Indian Journal of Engineering & Materials SciencesVol. 5, June 1998, pp.136-139

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Admixed binders and lubricants in ceramic powder pressing-A study ofzinc behenate additive

Palanki Balakrishna*, B Narasimha Murty, R B Yadav, M Anuradha, P S A Narayanan & R PramanikNuclear Fuel Complex, Hyderabad 500 062, India

andS Majumdar

Radiometallurgy Division, Bhabha Atomic Research Centre, Bombay 400 085, India

Received 24 December 1997; accepted 28 April 1998

Admixed lubricants are used to bring down compaction and ejection pressures, and to increasepacking efficiency in ceramic powder pressing. Admixed binders are used for increasing green strengthof the compact. The stronger and denser green compacts so obtained, sinter to high densities withoutdefects and with minimal shrinkage, leading to very high acceptance rates in large scale production.Zinc behenate powder has been used as a lubricant cum binder for uranium dioxide with very goodresults. Green density versus compaction pressure has been obtained. Weight loss with temperature offree and admixed zinc behenate and carbon and zinc loss in admixed compact and other data arepresented. On the other hand, the use of zinc behenate could lead to bloating and high nitrogen values insintered uranium dioxide under some conditions of processing, which have been identified.

Defects which appear after sintering have theirorigin in the arrangement of particles established bythe shaping process'. Green strength is important forhandling of powder compacts. A tensile strength of5 MPa is sufficient for powder metal bodies. Mostceramic green bodies are weaker than this, 1 or 2MPa being typical', Green strength can be enhancedby using a binder in the powder.

When the die-wall lubricant is too thin, it tends toflow down the die-wall and collect on the bottompunch. Excess lubricant on the bottom punch is aknown cause of bottom endcapping in the compact(Swaminathan N, personal communication). On theother hand, insufficient lubricant on the die-wallleads to friction cracks in the compact. Uniformavailability of lubricant becomes more difficult asthe number of punches in the press increases. Greendensity varies significantly from region to region inthe same compact due to inter particle friction',Problems associated with die-wall lubricant andvariations in green density within the compact canbe minimized by an additive uniformly dispersed inthe powder.

In the pressing of small diameter compacts, it isnot practicable to have automatic die-wall

• For correspondence

lubrication, as the punch does not have sufficientspace for a lubricant channel in it. In such cases alsothe powders are admixed with the lubricant. '

The goals of dry pressing ceramics are uniformcompact size and green density, consistent part topart green density and a strong and defect-freecompact", These may be achieved by using admixedlubricants or binders.

The common lubricants used in ceramic powdercompaction are paraffin wax, aluminium stearate,butyl stearate, lithium stearate, magnesium stearate,sodium stearate, stearic acid, zinc stearate, oleicacid, poly glycols, talc, graphite and boron nitride.Common binders are poly vinyl alcohol, clay,microcrystalline wax', Uranium dioxide powdershave been pressed with or without additives", Zincstearate and zinc behenate were found suitable? Amixture of I% zinc behenate and 1% naphthalenewas used in UC and (U, Pu)C compaction", Otherstearates have been used for deliberately loweringthe sintered densities9

• After heating to 500°C in air,a black tar-like residue remained in the case of polyvinyl alcohol and poly acryl amide; no residue wasfound in the case of polyethylene glycol'", Inprevious papers, it was stated that defect-free U02compacts could be made on laboratory scale? orproduction scale I I by using zinc behenate additive.

-BALAKRISHNA et al.: ADMIXED LUBRICANTS 137

Subsequently, a detailed investigation was carriedout, the results of which form the basis of this paper.

Green density as a function of compactionpressure with and without zinc behenate admixed inV02 powder is presented here. Weight loss and otherdata on zinc behenate, in free and admixedcondition, are also presented. A possible mode ofdecomposition of zinc behenate in a reducingatmosphere has been suggested. A serious problemof impurity and bloating that could be faced undersome circumstances has been described andpreventive measures suggested.

Experimental ProcedureThe V02 powder is prepared through the

ammonium diuranate route I 2. It has a BraunerEmmett Teller (BET) specific surface area of 3.0 -3.5 m2/g and a Fisher Average Particle Size of 2 to2.5 micrometres. The powder does not flow readilyinto pressing die cavities, unless precompacted andgranulated.

Zinc behenate Zn(Cz, HB COOn powder was sofine that it could reveal finger prints readily. Thepowder was hygroscopic. On drying at 80°C at 0.1mm Hg pressure for 30 min, the moisture evolvedwas 0.18% by weight. BET specific surface areawas found to be 7.12 m2/g. Fisher Average ParticleSize could not be determined. The weight loss of thezinc behenate powder in free state as well asadmixed in V02 compact was studied using athermal analyzer.

When the zinc behenate content is I% or more, aseparate dewaxing furnace is required beforesintering. When it is 0.3% or less, the compacts maybe directly sintered, without dewaxing. It may beadded to the virgin powder or to granulated powder.With virgin powder, I% of the additive was theminimum required without the use of a die-walllubricant. At 0.5%, screeching noise could be heardduring compact ejection without die-wall lubricant.Compacts thus made showed axial striations onsintering and finish grinding. However, when theadditive was used as a coating after granulation,0.3% was found to be sufficient, without needingdie-wall lubrication against 1% required beforegranulation. The tumbling operation required foradding zinc behenate has an additional benefit ofhomogenizing the granules with respect to coarseand fine fractions (Katiyar, NPS, personalcommunication). This is necessary for achievinguniform die fill in all the die holes of the press, The

I~ 5.e I

f 5.75 ." I.!1

I::14

;__ ___".._"_"._-+1-- ~__j1M 2~ ~6

Compaction PreuII_, UPo277

Fig. I--Green density compaction pressure vs (data based onthe experiments with U02 powders (a) with and (b) withoutzinc behenate)

homogeneity of the binder mixed uranium oxide isverified by analyzing various samples for zinccontent.

U02 powder was precompacted at 70 MPa toform cylindrical slugs which were crushed andsieved to get granules between 14 and 60 Tylermesh. A part of the granules was final compacted atpressures ranging from 180 MPa to 280 MPa in ahydraulic press into cylindrical compacts ofdiameter 18 mm. Another part of the granules wascoated with 0.3% zinc behenate using a ribbonblender before' final compaction into cylindricalcompacts of the same diameter. In both cases, oleicacid was used as die-wall lubricant. Compactsamples were dewaxed at different temperatures andatmospheres and were analyzed for retained carbonand zinc contents.

ResultsFig I shows the green density of U02 compact as

a function of compaction pressure with and withoutzinc behenate. The weight loss of zinc behenate withincreasing temperature is shown in Fig. 2 for freebehenate and 1% behenate admixed in V02 pellet.The atmosphere was argon - 8% hydrogen mixture.In the case of oxide pellet, the weight loss begins atabout 500°C and is complete at about 1000°C.

The retained carbon and zinc contents in theoxide pellet in the same atmosphere are shown inFig. 3, Table I gives carbon and zinc contents of de-waxed pellets initially with I weight percent zincbehenate in different atmospheres at 500°C.

-138 INDIAN 1. ENG. MATER. SCI., JUNE 1998

Table l=-Carbon, zinc and total gas in dewaxed pellets

Condition carbon zinc Total gasppm ppm cc/g

No dewaxing 7410 850Ar+ 8% Hz 2900 800 2.417vacuum 2697 930 2.777argon 3800 535 1.607COz 2752 810No additive 50-\ 50 <200

DiscussionDecomposition of binder-In a reducing

atmosphere, zinc behenate decomposes into behenicacid and zinc,

Zn(C2l lit3 COO)2 + H2 ---> 2 C2l n., COOH + Zn

Behenic acid, in turn, is likely to get reduced tobehenic alcohol,

C2l lit3 COOH + 2 H2 ----> C2l H43 CH2 OH + H20

Behenic alcohol on pyrolysis gives lower chainalcohols and alkanes. The carbon escapes in theform of methane, carbon monoxide or carbondioxide.

Endcapping+' There may be some problemsassociated with the use of binder or lubricant.According to ThompsonJ3

, the use of admixedlubricant increased the tendency to form a defectcalled endcapping. In the present case, the use ofzinc behenate greatly reduced endcapping,indicating its binding property.

Bloating--In the course of sintering, evolvedgases must be eliminated while the surfacepermeability of the compact is high. Additiveburnout is very dependent on the composition andstrength of the additive material, the composition ofthe gas surrounding and in the pores of the product,and the rates of diffusion of the decompositionproduct gases and furnace gas through the product,which is dependent on permeability of the greenbody'.

Low density--When zinc behenate is mixed inthe powder before granulation, care should be takento keep the precompaction pressure low, to avoidformation of hard granules with low apparentspecific volume. The breaking strength of compactsmade from granules decreases as the apparentspecific volume of the granules is decreasedl4. Hardgranules retain their identity in final compaction andin sintering. Instead of the final compact sintering as

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j.ell!---::.r:~-------> I

•20 20

JIG '10 710T--,,","<'C) .f _ •••••

Fig. 2-Weight loss with temperature under Ar-8% Hzatmosphere

,:to (wt) me bel'lenate in uo,peUe1:

Atm~h«. or d_olCing: '" - ",. H2atmo~h •.•10

1 _

~ 11---------

i61 <-------- ,••.•••----

~ I

i·1: I

21I

oL---------------------------~~wo260 ''0 700

Te"'perature( ·e) of O.woxlng

Fig. 3-Retained carbon and zinc in the dewaxed pellet

a whole and shrinking towards its centre, eachgranule shrinks towards its own centre, leaving anoverall low density framework on sintering.Increasing the final compaction pressure toovercome the low sintered densities leads to stressrelief defects such as cracking and endcapping. Inview of these problems, it is advisable to add zincbehenate after granulating the powder.

Nitrogen impurity-On the basis of a process forthe preparation of uranium mononitride", nitrogenimpurity in sintered V02 appears to have beencaused by the reaction of V02 with nitrogen fromthe sintering atmosphere cracked ammonia in thepresence of trapped carbon. At 1400 - 1600°C,

2U02+5C+N2+2Hr--->2UN+4CO+CH4

This cannot happen when the furnace atmosphereis hydrogen or argon-hydrogen mixture, that is,when it is free from nitrogen. The carbon comesfrom binder or die-wall or powder admixedlubricant. Its bum-out depends on its quantity,

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BALAKRlSHNA et al.: ADMIXED LUBRICANTS 139

temperature and oxygen potential or moisture levelin the sintering furnace atmosphere. The problemexists only when short length continuous sinteringfurnaces and short pushing intervals correspondingto fast heating rates are used and absent in otheroxide ceramics sintered in air. To avoid nitrideformation, the U02 sintering furnace profile andpushing interval are so adjusted as to allowsufficient time for volatilizing of the binder beforeits decomposition and carbon burn out afterdecomposition at a low temperature when thecompacts are still permeable and allowcarbonaceous gases to diffuse out. This waysubsequent nitride formation at 1400°C can beavoided. Admixed lubricants and binders may alsobe avoided altogether and minimal die-walllubricant used. It has been found that polyethyleneglycol is superior to oleic acid as the die-walllubricant in respect of nitride formation inU02 sintered pellet. Care should be taken however,to adjust its viscosity to enable complete lubricantfilm formation over the die-wall. In a laboratorythermal analysis, evaporation of polyethyleneglycol was found to be complete by 423°C whileoleic acid required a higher temperature 495°C'Balaji Rao Y, personal communication).

Compacts made from higher specific surface areapowders tend to pick up more nitrogen as reactivityof a powder increases with increasing surface area.A high green density means finer interconnectedporosity and more tortuous path for diffusion oforganic materials out of the body. The mean poresize in a packed ceramic body is mainly a functionof particle size and powder volume fraction", Theporosity of a green compact can be increased bylowering the green density. A large size of the porechannels means easy diffusing out of organicmaterials and their decomposition products.

Zinc deposition--If zinc behenate is used inexcessive quantity, a separate dewaxing furnacewould be required. Otherwise, the decompositionproduct zinc in a reducing atmosphere, depositing inthe cooler parts of the sintering furnace may causeproblems.

ConclusionZinc behenate may be used in small

concentrations (up to 0.3 % by weight withoutdewaxing or 1% with dewaxing) as a. binder in U02

(or mixed oxide) powder compaction, with theadvantage of higher green strength, higher anduniform green and sintered density, uniformshrinkage and greater final acceptance levels. Careshould be taken, however, in that the binder ishomogeneously dispersed in the powder and thesintering furnace atmospheres and temperature-timeprofiles are such that bloating and nitriding insintered U02 are avoided.

AcknowledgementsThe authors are thankful to Mr. K.K.Sinha, Chief

Executive and Mr. U.C.Oupta, Deputy ChiefExecutive, NFC for their encouragement in thiswork and for granting permission for publishing inthe form of a paper, to Messrs R.M.Kartha, K PChakraborthy, R N Jayaraj and M Suryaprakash fortheir useful discussions, to Messrs K.Ramachandran,V.Ravichandran, U.Narayana Rao, R.R.K.Raju,A.K.Dubey and K.Sarat Babu for their assistanceand to Mr.V.Pushpangadan for preparing themanuscript.

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