High Temperature (>500o C) Ultrasonic Transducers: An Experimental Comparison among Three Candidate Piezoelectric Materials

High Temp Ultrasonic Transducers

D. Parks, K. Sinding, S. Zhang and B. Tittmann

Penn State University, University Park, PA 16802

Outline

[1] Motivation

[2] Objective

[3] Work in Progress

[4] Results

[5] Conclusions

Motivation

Ultrasonic techniques have been widely applied for non-destructive evaluation (NDE) of material properties and structural integrity.

The ability to apply typical ultrasonic techniques in high temperature environments is desired for the

monitoring of process variables, such as viscosity of melts

evaluation of structural/material integrity in harsh environments, e.g.,

turbine blades,

combustion engines and

nuclear reactors.

Problem Statement

Aircraft engine turbine and fan disk cracks have led to disk burst and catastrophic engine failures:

United Airlines 232, Sioux City, Iowa, (1989) 111 fatalities

Delta Air Lines 1288, Pensacola, Florida, (1996) 2 fatalities

Air Florida Airlines 2198 takeoff aborted

Damaged Engine of N927DA

Part of the DC-10's fuselage after the crash.

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Jet engine rotor failures may occur from fatigue (high and low cycle), material defects, assembly errors, and secondary causes such as bearing failures, over temperature induced rubbing an foreign object ingestion

Although rare in occurrence, a disk rupture in an aircraft engine can lead to a catastrophic failure. In 1989 United Airlines flight 232 crashed during an attempted landing at Sioux Gateway Airport, Iowa. The separation, fragmentation, and forceful discharge of the stage one fan rotor from the number two engine led to leaks in all three hydraulic systems, resulting in loss of flight controls. The failure was due to a fatigue crack in a critical area of the fan disk. Here 111 fatalities occurred.

What are the Applications?

Characterization of flaws is an important goal of ultrasonic NDE in aircraft

structural health monitoring to achieve retirement for cause.

Aluminum wings/fuselage

Composite components

Turbines

Steam pressure pipes/vessels

Aircraft Disc brakes

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Application: develop in-situ monitoring techniques for a nuclear reactor environment

Research reactors:

Material specimens

LE fuel specimens

Core

Commercial reactors:

critical structures

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Fukushima Daiichi, Japan

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Background

High temperature piezoelectric materials have been surveyed previously.

For example, the commercial piezoelectric material PZT possesses a Curie temperature of about 350o C but has a maximum recommended operation temperature of 150-250o C.

As a result, buffer rods/ultrasonic guided waves have been utilized to keep the piezoelectric materials out of the high temperature region for extended periods of time.

Limitations

Limitations of a guided wave system include, depending on the required waveguide geometry, the following limitations:

Dispersive behavior limiting bandwidth

Sensors size

Cumulative attenuation losses over the waveguide propagation length

Losses due to material discontinuities along the ultrasonic propagation path

New Materials

The limitations imposed by this methodology have been one of many driving forces that are leading to the development of new piezoelectric materials with operational temperatures exceeding 350o C.

The relatively new high temperature capable materials include high Curie temperature ferroelectric materials and non-ferroelectric single crystals.

Additionally, depending on the intended operating frequency, electrical resistivity becomes problematic for high temperatures and low frequency operating conditions

Candidate Piezoelectric Materials

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High Curie Temperature

Few stable phase transitions

High electromechanical coupling

No problematic atomic species

Reactor waste requirements

Kaz

Show that AlN performs in up to 18.7 MGy

AlN

The group III-V material Aluminum Nitride (AlN) belongs to the Wurtzite structure and point group 6mm and is not ferroelectric.

No phase transitions exist aside from the melting point at 2800o C

resistivity of high quality crystalline samples is on the order of 10 Mcm at 200o C. The dielectric constant has been measured to be 8.5 and the piezoelectric strain constant has been estimated from thin films to be roughly 5 pC/N.

Reactor waste requires relatively short half life isotopes (Al 6061 primary material)

Aluminum foil coupling is used after heat treatment for a Brazed contact

Waveguide Al 6061 for in-situ characterization

Develop radiation tolerant BNC cable

Fused-Quartz insulation

Aluminum conduit sleeve/inner conductor

50ft Low Loss RG 213 U

Carbon-Carbon backing

Monolithic bulk z-cut single crystal AlN centered at 13.4 MHz

Transducer Design

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Test Setup

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Purpose: Understand Sensor Behavior

Does radiation reduce piezoelectric effect tending towards an isotropic state?

Is there increase dielectric loss due to defect generation and ionization

Is there mechanical failure due to transducer design?

Measurements

Impedance (tan)

Pulse-echo A-Scan through aluminum 6061 waveguide

In-Situ Test Time : 3 Months

Fast Neutron: 1.85x10^18 n/cm^2

Thermal Neutron: 5.8x10^18 n/cm^2

Gamma:26.8 MGy

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Results

Pulse-echo amplitude measured in-situ

Reactor-on status measured by loss in signal amplitude

Pulse-echo amplitude stable during off-state

Amplitude relatively stable 15% deviation

Fluctuations need to be isolated from changes in transducer mechanics

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Lithium Niobate

Ferroelectric material with a Curie temperature in excess of 1000 C depending on the stoichiometry .

3m crystal symmetry with the corundum structure. The 36 rotated Y-cut is quite sensitive in the longitudinal mode of vibration with a coupling coefficient of 0.48.

known to lose oxygen at elevated temperatures, particularly at low oxygen partial pressure.

decomposes at 600 C even in oxygen at atmospheric pressure.

oxygen loss is an activated process so for 170 hours before observing their decomposition.

YCOB

Oxyborate crystals with general formula ReCa4O(BO3)3 (Re = rare earth element, abbreviated as ReCOB) used for nonlinear optic applications.

no phase transitions occur prior to their melting points, those being on the order of 1400-1500 C.

ultrahigh electrical resistivity at elevated temperatures. For example, YCa4O(BO3)3 (YCOB) possesses a resistivity of 2108 cm at 800 .

The dielectric permittivity, piezoelectric strain constant, and electromechanical coupling factor of the (XYlw -15 /45) cut were found to be on the order of 11, 6.5 pC/N, and 0.12, respectively, with little variation in the range of room temperature to 950o C

Test Fixture for High T

Normalized echo amplitude while at 550oC continuously for 60 hours in an open tube furnace

Thermal Ratcheting Experiments

Pulse-echo Waveforms for YCOB

Sol-gel Spray-on Fabrication of BiTi transducers

Fabrication method in a nut shell:

Powders of ferroelectric materials are added to aqueous solution of mixed oxide precursors.

Powders increase viscosity such that sol-gel can be sprayed with an air gun onto surface.

Substrate is sprayed, then pyrolyzed.

Repeated until desired thickness is achieved.

Substrates can be metallic.

Sinter, followed by electroding and poling.

Sintering

Typical sintering methods have proved to be destructive and difficult to handle for the

sol-gel deposition method,

Induction sintering has proved most effective

Microwave sintering

Blow torch sintering

Corona Poling

Corona Poling

Poling method which does not require electrodes

Metallic substrate is grounded and heated (150-200o C), metallic needles are brought within proximity of sample surface

High voltage ( ~10 kV) applied to needles, resulting electric field aligns electric domains within ferroelectric material

Comb Transducer Fabrication directly on Pipe

Laser ablation has been effective at creating

an accurate electrode pattern for launching guided waves in pipes

Bi4Ti3O12 High Temperature Response

Amplitude remained relatively constant until 625 oC followed by sharp decline.

Bi4Ti3O12-LiNbO3

Ultrasonic response was lost after 1000 oC

OBSERVATIONS

the long-term in-situ testing indicates that all four materials are suitable for operation at 550 C for at least 55 hours.

carbon-carbon backing material is a limiting component. A solution based on a porous Al backing was found but thorough testing on this backing remains to be completed. This backing material is easily created by compressing Al foil at 550 C under 150 psi for 5 hours.

Fixture Details

Capacitance and Attenuation

OBSERVATION

The YCOB crystal exhibited a much less pronounced change in dielectric properties after heat treatment.

It is expected that YCOB is more stable at high temperatures than LiNbO3 which is known to deplete its oxygen particularly at low oxygen partial pressure.

YCOB, AlN and LiNbO3 exhibit stability in ultrasonic performance through the heat treatment of 950 C for 24 hours and 1000 C for 48 hours. Any variations observed were less than the experimental error.

Summary

High temperature piezoelectric crystals, including YCa4O(BO3)3, LiNbO3 and AlN, have been studied for use in ultrasonic transducers under continuous operation for 55 hours at 550 C.

Additionally, thermal ratcheting tests were performed on the transducers by subjecting the crystals to heat treatments followed by ultrasonic performance testing at room temperature and 500 C.

.The changes due to the heat treatments where less than the statistical spread obtained in repeated experiments and thus considered negligible.

Finally, in-situ measurements up to 950 C, of the pulse echo response of YCa4O(BO3)3 were performed for the first time, showing stable characteristics up to these high temperature.

Sol-gel Bi4Ti3O12-LiNbO3 gave good performance to almost 1000 C

CONCLUSION

At atmospheric oxygen partial pressures, 48 hours of exposure to 950 C or 24 hours exposure to 1000 C had no significant effect on the efficiency of ultrasonic transduction of LiNbO3, YCa4O(BO3)3 and AlN.

The oxidation of AlN, known to occur at these temperatures, had no significant effect on the ultrasonic transduction efficiency, but the difficulty to achieve high quality AlN single crystals limit their applications greatly.

YCOB crystal was found to be capable of efficient ultrasonic transduction to about 1000 C.

OUTLOOCK: From Naviers equation, the temperature dependence of guided wave propagation depends on the material properties.

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Theoretical Dispersion Curves

Effect of thermal expansion (cross-section)

CTE 6 /K in Zircaloy

295K (70K) vs. 700K (800F)

thickness = 0.015 mm

width = 0.05 mm

Negligible at 150 kHz

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Theoretical Dispersion Curves

Zircaloy bar specimen:

shear=19.7 mm @ 150 KHz

Bar width = 20.625 mm

Cannot use Rayleigh-Lamb equations for bar!

Solve via SAFE technique

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2D Semi-Analytical Finite Element (SAFE)

Define analytical solution in the wave propagation direction

Orthogonal function

Discretize cross-section; Boundary conditions

Eigenvalues of system = wavenumbers

Wavenumbers dispersion curves

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Group Velocity (SAFE)

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Group Velocity vs. Temperature (150kHz)

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Thank you for your attention

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