total 26 3.65 3.08 3.58 4.04 3.50


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Holder Design

Next, it was necessary to design aninterface for attaching the reactor to the stand.Multiple ideas were considered that wouldallow the reactor to be easily connected to and

removed from the stand. Due to its simplicityand effectiveness, the idea of a horizontal trackinto which the bottom reactor cap could slidewas quickly adopted, Figure 10. A slot heightof 0.25 was picked so that it could be cut in asingle pass of an end mill. The tab wasdimensioned for a tight clearance fit.

Unfortunately, a horizontal slide couldnot be used to hold both caps because the topcap must be lowered onto the reactor fromabove rather than slid on from the side.Therefore, the idea of not having a connectionbetween the top cap and the holder wasconsidered. However, it was ultimately decidedthat both caps should interface directly with theholder to ensure that they remained parallel toeach other. Therefore, a simple cup wasdesigned to hold the top cap, Figure 11. Theouter lip was added to the design to ensure thatthe reactors horizontal position would be sufficiently constrained.

The remainder of the holder was designed to ensure that no metal piece would approach theinduction coils too closely, causing them to couple with the induction field and become hot. The threemost desirable configurations are depicted in Figure 12.The double-sided stand has the advantage ofbeing the most rigid structure. However, it requires much more space and many more parts than the othertwo configurations. The bottom supported stand is somewhat less rigid, but much more compact than the

Tab in

bottom capSlot cut

into holder

Figure 10: Interface for bottom cap. Track allowsfor horizontal motion.

Constrains

reactor

horizontally

Keeps

caps

parallel

Figure 11: Interface allows top cap to be loweredonto the reactor from above.

Figure 9: Finalized stand design. The linear stage is kept vertical usin

two aluminum angle plates.

Figure 9: Finalized stand design. The linear stage is kept vertical using taluminum angle plates.


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double-sided stand. However, it presents a safety concern in the event of an uncontained reactor failure. If

a significant amount of molten metal escaped from the bottom of the reactor, the stand could losestructural integrity, causing it to collapse. The cantilevered configuration is therefore a compromisebetween the other two designs. It requires less parts and space than the double-sided configuration, but itdoes not have any parts directly below the reactor like the bottom supported configuration. However, itdoes lack some of the rigidity of the other two configurations. Regardless, it was deemed the mostappropriate for the current design.

The remaining holder was created by merging the cantilevered configuration with the slotted andcupped interfaces described earlier. It was important to carefully consider the rigidity of the final design.Sufficient rigidity was necessary to ensure that the eccentricity of the anode relative to the cathode wouldnot become too great. Because the linear actuator had to be at least 6 above the cathode to preventinductive coupling, even a relatively small deflection in the holder could cause a collision between theanode and cathode as shown in Figure 13. Such a collision would short-circuit the electrolysis cell,

possibly resulting in damage to various pieces of expensive equipment. An analysis of the anode andcathode geometry suggests that the deflection, x, can be up to 0.063 before the anode and cathode are atrisk of colliding. However, there are a variety of reasons that it is desirable to keep the predicted staticdeflection far below this level. First of all, the static loading is only due to the relatively small weight ofthe reactor, linear actuator, etc. It is also important that the reactor not fail if it is accidentally jostled by alarger force during operation. Furthermore, maximum reactor efficiency occurs when the anode andcathode are perfectly aligned, making it is desirable to keep deflection to a minimum, even underrelatively small loading conditions.

Figure 12: Stand configurations originally considered. The cantilevered configuration was used due to its relative simplicityand low cost.


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To maximize the holders rigidity, it wasdesigned with a C shaped cross section. Thecross section dimensions were established byrelating superposition calculations to a maximumtolerable deflection under static loading. Themaximum tolerable deflection used for this

calculation was 0.002. This deflection is on thesame order as the machining tolerances used tomanufacture the holder and can therefore beconsidered negligible. The calculationssupporting this part of the design are shown inthe appendix, p. xxvi. and the resulting crosssection dimensions are shown inFigure 14.

Combining these different aspects of thedesign, the holder design in Figure 16 wasfinalized. The dimensions of the horizontal armsof the holder were set to leave roughly 4.5between any part of the holder and the induction

coils. This was a conservative spacing basedupon past work by Paul Sander in which he foundthat objects further than about 3 would notcouple with a similarly sized induction coil. Theheight of the holder was based upon the height ofthe reactor which was governed by the sameconstraint, and by the size of quartz tubesavailable for purchase. Finite element analysiswas performed using Abaqus to confirm that thedeflection under static loading conditions wouldbe on the order of 0.002.

Figure 13: Diagram showing the risk of a collision between theanode and cathode due to deflection in the holder.

F

Figure 14: Final dimensions (in inches) of the cross section used toconstruct the reactor holder. Deflection under predicted loading was

calculated to be 0.002".


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Reactor/Stand Interface

With the reactor holder and stand bothfinalized, the only remaining design was theinterface between the two parts. As discussedpreviously, this interface must be rigid and allowthe reactor to travel horizontally. Due to therelative low cost and ease of production, it wasdecided that this motion would be provided by an80/20 T-slotted bar with a corresponding linearbearing. The best method of mounting the T-slotted framing to the stand was not immediatelyobvious. Therefore, a number of mountingschemes were compared,Figure 15.The differentmethods where compared using the criteria ofstiffness, weight, cost, and complexity (Table 3).It was found that directly mounting the 80/20

framing to the linear stage (5) was the mostdesirable method. Initial attempts were madeusing this method. However, this resulted in areactor that was mounted too high off of the labbench surface. Therefore, the design was changedto include the use of a spacing plate (4). Reactorheight was not a consideration that appeared inthe original decision matrix, resulting in theultimate use of what initially appeared to be thesecond best method. The final method employedto connect the holder to the stand is shown inFigure 17. In this figure, the holder is on the

right and the linear stage is on the left. The holderis attached to the linear bearing which rides alongthe piece of T-slotted framing. This framing isattached to a reinforced angle bracket which is inturn connected to the carriage of the linear stage.A spacing plate rests in between the angle bracketand the carriage to create clearance for the sidesof the linear bearing. The final stand, holder, andreactor assembly is depicted inFigure 18.It canbe seen that the stand allows for horizontalmotion of the reactor via the 80/20 T-slottedframing and for vertical motion via the linear

stage that makes up a large portion of the stand.The reactor is positioned at the end of thecantilevered holder which allows for plenty ofclearance to eliminate any danger of couplingwith the induction field. The linear actuator ispositioned above the reactor, so that the anode islowered straight down into the reactor.

Figure 16: Final design for the reactor holder. The design was

optimized for rigidity by using a "C" cross section.

Figure 15: Side view of possible mounting schemes for attaching thereactor holder to the stand. The square in each image is a head-on view othe T-slotted framing and the C shape above it represents the linea

bearing. 1) Vertical plate with countersunk holes. 2) Two angle bracketsupporting cantilevered plate. 3) Two angle plates forming horizontasurface. 4) Spacing plate and direct connection. 5) Direct connection withcountersunk screws.


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Material Considerations

Both the stand and the holder areconstructed of aluminum alloy 6061-T6. A

metallic material was used despite the risk ofinductive coupling simply because no otherclass of material is suitable for thisapplication. Plastics or composites cannot beused due to their poor thermal properties andlow melting points. Ceramics cannot be useddue to their low toughness and poor tensileproperties. Aluminum was used rather thansteel due to its low density and corrosionresistance. Low density is preferable for acantilevered configuration in which weight isvery important. Corrosion resistance

provided by the protective oxide layer meansthat, unlike steel, aluminum does not need tobe oiled. The 6061-T6 alloy was used due toits low cost, high availability, andmaximized aging.

Figure 17: Side view of scheme used to mount the holder to the stand.The bearing running along the T-slotted framing provides horizontal motion,

hile the carriage provides for vertical motion.

Table 3: Decision matrix used to pick method of connecting the holder to the stand. Option 4 was eventually useddespite scoring only second-best.

Rate

1(worst)-

5(best) Weight

(1) Direct

Connection

(2) Plate

and Angle

Brackets

(3) Angles

and Angle

Bracket

(4) Offset

80/20 Mount

(5) Direct

80/20

Mount

Stiffness 5 1 4.5 5 3.5 4

Weight 2 5 2 1 4 5

Cost 1 5 1 3 3.5 4

Complexity 3 5 2 1 4 5

Average

Score3.18 3.05 3.00 3.73 4.45


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Stress Calculations

A few key analyses were performed on the stand to ensure that the design contained no weaknessesthat had not been considered during the design process. Even a brief look at the design reveals that thearea most inclined to failure is at the mounting point where the holder connects to the stand. In this area,the entire weight of the reactor, holder, and linear actuator are held by just four bolts, all loaded in shear.Although the loading is relatively small, the concentration of stress in this area cannot be ignored.Therefore, two separate analyses were performed on this area. First, the shear in the screws wasconsidered and compared to the yield strength of an SAE Grade 5 -20 screw. The loading conditionsassumed for this calculation involve the holder resting on the far end of the piece of T-slotted framing,generating the maximum moment arm and thus the maximal shear on the screws. The calculations areshown in the appendix, p.xxvii.They show that shear in the screws is not even a remote concern. TheVon Mises stress in the screws is more than an order of magnitude below the yield stress of the screws.

However, it is also important to consider the aluminum into which the screws are threaded.Aluminum is a much softer material than steel, so it is important to check that the pull-out strength of thetapped holes in the aluminum is not exceeded. In this calculation it is assumed that only 3.5 threads areengaged and that 38% of the tensile force is held by a single thread. The full calculations are found in the

appendix, p.xxix.The factor of safety was found to be at least 15, meaning that thread pull-out in thealuminum plate is not a concern. If the threads in the aluminum are not in danger of pulling out, it is notnecessary to duplicate these calculations for the screws steel threads.

Figure 18: Final design of reactor stand and holder. The linear stage provides vertical motion while the T-slotted framing provides horizontal motion.


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Tipping Calculations

Although the preceding calculations suffice for investigating areas of high stress in the design, itis also necessary to consider one further mode of failure for the stand: tipping. When the holder is at thefar end of the T-slotted framing, the reactorsweight is concentrated many inches outside of the base ofthe stand. Therefore it is important to ensure that the stand will not tip over when the reactor is fullyextended. Tipping could feasibly occur in either the direction parallel to or perpendicular to the holder,since a cantilever exists in both directions. The calculations on page xxviii consider tipping in bothdirections by looking at the center of gravity relative to the edges of the base plates. Calculationsindicated that tipping was a possibility, so the base plate size was increased to that seen inFigure 18.Thefinal design gives a factor of safety of 1.6 in each direction.

Reactor Caps

Overview of Analysis

The reactor sealing mechanism used in this reactor is based upon that used in Sanders furnace.The cap parts were up-scaled to meet the requirements of the larger reactor (see Appendix 1,Figure 40-43). During research previously conducted by Brad Nakanishi, experiments with the previous furnacewere terminated prematurely due to cap temperatures approaching the O-ring failure temperature. A two-channel water cooling system was proposed for the new generation reactor to correct for this problem.The following is an analysis of the effectiveness of said system. Figure 19 outlines the analysisperformed.

Figure 19: Water-cooled cap modeling flow chart.

Incident Radiation

The cap and crusceptode system was analyzed to determine the steady state heat flux from thecrusceptode top to the bottom of the cap via radiation. The aluminum cap is assumed to be at ambienttemperature (300 K) and the molybdenum crusceptode at its maximum operating temperature (2600 K).Only three surfaces are considered: 1.) the bottom of the aluminum cap, 2.) the top of the molybdenumcrusceptode, and 3.) the inner surface of the quartz tube. Quartz is opaque to lower energy thermalradiation and nearly a perfect transmitter of radiation in the upper range of infrared and higher energy

DetermineIncident

Radiation

Analytical

2 SimplifiedCases

Considered

Channel Flowand Cooling

Analyis

Analytical

Internal PipeFlow

COMSOL

Modeling

Determinewater coolingeffectiveness


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radiation4. Thus, two cases were considered for modeling purposes: 1.) clear quartz tube, transmissivity =1 and 2.) opaque quartz tube, transmissivity = 0.

Case 1: Clear Quartz Tube, transmissivity = 1SeeFigure 20below for the geometry diagram and nodal network analyzed. View factors are

calculated and two equations with two unknowns (J1and J2) are the result.4

Figure 20: Left: Diagram of crusceptode (red) and cap (gray). Right: Heat transfer nodal network.

Case 2: Opaque Quartz Tube, transmissivity = 0See Figure 21below for the geometry diagram and nodal network. The view factors are

calculated and 3 equations with 3 unknowns (J1, J2and J3) are the result.

Figure 21: Left: Diagram of crusceptode (red), cap (gray), and quartz tube (blue). Right: Heat transfer nodal network.

A plot generated in Matlab (seeFigure 22)summarizes the results of the two cases. The incidentheat flux on the cap is plotted against crusceptode-cap distance. This plot can be used for quicklyestimating the heat flux on the aluminum cap if the distance between the crusceptode and cap are known.


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It was found that the presence of an opaque quartz tube increases the incident radiative flux byapproximately ten percent.

Figure 22: Plot of the various cases used in analytical modeling incident radiation on the cap versus distance between thecap and crusceptode.

Channel Flow and Cooling

A convective heat transfer analysis was performed to determine the heat transfer coefficient, h,and the mean temperature distribution in the water-cooled channels. SeeFigure 23below for a diagram.The maximum mass flow rate of the Neslab RTE7 chiller (0.25 kg/s), the diameter of each channel (0.250inch), and the inlet temperature (298 K) are known. It is assumed in this analysis that the heat transferredto the bottom cap equals that transferred to the water cooling channels. By design, the flow from thechiller will be divided equally between the two channels. Based on the Reynolds number for waterflowing through a 0.250 inch diameter pipe at 0.25 kg/s (~0.05), this flow is definitively laminar (RE


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Figure 23: Diagram of aluminum cap showing heat fluxes (orange) to the cap, qc, with incidence area Ac and to the water-cooled channels, qs, with area 2DL. Parallel flow is indicated by blue and red arrows.

Figure 24: Plot of cap channel surface temperature (solid lines) and mean water temperature (dashed lines) for variouswater mass flow rates generated in Matlab. It can be concluded that an incident heat flux of 7 kW/m2 corresponds to a 3.5 inch

distance between the cap and crusceptode, a worst case scenario. is the combined mass flow rate through both channels.

COMSOL Modeling

The results of incident radiation and channel flow analyses are combined in a COMSOL model of

the cap. Convective boundary conditions are used in the water channels and a constant radiative heat fluxboundary condition is used on a circular region at the bottom of the cap. The remaining boundaries areradiation to ambient. The results of this model with and without water cooling are shown inFigure 25andFigure 26below, respectively.


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Figure 25: Surface temperature plot of the cap without water cooling generated using COMSOL. The radiative heat flux

used in this simulation was 7 kW/m2, and the emissivity was 0.19 (oxidized aluminum). The surface temperature range foundusing this model was 248-249C.

Figure 26: Surface temperature plot of the cap with water cooling generated using COMSOL. Again, the radiative heat fluxused in this simulation was 7 kW/m2, and the emissivity was 0.19 (oxidized aluminum). This time, water cooling was added witha 0.25 kg/s mass flow rate and 25 degrees Celsius inlet temperature. The surface temperature range found using this model was

43-44C.

Discussion

This engineering analysis verified the necessity and effectiveness of the water cooling designprior to ordering materials and manufacturing the reactor caps. Several simplifying assumptions weremade, but the results provided an effective guide for implementing a method for water cooling the caps.Tests of the reactor have demonstrated the effectiveness of the water cooling design, and modificationscan easily be made if more cooling is necessary in the future.


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Inner Reactor Analysis

One of the most challenging aspects of this project was the design of the inner reactor. Anenormously high temperature was to be maintained at the center of the reactor. Based on the engineeringspecifications, this temperature needed to be 2300-2400oC. Nonetheless, the entire reactor was containedin a quartz tube, which has been found to melt at approximately 1700oC. Moreover, the quartz tube is

sealed vertically by two aluminum caps. Aluminum has been found to melt at 660

o

C. A more pressingconcern comes from the O-rings which maintain an airtight seal between the aluminum caps and thequartz tube. These O-rings are rated to function only up to 200oC. Finally, the materials used inside thereactor itself were all chosen to be ceramic (other than the crusceptode). Ceramics were chosen becauseany metallic elements would couple with the induction field. Unwanted heating could then be caused tothe supporting structure. While ceramics are beneficial because induction coupling would not occur, thesematerials are very susceptible to cracking under high thermal gradients.

Thus, with these considerations in mind, it was necessary to create a design to slow thedissipation of heat from the crusceptode through the radiation shields. The materials to be used in thereactor were often expensive, such as the high-temperature ceramics, or custom manufactured, such as thealuminum caps. Thus, to avoid destroying any equipment, it was necessary to perform a series of analyseson the heat transfer in the reactor before running any tests. These analyses were performed using

COMSOL Multiphysics Software.In order to construct an accurate model representing the heat transfer inside the reactor, it wasnecessary to input material properties for all materials to be used, including surface emissivity (), density(), thermal conductivity (k), and specific heat capacity (Cp). The materials included: molybdenum (thecrusceptode), quartz (the tube), aluminum (the two caps), aluminum oxide (a.k.a. alumina, Al2O3), andmagnesium oxide (a.k.a. magnesia, MgO). Due to the relative obscurity of these ceramics, many of thematerial properties came from current research papers. Moreover, all of these materialsproperties behaveas a function of temperature. Thus, it was necessary to provide each COMSOL simulation with as manyvalues of each material property as possible at each corresponding temperature so that the software couldperform the appropriate interpolation. These values were calculated or taken from the thesis of PaulSander.7

Three design iterations were simulated and analyzed in COMSOL before the final reactor designwas settled. These iterations are presented and described in detail below.

1. The first design iteration was built upon the previous reactor design upon which this project isbased. A smaller tube was featured in this previous reactor, and thus there was room for onlytwo heat shields. The first design iteration of this reactor also featured this two-shield design.A lower pedestal was also featured on the bottom of the reactor in an attempt to limit thermalgradients in the outer heat shield by essentially splitting it into two separate components. Thegeometry was modeled in SolidWorks which was then imported into COMSOL. In thismodel, all shields were modeled as magnesia (MgO). A heat transfer analysis was thenperformed in COMSOL. From this analysis, it was clear that the two-shield model wasallowing too much heat to escape to the outside of the reactor at the tube and the caps. Thecaps were calculated to reach a temperature of about 500K (227oC). This was well above thetemperature for which the caps O-rings were rated. And so, because there was plenty ofspace in the larger reactor, it was decided that a model be analyzed with a third heat-shield.


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Figure 27: Inner reactor design iteration 1. Two nested radiation shields with two nested pedestals.

Figure 28: Inner reactor design iteration 1. Close up of "hot zone"


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2. Three heat shields were featured in the second design iteration. Again, pedestals were used tominimize the thermal gradients of each individual heat shield as an attempt to minimize onstress from thermal expansion and limit cracking. The goal of this model was to limitradiation out from the center of the reactor towards the quartz tube and the caps. As in theprevious iteration, the geometry was modeled in SolidWorks and imported into COMSOL fora heat transfer analysis. Once again, in this model, all shields were modeled as magnesia

(MgO). This model was successful in reducing radiation in the horizontal direction, as can beseen by the lower temperature of the quartz tube. In the previous model, the quartz tube wascalculated to maintain a temperature of approximately 1200K (927oC) at the location closestto the crusceptode. The temperature computed in the three-heat-shield model at this samelocation was about 900K (627oC). However, the temperature of the caps was not significantlylower than in the two-heat-shield model. This would be the goal of the third iteration,presented below.

Figure 29: Inner reactor design iteration 2. Three nested heat shields with three nested pedestals.


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Figure 30: Inner reactor design iteration 2. Close up of "hot zone"


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3. Use of magnesia frit was considered in the third design iteration of the reactor. Thehypothesis to be tested in this iteration was whether adding frit into the empty space insidethe bottom pedestal would increase or decrease heat transfer to the bottom cap. It washypothesized that this would decrease heat transfer. Despite the fact that the frit provides adirect conduction path to the bottom cap, it was speculated that the loss of radiative transfer

would overcome the potential increase due to thermal conduction. This could be explained byexamining the corresponding heat transfer equations for a black body. In heat transfer byconduction,

which shows that heat transfer is linearly proportional to temperature and scaled by thethermal conductivity, k. While in radiative heat transfer, heat transfer is related totemperature to fourth power (T4)

thermal conductivity, k, of grainy solids such as sands and frits is generallyvery low, so a great deal of heat is able to be absorbed without transferring much heat to theopposing surface.

Material properties for magnesia frit could not be found in a literature search.The material density (), was measured experimentally in the lab and determined to be 1600kg/m3. The values of surface emissivity (), thermal conducivity (k), and specific heatcapacity (Cp) were estimated based on similarities to other materials. Thus, it was importantto consider in this final model that it would be critical to monitor a real test carefully as itwould be possible that these estimations would be incorrect. The geometry was constructed in

SolidWorks and imported into COMSOL for heat transfer simulation. In this model, theinner-most shield and two inner-most shields were modeled as magnesia (MgO), while theremaining shields were modeled as alumina (Al2O3). The materials actually available for atest were reflected in this modeling because of the obscurity of magnesia and the difficulty toobtain more of it in the time-scale presented in this project. Magnesia was used for the innerheat shields because of its very high melting temperature (3125K, 2852oC). Thus, it wasdeemed safe to be in direct contact with the crusceptode.

From the thermal model (presented below inFigure 32), it was shown that the inclusion of frit did significantly minimize the heat

transfer to the lower cap. The top cap was modeled with the updated properties reflecting thewater-cooling system which had been designed and developed since the first and seconditeration. In the first two models, it had been calculated that the bottom cap would reach a

temperature of more than 550K, which is alarmingly close to the point of failure of the cap O-rings. With the addition of frit, the bottom cap was computed to maintain a temperature ofapproximately 320K (43oC). This value was deemed acceptably far below the failuretemperature of 200oC, and the design was implemented for a live test.


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Figure 31: Inner reactor design iteration 3. Three nested heat shields with pedestals, magnesia frit, and water-cooling.

Figure 32: Inner reactor design iteration 3. Close up of hot zone showing added frit and pyrometer sight.


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Temperature Measurement

Accurate temperature measurements are critical when working with ultra-high temperaturesystems such as this. Due to the nature of this project and the materials being used, there was fairly littleroom for error. Thus, it was defined in the engineering specifications that temperatures should be knownwith an error of less than 50oC.

The temperature was measured by two independent methods. First, temperature was measureddirectly, via contact, using thermocouples. During tests, two type K thermocouples were placed in contactwith each of the caps to be sure that their temperatures would not cause O-ring failure. A type Cthermocouple was inserted into the crusceptode. The melt temperature could be measured directly withthis thermocouple up to its point of failure at 2320oC, just above the desired melt temperature of 2300oC.

The temperature of the crusceptode was also measured using a pyrometer. An alumina tube wasused to block radiation coming from sources other than the crusceptode. Based on the geometry of thesystem, it was determined that a longer sight would provide the most accurate representation of thetemperature of the crusceptode. This is because the majority of the photons received by the pyrometerwould be emitted by the bottom of the crusceptode, rather than the surroundings. Thus, the pyrometersight was designed to be as long as possible while still fitting into the physical design of the reactor andstand. The raw pyrometer reading must be calibrated against a known value. The data from the inner

thermocouple can be used for this purpose.However, thermocouples inside the reactor are affected by the electromagnetic field generated bythe induction coils. The field creates a stray voltage in the thermocouple lead, which creates an error thatincreases with field strength. Nonetheless, the actual reactor temperature can be observed by shutting offthe power to the induction coils. When this is done, the thermocouple reading immediately jumps to adifferent value, which is hypothesized to be the true temperature. Testing shows this difference intemperature to be between 20oCand 70oC. With more tests, it maybe possible to determine a scalingfactor, although it is unclear thatthe value would be consistentfrom test to test. See Future Worksection for more.

Figure 33: Schematic showing that a longer pyrometer sight tube blocks out radiationfrom sources other than the crusceptode.


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Engineering Solution

Construction Process

The final design is comprised of two subassemblies: The reactor itself, which must withstandhigh temperatures, and the reactors stand, which must be extremely rigid. Because of the difference inspecifications between the two assemblies, different approaches were taken for their construction.

Stand Construction

The stand was designed to be fabricated almost entirely in-house, in the Tufts mechanicalengineering machine shop. Exceptions include the linear stage and linear actuator, which were taken withpermission from Bray Laboratories. The rest of the stand was designed around these two components.This saved money over purchasing new components, but the main incentive was saved time: Buying anew stage or actuator would have a lead time.

Once the linear stage was acquired, the stand design was finalized, and machining began. Moststand parts were simple plates with tapped or through holes, which could be created in rapid successionon a milling machine. One exception is the large C-shaped reactor holders, which were completed viaCNC machine. This is because the holders are very large and difficult to machine manually. To do sowould require more than a day of continuous work by an inexperienced machinist. Therefore, a mistakewould be very costly, in terms of time and money.

Both holders were cut from one 18x18x plate, but the CNC machine only had 15 of travelalong the x-axis. To accomplish this, the pieces had to be laid out such that each fit in the others negativespace. The plate was first cut with a band saw in an N-pattern. To allow the band saw to make tight-radiusturns, through-holes were cut in the plate at the turning points. Once the plate was separated, each piecewas individually programmed into the CNC.

A mistake was made in programming theCNC for one holder. The slots for the reinforcingplates were cut deeper than intended, and the slotfor the reactor bottom cap was cut through the plate.This error could have been costly, necessitating thepurchase of another $160 plate and several daysworth of lost time, but it was determined that the partcould be salvaged with some modifications.

Because the deeper slots would cause thepieces to sit closer together, clearance for the reactorwas provided by milling the surfaces that contact thereactor caps. This allows the reactor to fit in theholder, albeit slightly off-center.

An additional error was made whenmanufacturing these parts. Previously, all parts weredesigned in SolidWorks to have zero clearance. All

clearances were indicated on drawings, which werethen manually implemented on milling machines.Because the CNC machine takes .dxf files rather than

SolidWorks drawings, the top cap slot was sized to the exact size of the cap. A small modification had tobe made in order to give the cap proper clearance.

Both of these modifications were made at the Colby machine shop, due to the ME machine shopbeing closed the week of 11/12. Modified drawings were made to indicate the exact changes being

Figure 34: Preliminary bandsaw cut, including drilled holes.


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requested, and the drawings were explained in person before work was performed. The parts returnedwith the requested modifications, allowing the reactor holder assembly to be assembled.

The linear stage was mounted upright using two right-angle pieces cut from a single section.When ordering the right-angle section, it was known that it would not be perfectly square. However, thegiven tolerances were deemed suitable, as the pieces would be attached via #8-32 screws to a base platewith through-holes larger than the screw diameter. In the event that the angles deviated more than the

stated tolerance, the through holes could be slotted to accommodate. Fortunately, this was not necessary.The reactor holder assembly was attached to the linear actuator via a piece of T-slotted aluminum

framing, with an attached linear bearing. The piece of framing was already on-hand in the lab, but thebearing, mounting bracket, and screws were ordered through MIT. Due to a shipping error, these were thelast components to arrive, pushing back the first furnace test.

Reactor Construction

The reactor assembly is primarily composed of brittle materials thatare difficult to machine. The quartz tube and ceramic radiation shields weretoo fragile to modify with normal machining techniques. Therefore, mostmaterials were used as-delivered.

The pyrometer sight holes are the notable exception. To allow the

pyrometer to view the bottom of the crusceptode, it was necessary to drillholes in the ceramic crucibles being used as radiation shields (Figure 35). Todo so, the crucibles were shrouded in bubble wrap and held in a vise, thencut with a diamond drill bit. The drill press is equipped with water cooling,so as not to shatter the ceramic.

The aluminum reactor caps were also machined in the ME machineshop, using the same CNC process as part of the stand. The threaded holes inthe caps hold steel Heli-Coil inserts, so that repeated screw fastening doesnot damage the soft aluminum.

Reactor Tests

A variety of reactor tests were performed in order to confirm theoperability of the reactor created for this design project. These tests alsoallowed the team to troubleshoot problems that arose. Not surprisingly, avariety of small issues were found and corrected while testing the reactor. Asummary of the four tests follows:

Test 1:

The crusceptode was filled with alumina powder for this test. The reactor was heated at a rate of2C/s. At a thermocouple temperature of roughly 1400C, the temperature measured by the pyrometersuddenly dropped. The test was stopped and the reactor was allowed to cool. Inspection of the reactoronce it had cooled revealed that the heat shields had shifted during heating, causing partial obstruction ofthe pyrometer sight tube. The shift seems to have been caused by expansion in alumina balls that had

been used in place of magnesia frit for this test. Use of alumina as a frit material was discontinued afterthis test. The reactor reached a maximum temperature of 1650C.

Test 2:

The crusceptode was filled with alumina powder and heated at a rate of 2C/s. Magnesia frit wasused in place of the alumina frit used in test 1. At around 2000C the plastic tubing used for cooling thetop cap of the reactor began to leak. The reactor was shut down and allowed to cool. Inspection of the

Figure 35: Holes are drilled inceramic using a specialized drill

press, so that the pyrometer has adirect view of the crusceptode

bottom.


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crusceptode revealed that the alumina powder within the crusceptode had melted. The maximum reactortemperature recorded during this test was 2100C.

A slight change was made to the water cooling system as a result of this test. Originally, plastictubing used for water cooling was run directly to the top cap. Toreduce the temperature of the plastic, small sections of steel tubingwere connected directly to the top cap. The plastic tubing was then

connected to these steel tubes, increasing the distance betweenplastic and the hot, aluminum caps.

Test 3:

The crusceptode was filled with alumina powder and thereactor was heated at a rate of 2C/s. At a temperature of 1000C, theinner radiation shield developed an axial crack and lost structuralintegrity. The test was abandoned.

Prior experience with the last generation of induction heatedMOE reactors indicates that it is common for radiation shields tocrack circumferentially due to thermal gradients. However, this wasthe first shield ever to crack axially. Therefore, it is unclear what

exactly caused the shield to crack. It was observed that the holedrilled in the bottom of this shield for the pyrometer sight tube wasvisibly off center, but the crack did not propagate from the hole.Thus, stress concentrations due to the off-center hole are an unlikelysource of the crack. It is also possible that the shield simplydeveloped too large of a thermal gradient, but this does not explainthe axial orientation of the crack. More data must be collected todetermine if radiation shield cracking is a large problem for thisgeneration of MOE reactor.

Test 4:

The crusceptode was filled with alumina powder for this test. The reactor was heated at a rate of

1.5C/s. After 20 minutes of heating, the goal temperature of 2300C was achieved. Shortly afterwards,the type C thermocouple failed, as expected at this temperature. No issues were reported in this test, andthe alumina inside the crusceptode had clearly melted.

Team Responsibilities

The responsibilities were nominally distributed as follows. However, weekly meetings, as well asfrequent unscheduled conversations, ensured that each team member assisted one another with all areas ofwork.

Nathaniel Eckman was responsible for the stand design. He designed the majority of SolidWorksmodels for the project. Nate did a large portion of the machining for the stand, as well as the assembly.

Jack Carter was responsible for the linear actuator design, fabrication, and programming. Hewired the electronics required to drive it, and integrated the controls into the existing LabView interface.

Tyler Andrews was responsible for all heat transfer modeling. He created the Comsol model andanalyzed the impact of various heat shield configurations.

Brad Nakanishi was responsible for instrumentation and designing the reactor caps. He was alsoresponsible for all tests in the lab. He was the only team member trained to use the lab equipment, and heguided the rest of the team throughout the experimentation process.

Any responsibilities not listed here were shared among team members equally.

Figure 36: Reactor during testing


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Conclusion

In conclusion, most of the original goals outlined in the proposal of this project have beenaccomplished. To review, these goals were:

Increase cap area for sensor ports Improve temperature measurement capability Implement actuator for anode Increase ease of assembly Achieve melt temperature of 2300oC Demonstrate electrolysisFirst, it was desired to increase the cap area to allow

for more room for instrumentation ports. This goal wasaccomplished by two methods. First, the diameter of thereactor was increased, thus increasing the cap diameter.More importantly, the fundamental reactor design was

changed from a one-cap tube with a rounded bottom to atwo-cap straight tube. With this, the effective cap sizewas doubled, allowing for instrumentation ports on bothends.

The temperature measurement capability was alsoimproved by the addition of these extra instrumentationports. More thermocouples were able to be inserted intothe reactor, giving accurate measurements of multipleparts of the reactor. Also, the pyrometer was able to beattached via the bottom of the reactor. A more accuratetemperature reading could be obtained by this methodwith the pyrometer facing the bottom of the

molybdenum crusceptode. The crusceptode, as a metalwith a very high thermal conductivity, is essentiallyisothermal during the melt, while small variances such as bubbles or nulls can occur during the melt in theslag itself.

Another important goal which was realized during thisdesign project was the implementation of a computer-controlled linear actuator for the insertion of the anode intothe slag. The anode was to be lowered into the molten slagwith an accuracy of 0.020 to 0.060 inches. The currentdesign features a computer programmed linear actuatedanode with a resolution of 0.001 inches.This makes thingssimpler and safer for the experimenter.

The ease of experimental assembly was alsosignificantly increased in this design. The reactor can belocked in place thanks to the square caps and the lockingmechanism of the reactor stand. This creates rigidity in theX-, Y-, and Z-directions. With this rigidity, experimental set-up becomes much more repeatable. Thus, experimental set-up is made significantly simpler because the elements arelocked into place as opposed to having to be adjusted by

Figure 37: Thereactor facility

Figure 38: Looking down into the reactor. Fromcenter: alumina powder, molybdenum crusceptode,radiation shielding, and aluminum collar.


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hand. The ease of set-up is also contributed to by thepreviously mentioned anode actuation. In previousreactors, the anode was lowered into the reactor by hand.The set-up is made safer and easier by this actuation.

Finally, the goal of achieving a reactor temperature of2300oC was achieved in testing. Four (4) test-melts were

conducted prior to the writing of this report. During two(2) tests, the test sample of alumina (Al2O3) was melted,implying that the reactor was above the melt temperatureof alumina at 2072oC. In the final test, the goaltemperature of 2300oC was reached in the reactor. Thetemperature was measured directly by a type-Cthermocouple while simultaneously being monitored bythe pyrometer. The thermocouple data became erraticvery close to the 2300oC mark, which is when failurewould be expected from thermocouples of this rating. Itcan be shown by scaling the pyrometer data after the testthat this was indeed the case and the reactor was over the

2300oC mark.The only goal not achieved was demonstration of

molten oxide electrolysis using the reactor designed inthis project. Given the depth and complexity of simply designing and building the reactor, performingelectrolysis was deemed beyond the scope of this project.

Figure 39: The reactor


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Future Work

While an enormous amount of work was devoted to this project and creating a functioning inductionreactor which could heat to 2300oC, much work is left to be done, both in improving the reactor designand in using the reactor as an experimental apparatus to perform molten oxide electrolysis of rare earthoxides. Several of these ideas for improving reactor design are presented below.

First, one of the most delicate and complicated aspects of the current set-up procedure is the arrangingof the nested heat shields and frit inside the reactor. The heat shields must be centered on the bottom capand on top of one another by hand. This is difficult and relatively inaccurate. An idea to improverepeatability of this would be to machine a groove in which the outer heat shield and pedestal could bemounted on the bottom cap. This would assure that at least these two shields are centered and in the sameposition for each test. Grooves could also be machined into the top of the ceramic crucibles used aspedestals. This would allow the pedestal above it to be mounted into the exact same spot. The machiningprocess would be significantly more difficult, but this could create a model of nested heat shields which issignificantly more rigid and repeatable.

Another idea to improve the set up process would be to have the bottom cap mounted to a piston. Thiscould raise the entire inner cap up out of the quartz tube which would allow for easier access to the innercomponents than the current method, in which tweezers are necessary to lower the final sample into theinner heat shield and then into reactor.

Once the reactor design has been perfected, a great amount of design and experimentation will benecessary to perform molten oxide electrolysis on rare earth oxides. A great deal of future work can befound here, though it is hard to speculate as to exactly what that might be at this stage because no MOEtests have been attempted. Some aspects which may be worthwhile for exploration include anode design(shape and material), choice of electrolyte solution, and current flow between electrodes.

Two group members, Tyler Andrews and Brad Nakanishi, will be continuing to work on variousaspects of the project in the Spring of 2013 as part of an undergraduate honors thesis. Graduate studentJay Gomes will be continuing to help with the project as well. All are excited to continue work on theproject and bring the current design to a fully functional molten oxide electrolysis reactor.


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Appendix #1: Engineering Drawings

Parts List

Inner Reactor

- 0.25ID x 0.500 OD x 0.75 Molybdenum crucible (Crusceptode)- 0.815ID x 0.945 OD x 8 Alumina tube (Outer heat shield)- 0.572ID x 0.675 OD x 4.070 Alumina tube (Bottom pedestal)- 0.669 ID x 0.797 OD x 3.572 with 0.500 hole in bottom alumina crucible (Second heat

shield)- 0.125 ID x 0.188 OD x 6 Alumina tube (Pyrometer sight)- 0.400 ID x 0.498 OD x 0.575 Magnesia tube (Second pedestal)- 0.399ID x 0.489 OD x 3.696 with 0.500 hole in bottom magnesia crucible (Inner heat

shield)- 0.263ID x 0.369 OD x 0.618 Magnesia tube (Inner pedestal)-

1.050ID x 1.130OD x 11.88 Quartz tube- Magnesia FritReactor Cap

- Aluminum Cap, Not Water-cooled (See p.iv)- Aluminum Cap, Water-cooled (Seeviip.vii)- Aluminum Bottom Collar (See p.v)- Aluminum Middle Collar (See p.vi)- 1 Length, 6-32 Thread, Black-Oxide Alloy Steel Socket Head Cap Screw (McMaster-Carr)- Length, 6-32 Thread, Black-Oxide Alloy Steel Socket Head Cap Screw (McMaster-Carr)- Viton fluoroelastomer O-ring, 3/32 x 2 ID x 2 7/16 OD (McMaster-Carr)- Viton fluoroelastomer O-ring, 1/16 x 2 ID x 2 5/8 OD (McMaster-Carr)-

.207 Length, 18-8 SS Standard Heli-Coil Insert 6-32 Internal Thread (McMaster-Carr)- Tube SS Fitting x MaleNPT Bore-Through (Swagelok)- 3/8 Tube SS Fitting x 3/8 Male NPT Bore-Through (Swagelok)- PTFE Ferrule Set for 3/8 Male NPT Bore-Through (Swagelok)- Various Ultra-Torr Vacuum Fitting to Male NPT Adaptors (Swagelok)- Various Ultra-Torr Vacuum Fitting to tubing adaptors (Swagelok)

Stand

- Floor Right (See p.xx)- Floor Left (Mirror of above)- Floor Brace (See p.xxiii)- Right Side (See p.xix)-

Left Side (Mirror of above)- Linear Stage (Thomson Linear: Thomson Superslide Systems, model unknown)- Carriage Plate (See p.xxii)- Angle Bracket (80/20: Part 4138)- 8-32x1/2 Flat Head Screws (x14)- -20x1/2 Flat Head Screws (x4)- -20x1 Round Head Screws (x4)- Washers (x4)- Nuts (x4)


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Stand/Holder Interface

- Carriage Spacer (See p.xxi)- 17 80/20 T-slotted framing (80/20: Part 2020)- End Caps (80/20: Part 2028) (x2)- -20x3/4 Button Head Socket Cap Screws (80/20: Part 3692) (x4)- -20x3/4 Button Head Socket Cap Screws w/ Slide-in Nut (80/20: Part 3393) (x4)- 80/20 Linear Bearing (80/20: Part 6423)- 80/20 Hex Bearing Break (80/20: P

rare earth reactor final report

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