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Journal of Alloys and Compounds 484 (2009) 920–923 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Preparation of a stabilized -cristobalite ceramic from diatomite Osman S¸an ∗ , Cem Özgür Dumlupinar University, Department of Ceramic Engineering, Kütahya 43100, Turkey a r t i c l e i n f o Article history: Received 19 February 2009Received in revised form 9 May 2009Accepted 15 May 2009Available online 23 May 2009 Keywords: CeramicsMicrostructureThermal expansion a b s t r a c t Roomtemperaturestabilized -cristobaliteceramichasgreatpotentialtouseinproductionofengineer-ingceramicmaterialsduetoitshighresistancetothermalshockandlowexpansioncoefficientwithhighchemicalresistanceandlowdensity.Thematerialwasinvestigatedfromthemixtureofpurifieddiatomitedopedwithaluminaandcalciumionsobtainedfromnitrates.Thediatomitepurificationwasachievedbyhot acid leaching and obtained high grade of silica powder (98wt.% SiO 2 ). The -phase composition wasdesigned as Si 1 − x Al x Ca x /2 O 2 where x varied 0.02–0.1. The powder was uniaxially pressed at 15kPa, andlatersinteredatdifferenttemperatures(1000 − 1300 ◦ C)andtimes(1minto72h).Thematerialspreparedfromthecompositioninwhich x =0.05andsinteringappliedatlowtemperaturefor24hhavepromisingengineering properties: the thermal expansion is almost linearly and found the thermal expansion coef-ficient as ∼ 11.42 × 10 − 6 ◦ C − 1 .Hightemperature sintering leadsto cristobalite crystallizationandthusthethermal stability of product sample was decreased.© 2009 Elsevier B.V. All rights reserved. 1. Introduction -Cristobalite is a high temperature and low pressure poly-morph of silica (SiO 2 ). The stabilized -cristobalite at roomtemperature has useful in engineering owing to its low and lin-early thermal expansion behaviour, besides other properties suchaschemicalinertnessandlessdensities.Thelowthermalexpansionbehaviourwithlinearchangesminimisestheoccurrenceofthermalstressesonceramicmaterialduringabrupttemperaturechange[1].These useful properties make the material a viable alternative toomany conventional ceramics used in harsh thermal environments.Conventionally, the starting powder for preparation of a stabi-lized -cristobalitematerialcomprisessilica[2],colloidalsilica[3], amorphous silica [4], silica gel [5] and silica sol [6,7]. The stuffing ions, such as Ca, Na, Cu, Sr and Li, are necessary for the stabiliza-tionof -cristobaliteformofsilicaatroomtemperature.Otherwisethe -cristobalite formed at high temperatures undergoes -formdisplacivephasetransition,whichoccursbetweentemperaturesof 170–270 ◦ C.Previously, the stabilized -cristobalite material has beeninvestigated by Saltzberg et al. [4] and given a stoichiometriccomposition such as Si 1 − x Al x M x/nn + O 2 , where Mn + represents thecations occupying the interstices of the framework. The phase sta-bility of -cristobalite material and its microstructural evaluationare largely determined by either gel- or wet-processes of pow-der synthesis, doping level and sintering conditions. Preparation ∗ Corresponding author. Tel.: +90 274 265 20 31/43 02; fax: +90 274 265 20 66. E-mail address:
[email protected] (O. S¸an). of -cristobalitematerialwhichischeapandhashighperformancestarting powder is the aim of this research. Diatomite is a potentialmaterial for consideration as it is widely used in industry owing toitshighthermalresistance.Therawdiatomitewasinitiallypurifiedfromtheirimpuritiesbyhotacidleachingtoobtainhighsilicacon-tentandlaterstabilizedas -cristobalitebysinteringofthematerialwiththeadditionofsomestuffingions.Thisstudyenhancedtheuseoflowgradediatomitematerialsinfabricationofhighperformanceceramic material for use in thermal environments. 2. Materials and methods -Cristobalite ceramic was investigated from diatomite powder after hot acidleaching. The material was stabilized with Al 3+ and Ca 2+ as stuffing ions. After thestabilized -cristobalite samples had been produced, the phases were identifiedby X-ray diffraction (XRD) analysis and characterised according to their thermalexpansion behaviour. Densities and SEM images of the materials were also takenand studied. 2.1. Preparation of diatomite powder The starting powder of -cristobalite ceramic was a raw diatomite obtainedfrom the Kütahya region of Turkey. The main chemical composition was as follows(wt.%):68.08SiO 2 ,17.99Al 2 O 3 ,4.22MgO,3.36Fe 2 O 3 ,1.32K 2 O,0.98CaO,0.67Na 2 Oand 0.29 TiO 2 . The sample was ground in an attrition mill for 1h using aluminaballs with an aqueous system. The slurry was dried in 105 ◦ C for 24h, and thensieved through an aperture of 45 m and stored for the leaching. For the leachingexperiments, the prepared diatomite powder was treated in HCl solutions at 75 ◦ C.The solutions contained 5M HCl acid and the treatment time by a magnetic stirrerwas applied as 240h. During leaching, 10g of diatomite sample was weighed andpouredin200mlsolution,thenthesolutionwasstirredcontinuouslyat500rpmanda thermostat was employed to keep the reaction medium at constant temperature.Aftertheleachingoperation,thesolidproductwasfilteredandwashedwithdistilledwater.0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.jallcom.2009.05.077 O. S¸an, C. Özgür / Journal of Alloys and Compounds 484 (2009) 920–923 921 2.2. Preparation of diatomite based ˇ -cristobalite The -cristobalite material was fabricated from the mixture of the purifieddiatomite powder and metal nitrates. The nitrates were obtained from Merck:Al(NO 3 ) 3 · 9H 2 O (CAS No.: 7784-27-2) and Ca(NO 3 ) 2 · 4H 2 O (CAS No.: 13477-34-4).The -cristobalitecompositionwasdesignedasSi 1 − x Al x Ca x /2 O 2 where x =0.02,0.04,0.05, 0.06, 0.07, 0.08, and 0.1. The nitrates were dissolved within water; the purifieddiatomite powder was then added and dried to obtain a powder mixture. The pow-dermixturewasagglomerated,uniaxallypressedat15kPaandsinteredatdifferenttemperatures(1000–1300 ◦ C)andtimes(1minto72h).Theheatingandcoolingrateof the furnace was 5 ◦ C/min. 2.3. Evaluation and characterisation The study of the samples included: (i) chemical composition measurement byX-ray fluorescence (Spectro X-LAB 200), (ii) crystalline phase identification by X-ray analysis (Rigaku Miniflex powder diffractometer employing CuK radiation in2 =10–65 ◦ at a ganiometer rate of 2 =2 ◦ /min), (iii) thermal expansion behaviourdetermined by dilatometry (Netzch DIL 402 PC) using 2.5cm long rods through thesamples heated at the rate of 10 ◦ C/min, (iv) microstructural analysis using a SEM(Zeiss Suprat 50), and (v) densities of the bulk materials determined by a helium-pycnometer (Qunatachrome-ultrapycnometer 1000). 3. Results and discussions 3.1. Crystal structure 3.1.1. The as-received diatomite The X-ray analysis of the raw diatomite powder before appli-cation of the acid leaching process is given in Fig. 1. It indicates a significant amount of crystalline phases such as quartz (JCPDS #46-1045), tridimite (JCPDS # 42-1401), aluminum silicates (JCPDS#25-0021,11-0046),magnesiumsilicates(JCPDS#39-0048,JCPDS# 41-1750) and plagioclase as albite (JCPDS # 19-0926), microcline(JCPDS # 09-0456) and orthoclase (JCPDS # 19-1227). 3.1.2. The diatomite after leaching The diatomite is a low grade material (68.08wt.% SiO 2 ). Thepurification achieved for leaching times as 240h and obtained ahighsilica-containingmaterial(98wt.%SiO 2 ).Phaseanalysisofthesample indicates a highly amorphous structure and the remainingphases are quartz and small quantity of tridimite (see Fig. 1). 3.1.3. The ˇ -cristobalite material The X-ray patterns of all compositions (Si 1 − x Al x Ca x /2 O 2 where x =0.02–0.1) prepared using the purified diatomite are shown in Fig. 1. Phase compositions of the as-received diatomite (a) and leached diatomite(b). Fig. 2. Phase compositions of the samples sintered at 1000 ◦ C for 24h through dif-ferent compositions ( x =0.02–0.1). Figs.2and3f orsinteringappliedatlow(1000 ◦ C)andathightem-peratures(1100–1200 ◦ C)for24h,respectively.Atlowtemperaturesintering, the addition of low amounts of stuffing ions (Al 3+ andCa 2+ ), such as x =0.02, is not sufficient for -cristobalite in whichpoor crystallization is obtained; the material has an amorphousstructure. The stuffing ions up the concentration so that x =0.05,then the crystallization is well formed and shows -cristobalitewithsmalleramountsofimpurities.Thestuffingionsaremorethan x =0.05, leading to a new phase as plagioclase. The excess stuffingionsdecreasethequartzcrystallization,anditcanbeconcludedthatthe plagioclase crystallization is the result of the reaction betweenthe excess stuffing ions and the Si 4+ ions belonging to the quartz.The high temperature sintering (at 1100 ◦ C; see Fig. 3a–c) decreased the amount of quartz crystallization where the com-positions varied as: x =0.02, 0.03, and 0.05, but this time theapplied temperature leads to occurrence of -cristobalite crystal-lization. At temperature increases up to 1200 ◦ C (see Fig. 3d), the -cristobalite crystallization occurs in a serious manner. The -cristobalite crystallization is totally undesirable for material usedin thermal environments in which nonlinear thermal expansionoccurs during the -form displacive phase transition.The above experimental results indicated that a lesser amountof impurity phases could be obtained with x =0.05 and sinteringof the material at low temperature (1000 ◦ C) for 24h. The materialwassinteredatthesamelowtemperature,butthistimeforalongertime,upto72h,anditwasobservedthattherewasnochangeintheimpurities,asshowninFig.4a.Theexperimentsalsoconductedon the sintering achieving at higher temperature such as 1250 ◦ C for30min (see Fig. 4b) and 1300 ◦ C for 1min (see Fig. 4c). Again, the Fig. 3. X-ray results of the samples prepared by x =0.02 with sintering at 1100 ◦ C(a), x =0.04 with sintering at 1100 ◦ C (b), x =0.05 with sintering at 1100 ◦ C (c), and x =0.05 with sintering at 1200 ◦ C (d). 922 O. S¸an, C. Özgür / Journal of Alloys and Compounds 484 (2009) 920–923 Fig.4. X-rayresultsofthesamplespreparedas x =0.05withsinteringat1000 ◦ Cfor72h (a), at 1250 ◦ C for 30min (b), and at 1300 ◦ C for 1min (c). -cristobalite crystallization appeared. But these conditions pro-duced only a small quantity of -cristobalite crystallization and aninsignificant amount of quartz.Giventheabovesinteringconditions,itcanbestatedthattwo -cristobaliteceramicmaterialswithdifferentimpuritycontentsmaybe suitable for use in thermal environments: (i) low temperaturesintering with high soaking times (at 1000 ◦ C for 24h) produced asamplewithasignificantquartzphaseandwithminorconstituentsof plagioclase and (ii) high temperature with less sintering time(1300 ◦ Cfor1min)produced -cristobaliteceramicwithverysmallquantities of quartz, feldspar and -cristobalite crystallization. 3.1.4. Sintering of diatomite without stabilization Purified diatomite powder was used directly for the fabricationof ceramic material. After shaping, the sintering was achieved at1300 ◦ Cfor1min.Fig.5showsXRDpatternsofthecomponentsin- terinwhich -cristobalitewithaverysmallquantityofquartzwasdetermined.Thistypeofcrystallizedmaterialisnotsuitableforusein a hot environment because of the thermally unstable behaviourof the -cristobalite crystallization. 3.2. The microstructure The microstructure of diatomite particles after purification anditssinterceramiccomponentsassuchwasinvestigatedbythescan-ning electron microscope (SEM). Naturally, some impurities weredeposited on the raw diatomite particles [8,9]. The leaching oper- ation removed the impurities and thus the typical diatomite typemicroscopic structure was obtained (see Fig. 6). Fig. 5. X-ray results of the sintered sample obtained from the purified diatomitewithout stabilization. Fig. 6. SEM picture of the diatomite particles after leaching. Fig. 7 shows the SEM micrograph of a typical facture edge of the ceramic materials obtained using the stabilized diatomite. Thesintering temperatures were of 1000 ◦ C for 24h (a) and 1300 ◦ Cfor 1min (b). The micrographs show the better microstructuralfeatures in which the homogenous pore distributions could beobtained. 3.3. Thermal behaviour Poor thermal stability of -cristobalite material is well-known;sudden increase of thermal expansion occurs at temperatures Fig. 7. SEM photographs of the porous materials produced by sintering at 1000 ◦ Cfor 12h (a) and at 1300 ◦ C for 1min (b). O. S¸an, C. Özgür / Journal of Alloys and Compounds 484 (2009) 920–923 923 Fig. 8. Linear thermal expansion curves for the materials obtained from the -cristobalite (a) [3], diatomite based -cristobalite obtained sintering at 1000 ◦ C for12h (b), and also sintering at 1300 ◦ C for 1min (c). between 170 and 270 ◦ C. The ceramic material obtained fromdiatomite powder without stabilization has almost fully -cristobalite crystallization and thus its thermal behaviour was notinvestigated. The thermal expansion behaviour of -crystallizedmaterial has been shown in Perrota’s studies [6] and is presented in Fig. 8a. The same figure also shows the thermal behaviours of presently fabricated ceramic materials in which the sintering tem-peratures were of 1000 ◦ C for 24h (b) and 1300 ◦ C for 1min (c).The material sintered at low temperature has promising engineer-ing properties: the thermal expansion is almost linearly and foundthethermalexpansioncoefficientas ∼ 11.42 × 10 − 6 ◦ C − 1 .Themate-rial sintered at high temperature is quite different: it has a smallquantityof -cristobalitecrystallization(seeFig.5).Thisstudyalso emphasize that a small quantity of -phase in the compositionadversely influenced the thermal stability of the product samples. 3.4. Density of the diatomite based ˇ -cristobalite material Low temperature sintering applied to the diatomite basedmaterialshowspromisingengineeringpropertiesforthermalenvi-ronment, thus its density is determined and compared with thedensityofpure -cristobalitematerial.Thepure -cristobalite[10]and presently fabricated diatomite-based material show close val-ues such as the density being 2.24 and 2.25g/cm 3 , respectively. 4. Conclusions This study indicated that the possibility of fabrication ther-mally stable ceramic material from a low grade diatomite. Thesuccess can be explained by the purification of the sample andlater stabilization using aluminum and calcium ions. The purifi-cation of diatomite was achieved by acid leaching and obtaineda high silica-containing material (98wt.% SiO 2 ). The impuritiesare Al 2 O 3 , CaO, K 2 O, MgO, Fe 2 O 3 , and TiO 2 . The thermally stableceramic material designed as -cristobalite using the compositionSi 1 − x Al x Ca x /2 O 2 where the best being x =0.05. The required stuff-ingionsweredeterminedbyconsideringtheimpurities(AlandCa)of the diatomite. The others were neglected. When the neglectedimpurities attended, the formula of -cristobalite changed as:Si 0.937 Al 0.049 Ca 0.024 Na 0.005 K 0.004 Mg 0.008 Fe 0.001 Ti 0.003 O 2 . But, theimpurities have no significant influence on the thermal behaviourof the ceramic material where the thermal expansion coefficientwas found as ∼ 11.42 × 10 − 6 ◦ C − 1 . Acknowledgement The authors acknowledge with sincere gratitude the financialsupportprovidedbytheTurkishStatePlanningOrganization(DPT)project 2003K120380. References [1] C.H. Chao, H.Y. Lu, Materials Science and Engineering A 328 (2002) 267–276.[2] A.M. Venezia, V.L. Parola, A. Longo, A. 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