Gelcasting of alumina ceramic components in nontoxic Na-alginate–CaIO3–PVP systems
- 格式：pdf
- 大小：416.32 KB
- 文档页数：6
Gelcasting of alumina ceramic components in nontoxicNa-alginate–CaIO 3–PVP systemsJingtao Maa,b,*,Zhipeng Xie b ,Hezhuo Miao b ,Baoqing Zhang a ,Xuping Lin a ,Yibing Cheng caInstitute of Nuclear and new Energy Technology,Tsinghua University,Beijing 100084,P.R ChinabState Key Laboratory of New Ceramics and Fine Processing,Tsinghua University,Beijing 100084,P.R China cSchool of Physics and Materials Engineering,Monash University,Clayton,Victoria 3800,AustraliaReceived 7November 2003;accepted 15June 2004Available online 31July 2004AbstractGelcasting of ceramic components with the soldium alginate (Na-Alginate)as gelformers has received increasing attention.The aim of the present study is to improve the flexural strength of green body gelcast from nontoxic Na-alginate system by introducing a proper amount of water-soluble polymer,PVP.The rheological properties and gelling characteristics of PVP–Na-Alginate/Al 2O 3suspensions were examined.Ceramic parts with uniform and homogeneous structure were fabricated as well as flexural strength and microstructure of the gelcast green and sintered bodies were highlighted.Ó2004Elsevier Ltd.All rights reserved.Keywords:Polyvinylpyrrolidone;Sodium-alginate;Gelcasting;A 2O 31.IntroductionIn recent years,gelcasting received increasing atten-tion as a new in situ colloidal forming technique.It has the following features:slurries with high solids load-ing and low viscosity,dried bodies containing less than 4wt%binder,and the ability to fabricate complex shaped bodies [1].Hewever,despite obvious advantages,indus-try was reluctant to use gelcasting because acrylamide,the main component of most of the monomers used,is a light neurotoxin [2].Therefore,its gelformer system was developed from acrylamide system with a neurotox-in to methacrylamide system with low-toxicity or non-toxic crude polymer Na-Alginate system [3–6].Sodium alginate solutions are viscous and gel in the presence of several cations,mainly Ca 2+[7].Gelation oc-curs by chelation of the divalent ions and alginate.Al-though there are many salts containing calcium ion,such as CaCl 2Æ4H 2O,CaC 2O 4,Ca(C 6H 11O 7)2,which can react with alginate and crosses link together.However,it is generally very difficult to control the reaction rate for most of them.This makes it impossible to complete casting processing at certain period.Therefore,the diva-lent salts with a controlled reaction rate with alginate have to be considered.Xie Zhipeng et al.[5,6,8]has studied gel-casting in Na-alginate/C 6H 10O 4/Ca 3(PO 4)2/(NaPO 3)6system,Na-alginate/C 6H 10O 4/CaCO 3/(NaPO 3)6system and Na-alginate/Ca(IO 4)3system,etc.Although it was applied to alumina and silicon carbide,there was a lower green strength gelcasted.Examining the solubility of sodium iodate,we found that Ca(IO 3)2Æ6H 2O is good for gelcasting processing because it has a lower solubility (0.17wt%)at room tem-perature and a high solubility (1.38wt%)at increased temperature of 60°C.Reaction of sodium alginate and0261-3069/$-see front matter Ó2004Elsevier Ltd.All rights reserved.doi:10.1016/j.matdes.2004.06.011*Corresponding author.Fax:+86-10-627-83763.E-mail address:majingtao@ (J.Ma)./locate/matdesMaterials and Design 26(2005)291–296Materials &Designcalcium iodate at the temperature of60°C was as shown [9]CaðIO3Þ2Á6H2O!Ca2þþ2IOÀ3þ6H2Oð1Þ2NaÀalginateþCa2þ!CaÀðalginateÞ2þ2Naþð2ÞBased on the previous study,this paper utilizes gel-ling characteristic of nontoxic crude polymer Na-Algi-nate and calcium iodate as well as realizes gelcasting of ceramic powders.The green strength was increased via adding a proper amount of water-soluble polymer, PVP.2.Experimental procedure2.1.MaterialsThe starting high-purity a-Al2O3powder(APA-0.5 w/MgO,>99.9%purity)used in this experiment with an average particle size of0.36l m and specific surface area of8.0m2/g were produced by American Ceralox Company.Controllable casting and consolidation are through the use of sodium alginate and calcium iodate (CaIO3).Sodium alginate was used as a gelation rea-gent,which dissolves in deionized water at room temper-ature.When the sodium alginate and calcium ions react, the molecular chains attract each other to form a three-dimensional network.Ammonium citrate was selected as the dispersant.Polyvinylpyrrolidone(K-30,Aldrich Chemical Company,Inc.,USA)used in the study has an average molecular weight of10,000.All reagents are chemically pure.2.2.Preparation of suspensionsThe forming processflow chart is shown in Fig.1.So-dium alginate and was dissolved in the deionized water to form a solution,which had been decomposed at60°C for48h to reduce solution viscosity.The3.0wt% PVP(based on alumina)wasfirst completely dissolved in the solution.And then alumina powder and disper-sant were added into the solution,the resulting slurry was then ball milled24h.After degassing for8–10 min in a rotary evaporator under vacuum,calcium io-date was added into this slurry before the slurry was cast into a nonporous mould,and consolidation occurred. After several hours,the wet green body was demoulded and dried at room temperature.Binder burnout and subsequent sintering were carried out in stationary air and conducted separately.Binder burnout was operated at600°C for2h,with a heating rate of2°C/min and a naturally cooling.Sintering was carried out at1600°C for2h with a heating rate of2°C/min from room tem-perature to1200°C and a rate of1°C/min from1200to 1600°C,followed by natural cooling.2.3.Methods and measurementsParticle size of alumina was measured by laser scat-tering method(Mastersizer,Malvern,Germany).The specific surface was measured via Brunauer–Emmett–Teller(BET)nitrogen-gas adsorption(NOVA4000, USA).The shear viscosity is measured using a Modular Compact Rheometer(MCR300,Paar Physica,Germa-ny)with a concentric-cylinder,CC27,having gap be-tween the inner and outer cylinder walls of1.33mm. The sample volume was$23ml offluid.Steady state shearflow curves were performed at0.1–250sÀ1by step-ping up and down in shear rate.Measurement of the storage modulus of slurries was conducted at the tem-perature of25°C,the strain amplitude of1%and the frequency of1Hz.Theflexural strengths of bars of green and sintered bodies(in the present research,six samples have been tested),were examined by three-pointflexure test with a span of30mm at a loading rate of0.5mm/min.The bars of green and sintered bodies were5·6·42and 3·4·36mm3,respectively.Fracture surfaces of the green and sintered specimens were examined with a scanning electron microscope(JSM-6460LV,JEOL,Japan).3.Results and discussions3.1.Dispersion of alumina particleFig.2illustrates the effect of the amount of dispersant on50vol%Al2O3suspension containing3wt%PVP at the pH of9.4–9.8.It can be found that there was the lowest viscosity of the suspension(the viscosity is0.75 PaÆs,when the shear rate is100sÀ1),when0.25wt% dispersant(based on alumina)was used.It was be attrib-uted to that when the amount of dispersant is less than 0.25wt%the viscosity of suspension was reduced due to adsorption of citric acid-based ions on the surface of alumina particles,resulting increase in the density of electron cloud on the surface of alumina particles and static exclusive force.When the amount of dispersant is higher than0.25wt%the viscosity of suspension was increased because there was low amount of adsorp-tion of citric acid-based ions on the surface of alumina particles under alkalescent condition.The free ions were increased in solution resulting in increase in viscosity of suspension.Thus,the viscosity of suspension exists a best value with the variation in the amount of disper-sant.Based on above experimental result,the amount of dispersant used in this paper is0.25wt%.3.2.Rheological behavior of suspensionsThe rheological behavior of alumina suspensions without and with an addition of3wt%PVP was shown in Fig.3.It can be seen from Fig.3(a)that the viscosity of suspension containing mixed alginate and3wt%PVP was increased obviously,compared to the suspension containing Na-alginate sole.Both the suspension con-taining Na-alginate sole and the suspension containing mixed alginate and3wt%PVP exhibited shear-thinning behavior.Also,it can be noted that the suspensions be-fore and after adding3wt%PVP is relative stable and the lag has not occurred.Fig.3(b)shows the corresponding variation of shear stress as a function of shear rate for the suspensions with and without PVA of Fig.3(a).The rheological behavior depicted in Fig.3(b)has been analyzed using the Hers-chel–Bulkley model[10]s¼s yþk c n;ð3Þwhere s y is the yield stress,s is the shear stress,n is the shear rate,n is the shear rate exponent and k is a con-stant.The yield stress values presented in Table1are calculated from this model.The suspension with addi-tion of3wt%PVP exhibits obviously a higher yield stress compared to the suspension withoutPVA. Table1Hershel–Bulkley parameters s0and n of alumina suspensions before and after addition of3.0wt%PVP(50vol%,0.25wt%dispersant,25°C) Sample PVP content a(wt%)Yield stress,s0(Pa)Constant,k Shear rate exponent,n Correlation ratio,R x yA012.275 5.22480.40290.9910B 3.023.005 4.26260.55310.9974a Content of the polymer are based on alumina.J.Ma et al./Materials and Design26(2005)291–296293Increase in yield stress of suspension results in change in the interaction among particles and inner structure of suspensions.Fig.4shows the storage modulus (G 0)versus the strain amplitude (c )of a 50vol%Al 2O 3suspension with-out and with 3wt%PVP obtained after ball-milling 24h.It can be seen from Fig.4that the linear viscoelastic regime of the suspension containing 3wt%PVP was spread (c <0.6%),compared to ones of the suspension without PVP (c <0.4%).Also,the comparison in Fig.4clearly shows that the storage modulus of the Al 2O 3suspension with PVP is higher than ones of the Al 2O 3suspension without PVP.At small strain amplitudes,the gel is in the linear viscoelastic regime where G 0is constant with respect to c ,which indicated that the net structure in suspension have not been broken and exhib-it relatively strong elasticity,within a range of this strain amplitude.Beyond the linear region,with increasing strain amplitude,the net structures in the system were broken;when the strain amplitude is higher than the critical strain amplitude of the system,then the suspen-sion starts flow and exhibits fluid characteristics and the storage modulus decreases gradually.The viscoelastic response of a colloidally stable con-centrated suspension is strong when the average distance between the suspended particles if of the same order as the range of the repulsive interparticle potential [11].Hence the viscoelastic properties originate from the re-pulsive interparticle potential.With an addition of PVP,the average distance between the particles decreas-es,causing a progressively stronger overlap of the poly-mer layers.This overlap results in an increase in the elasticity of the suspension,reflected in an increase in the storage modulus compared to that in the suspension without an addition of PVP.3.3.Gelation of the suspensionEffect of the amount of calcium iodate on gelation time was shown in Table 2.It can be noted that a long time was demanded and green body with a certain dis-tortion has not solidified completely when 0.125or 0.25wt%CaIO 3is used.The good green body was ob-tained when 0.5wt%CaIO 3is used.But when 1.25wt%CaIO 3is used gelation rate of the suspension is very quick,which caused the green body with heterogeneity was obtained.Therefore,the optimum amount of CaIO 3is 0.5wt%.As is well known that the storage modulus,which is proportional to the elastic modulus of the gel,character-izes the elastic response of the gel.When the structure of the slurry is changed,the modulus of the slurry shows a gradual change with time.Thus,gelling characteristics of alumina slurries was characterized by the dependence of the storage modulus on time in this paper.The variation in the storage modulus of the 50vol%Al 2O 3suspensions (20ml)containing 1.5wt%Na-algi-nate and 0.2g CaIO 3before and after adding PVP at the temperature of 60°C was shown in Fig.5.When the galation time is less than 13min,there was no obvi-ous variation in the storage modulus of suspensions,which illustrates that the concentration of calcium ions is low and can not cause the transit of Na-alginate to gel.During 14and 18min,calcium ion was released heavily,followed polymerization reaction occurred quickly and the storage modulus of suspension was in-creased sharply.After 3wt%PVP was added into the suspension,the starting gelation time was postponed.Above 16min,the low increase in the storage modulus of suspension occurs.3.4.The green strength and SEMThe green strength of dried bodies with and without PVP was shown in Table 3.Green strength showed a sig-nificant increase after addition of 3wt%PVP.After ad-dition of 3wt%PVP the strength of the green bodywasTable 2Effect of amount of calcium iodate on gelation time of alumina suspensions with 3wt%PVP at the temperature of 60°C Amount ofsuspensions (ml)Content ofsodium alginate (wt%)Amount ofcalcium iodate (g)Starting gelation time (min)Gelation result after 2h20 1.50.0580Solidification not completes 20 1.50.1035Appreciable deformation 20 1.50.2016Good 20 1.50.308Good201.50.503Heterogeneity of the sample and part rigidification294J.Ma et al./Materials and Design 26(2005)291–296about 10.1MPa.Previous studies indicated that the strength of green body formed by gelcasting was provid-ed by the polymer gel [12,13].But for the Na-alginate so-lution with 3wt%PVP,as the reaction proceeds,the PVP is likely to form a more complicated network struc-ture through hydrogen bonding with the Na-alginate,resulting in an intimate mix of polymer chains (refer to [14]).The relatively high gel strength obtained for the Na-alginate/PVP system with respect to a pure Na-alginate system is attributed to the enhanced polymer network due to this cross-linking through hydrogen bonding.Hence,the flexural strength of the green body prepared from the suspension with addition of PVP in-creased significantly compared to those without PVP.Fig.6presents SEM of alumina green body obtained from the system before and after addition of 3wt%PVP.It can be seen that a satisfactory microstructure with very few pores has been achieved for both systems.3.5.The sintered strength and SEMThe flexural strength of alumina sample was shown in Table 3.It can be found that the flexural strength of alu-mina sample prepared from slurry containing mixed 3wt%PVP and Na-alginate was not reduced obviously compared to ones of alumina sample prepared from slurry containing Na-alginate sole.A dense and homo-geneous microstructure without developing a huge grain growth is observed for the alumina samples prepared from the slurries containing mixed PVP and Na-alginate (as shown in Fig.7).Table 3The flexural strength and density of alumina green and sintered bodies PVP content (wt%)Flexural strength of green body (MPa)Flexural strength of sintered body (MPa)0 5.56±2.23320.65±6.573.010.12±1.93308.08±4.59Fig.7.SEM of alumina sintered body:(a)without an addition of PVP;(b)with an addition of 3wt%PVP.Fig.6.SEM of alumina green body before and after addition of PVP:(a)without an addition of PVP;(b)with an addition of 3wt%PVP.J.Ma et al./Materials and Design 26(2005)291–2962954.ConclusionsA new low-toxicity gelcasting system was developed in this paper.Gelcasting of ceramic components with Na-Alginate,PVP and CaIO3as gelformers is possible. The alumina suspensions with50vol%in Na-Alginate, PVP and CaIO3systems was prepared and gelcasted. The green strength of gelcasted samples(10.1MPa), which is sufficient to allow green machining,was multi-plied two times due to addition of PVP,compared to the green strength of gecasted samples(5.5MPa)in NaAlg and CaIO3systems.AcknowledgementsThe authors thank the National Key Basic Research Development Program of China(973program: G2000067204-01)and National Science Foundation of China(two-base project,Grant No.50140120423)for the grants that support this research.References[1]Young AC,Omatete OO,Janney MA,Menchhofer PA.Gelcast-ing of alumina..J Am Ceram Soc1991;74:612–618.[2]Omatete OO,Janney MA,Nunn SD.Gelcasting:from lavoratorydevelopment toward industrial production.J Eur Ceram Soc 1997;17:407–413.[3]Janney MA,Omatete OO,Walls CA,et al..Development of low-toxicity gelcasting systems.J Am Ceram Soc1998;81(3):581–591.[4]Gilissen R,Erauw JP,et al..Gelcasting,a near net shapetechnique.Mater Design2000;21(4):251–2570264-1275.[5]Jia Yu,Kanno Yoshinori,Xie Zhi-Peng.New gel-casting processfor alumina ceramics based on gelation of alginate.J Eur Ceram Soc2002;22:1911–1916.[6]Xie Zhi-Peng,Huang Yong,Chen Ya-Li,Jia Yu.A new gel castingof ceramics by reaction of sodium alginate and calcium iodate at increased temperature.J Mater Sci Lett2001;20:1255–1257. [7]Gacesa P,Russell NJ.Pseudomonas infection and alginates.1sted..London:Chapman Hall;1900p.29–49.[8]Wang Xiu,Xie Zhi-Peng,Huang Yong,Cheng Yi-Bing.Gelcast-ing of silicon carbide based on gelation of sodium alginate.Ceram Int2002;28:865–871.[9]Lin SH.Gelation of sodium alginate in a batch process.ChemEng Sci1991;46(2):651–655.[10]Reed James S.Introduction to the principles of ceramic process-ing.New York:Wiley;1988p.227–252.[11]Herschel H,Buckley R.P Am SocTest Mater1926;26:621–623.[12]Asad UK,Brian JB,Paul FL.Interaction of binders withdispersant stabilized alumina suspensions.Colloid Surface A 2000;161:243–257.[13]Bauer W,Ritzhaupt-Kleissl H-J,Hausselt J.Micropatterning ofceramics by slip pressing.Ceram Int1999;25:201–205.[14]Ma Jingtao,Xie Zhipeng,Miao Hezhuo,Huang Yong,ChengYibing,Yang Weiyou.Gelcasting of alumina ceramics in the mixed acrylamide and polyacrylamide systems.J Eur Ceram Soc 2003;23:2273–2279.296J.Ma et al./Materials and Design26(2005)291–296。
