Journal of Alloys and Compounds 586 (2014) 581–587

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Journal of Alloys and Compounds

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Some studies on ceria–zirconia reinforced solvothermally synthesizedcordierite nano-composites

0925-8388/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2013.09.150

⇑ Corresponding author. Tel.: +91 9454730609.E-mail address: srivasabhinav@gmail.com (A. Srivastava).

Abhinav Srivastava ⇑, Vinay Kumar Singh, Vijay Kumar, P. Hemanth Kumar, Himanshu Tripathi,Ashish Chaudhary, Krit Asiwal, Rahul Pandey, Shyam Kumar SumanDepartment of Ceramic Engineering, Indian Institute of Technology (BHU), Varanasi, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 August 2013Received in revised form 12 September2013Accepted 20 September 2013Available online 1 October 2013

Keywords:X-ray diffractionScanning electron microscopySolvothermal processNano-composites

Nanostructured cordierite and 12 mol% ceria stabilized zirconia were chemically prepared separately.Pure cordierite was synthesized solvothermally, whereas [(Zr)0.88(Ce)0.12O2] termed as CeSZ wasprocessed with gelation and co-precipitation method. The evolution of crystalline phases and the micro-structures have been studied using X-ray diffractometer, and FE-SEM with EDX. Varying contents(0–20 weight%) of CeSZ were then mixed with cordierite to form sample blocks of desired shapes. Thesenano-composite blocks were fired at 1100–1400 �C for a soaking time of 3 h to observe their thermo-mechanical, micro-structural and physical properties. Results indicate that ceria not only acts as a stabi-lizer in zirconia ceramics but it also acts as a sintering aid in cordierite formation. CeO2 also improvescompressibility and structural properties of nano-composites. Sintering temperature also plays an impor-tant role in the behavior of nano-composite blocks. All properties show a major improvement thanprevious reported data’s.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction have been investigated extensively for structural applications.

In recent years, nano-crystalline ceramics and the application ofnano particles to improve the ceramic properties have attracted con-siderable interest, as the mechanical, optical, electrical and mag-netic properties are crystallite size sensitive. Natural cordierite(Mg2Al4Si5O18) is a rare magnesium–alumino-silicate mineraloccurring in nature [1–3]. The cordierite ceramics are known dueto their low thermal expansion coefficient, high chemical durability,low dielectric constants, high refractoriness and excellent resistanceto thermal shock. However, cordierite ceramics are susceptible tolow fracture toughness, which hinders their structural applications.Unfortunately processing of synthetic cordierite is difficult as it is al-ways accompanied by a spinel (MgAl2O4) phase, which impairs thethermal shock resistance of end product due to its high thermalexpansion coefficient. It is difficult to produce dense cordieriteceramics by the solid-state reaction without sintering aids becauseof the narrow sintering range near the incongruent melting pointof cordierite [4,5].

Ceria stabilized tetragonal zirconia (CeSZ) polycrystallineceramics possess distinct advantages over other conventionalstructural ceramic materials because of their better thermal stabil-ity in moist environments, wider range of solid solubility in tetrag-onal region [6]. Consequently, ceria stabilized zirconia ceramics

The grain size and microstructure of these materials are difficultto control in conventional technique. Gelation and co-precipitationprocess is a promising candidate for production of CeSZ particlesgiving a better control over size and shape of the synthesizednano-powders [7,8].

There is some data available on the cordierite synthesis by thenovel chemical processes but not for structural properties [9]. Theaim of this study was the synthesis of nanostructured cordierite bysolvothermal technique and investigation of the phase transforma-tion during heat treatment of precipitate. Addition of ZrO2 (mostly intetragonal form) in cordierite has been extensively studied but verylittle is known about its cubic polymorph reinforcement with effectof mineralizer additions on both the sinterability and the mechani-cal properties of cordierite bodies fabricated by wet chemical routes.The effect of CeO2 as mineralizer has been studied in the presentwork with the addition of cubic ZrO2 in nanostructured cordieritematrix. To do this CeSZ was incorporated in thus prepared cordieritematrix and blocks were formed to investigate their micro-structuraland mechanical properties.

2. Materials and methods

2.1. Synthesis of nanostructured cordierite by solvothermal technique

A solvothermal process [10–12] was used to synthesize the cordierite nano-powders. All reagents were analytical grade, purchased from LobaChem (Mumbai,India) and used as received. First appropriate metal-nitrate precursors; Mg(NO3)2-

�6H2O, Al(NO3)3�9H2O and orthosilicic acid (H4SiO4) were dissolved in deionized

Fig. 1. XRD plot of solvothermally prepared cordierite.

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water–ethanol (1:1 volume ratio) under rigorous stirring to form 0.5 molar solu-tion. The precursors were taken in a ratio to give a stoichiometric composition ofcordierite. To this solution 100 g of urea and 1 mmol sodium dodecyl sulfate(SDS) was subsequently added. The solution was sealed in a tightly closed stainlesssteel autoclave and heated at 120 �C for 12 h. Upon completion of the reaction, theproduct was centrifuged at 10,000 rpm for 5 min and washed with deionized waterand ethanol. The resulting precipitate was dried at 100 �C for 24 h and then calcinedat varying range of temperature (1100–1300) �C for a soaking period of 2 h to obtaincordierite nano-powders.

2.2. Synthesis of [(Zr)0.88(Ce)0.12O2] by gelation and co-precipitation method

AR grade ZrOCl2�8H2O and Ce(SO4)2�4H2O were dissolved in deionized waterwith vigorous stirring and mild heating to give 12 mol% ceria stabilized zirconia[(Zr)0.88(Ce)0.12O2]. For preparation of gel, ammonia solution was added drop wiseand pH of the solution was maintained over 9. A point was reached when the gelbecame fully viscous and the process of stirring was stopped due to the viscous nat-ure of the gel. This gel was oven dried at 100 �C for 24 h and finally calcined at750 �C for 5 h to give super fine CeSZ [13–15].

2.3. Preparation of nano-composites

Batches were formed employing above processed nano-powders of cordieriteand CeSZ in the varying ratio. Nanostructured cordierite formed at 1300 �C was se-lected for the formulation of nano-composites as by the X-ray diffraction analysis itshowed the lowest content of spinel phase and highest content of a-cordierite. Azero weight% CeSZ containing cordierite mix was named CZ00. Similarly 5, 10, 15and 20 weight% CeSZ containing cordierite mixes were named CZ05, CZ10, CZ15and CZ20 respectively. These batches were pulverized in a high energy planetaryball mill. The jar and grinding media were of titanium-coated stainless steel mate-rial. At one time 250 g of a batch was taken in a jar and milled for 45 each min at600 rpm. Similarly, it was processed to complete the grinding of all batches. Thesenano-powdered batches were then cold pressed into desired blocks as per ASTMstandards by a uniaxial hydraulic press machine under a constant 10 tones pressureload. Several blocks of each composition were made and sintered in a temperaturerange of 1100–1400 �C for a soaking period of 3 h. Sintered and dense compositeblocks were investigated for their thermo-mechanical and micro-structural proper-ties [16,17].

2.4. Characterization of the nano-composites

Powder X-ray diffraction patterns were recorded using a Rigaku high resolutionpowder X-ray diffractometer employing Cu Ka1 radiation and Ni-filter. Data werecollected in the 2h range from 0� to 90�.

Synthesized and sintered blocks of formulated composites were examinedusing a FEI Quanta 200F Field Emission Scanning Electron Microscope (FE-SEM).The samples were metalized by gold sputtering for better image definition.

Apparent Porosity and Bulk Density of sintered nano-composite were investi-gated according to ASTM C20-00.

The Cold Crushing Strength (CCS) is the capacity of a material to withstand axi-ally directed pushing forces. Cubic test specimens of 51 mm size were prepared andthe value of maximum uniaxial load (in N) was noted when the sample block failedcompletely. Finally CCS value was calculated using the method stated in ASTMC133-97.

Cold modulus of rupture (CMOR) measurements was carried out under three-point bending tests (ASTM C133-97) using 152 mm � 25 mm � 25 mm samples.

Sample briquettes were measured for their mechanical strength in terms of coldcrushing strength (MPa) and modulus of rupture (MPa) according to ASTM C133-97.

Fig. 2. XRD plot of gelation and co-precipitation processed 12 mol% ceria stabilizedzirconia.

3. Results and discussions

Fig. 1 shows the XRD plot of as prepared cordierite nano-pow-ders. Two primary factors directly determine the crystalline sizeof the samples, namely, nucleation and crystal growth processes.At lower temperature 1100 �C the l-cordierite starts to form butstill the major phase is recognized as a magnesium aluminatespinel (MgAl2O4). Spinel occurs as an intermediate phase whichultimately reacts with l-cordierite at high temperatures to pro-duce a-cordierite; a-cordierite itself is formed at temperatures>1200 �C. Here inset figure illustrates the complete formation ofcordierite focusing on a specific range which clearly depicts intenseand sharp peaks corresponding to a-cordierite formation {110}plane at 2h = 10.5�. In sample calcined at 1200 �C {241} plane at2h = 29.5� portray improvement in crystallinity of cordierite dueto rise in synthesis temperature. Cordierite peaks were matched

with JCPDS data card 76-1794 and spinel peaks were identifiedwith JCPDS data card 77-1193.

Presence of broad peaks in XRD patterns of calcined powdersshow that particle size is small. Crystallite size, d of calcined pow-der was calculated from X-ray line broadening analysis usingScherrer’s formula.

d ¼ 0:9k=bCosh

where b is the full width at half maximum (FWHM) intensity of aBragg reflection excluding instrumental broadening, k is the wave-length of the X-ray radiation and h is the Bragg angle. b is taken forthe strongest Bragg’s peak corresponding to 2h. All the three cordi-erite powders calcined have an average crystallite size in the range20–35 nm.

Fig. 2 reveals the formation of gelation and co-precipitation pro-cessed 12 mol% ceria stabilized zirconia. No evidence of crystallineimpurities is found in the pattern. Cubic ZrO2 peaks matched ex-actly with JCPDS data card 65-0461. These results indicate thatthe cubic ZrO2 with high phase-purity can be obtained in the pro-cess reported. In addition, according to the Scherrer’s formula, theaverage crystallite size of the sample is evaluated to be approxi-mately 18 nm from the half-peak width of the {111} diffractionpeak at 2h = 30.13�.

Particle size distribution of as prepared Cordierite and CeSZ isshown in Fig. 3. IJ1.46 tool was used to identify and count thenumber of particles as well as their mean diameters. Maximum

Fig. 4. XRD plot of composites prepared and fired at 1300 �C.

Fig. 5. Bulk density and apparent porosity of composite fired at 1300 �C.

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Cordierite particles lay in the range 0.02–0.06 l and for CesZ it wasdispersed from 0.01 to 0.08.

The examination of the fabricated cordierite-zirconia, nano-composite by Fig. 4 enabled us to identify the component-phasesformed after sintering, which shows an overall improvement incrystalline behavior of zirconia peaks. This can be related to theincreasing content of stabilized zirconia and relatively increasingcontent of ceria. In contrast to previous works of similar composi-tions [2,7,9] where mullite and zircon diffraction peaks appearedas one of the major phases of final sintered samples, in presentstudy none of them are detected, this could be attributed to theceria addition, giving stabilization through formation of cubic cer-ium zirconium oxide.

As expected ceria not only stabilized zirconia but also improvedthe crystalline nature of a-cordierite peaks, suppressing any resid-ual phase formation. Sharp crystalline peaks of composite wereidentified and matched with JCPDS data card 65-0461 and 76-1794. More of indialite (a-cordierite) peaks appeared in contrastto evanescing l-cordierite peaks due to increase in cubic ceria–zirconia content. Ceria acts as a dissolution agent for magne-sium-aluminate spinel (MgAl2O4) and its peaks vanished whenCeSZ content was increased.

In Fig. 5 the theoretical density, bulk density and apparentporosity is portrayed. Variations of the density of the obtainedmaterials depend on the amount of CeSZ in the green body as wellas the selected level of sintering temperature which is apparentlyillustrated in the figure. It is worth noting that the present dataare consistent with these demonstrated by Hirvonen et al. [3] forcordierite/ZrO2 composite which indicates that composite densitysystematically increases with the content of zirconia in the sin-tered body.

Dual representation in Figs. 6 and 7 which is representation ofthermo-mechanical behavior of the composites prepared providesa better understanding of thermal as well as compositionalchanges occurring. The curves representing CCS and Cold MORhave similar shapes and are regularly spaced. These curves havesteep slopes in the initial stage, indicating an outstanding improve-ment proceeding rapidly with respect to sintering temperatures.Both the properties cold crushing strength and modulus of rupturehave an increasing trend with increase in the CeSZ content. Sinter-ing temperature has a positive effect in densification and strength-ening behavior up to 1300 �C after which it decreases. Here again,however the attainable strengths at highest temperature is not sat-isfactory but a maximum service temperature can be reckoned. Thissudden drop in mechanical strength with increasing temperature is

Fig. 3. Particle size

subjected to increasing content of liquid content and appearance ofamorphous phases. At 1400 �C an estimated 43 volume% liquidphase was formed [18].

Fig. 8 is the powder micrograph of as prepared cordierite andnanostructured cubic ceria doped zirconia which consists of veryfine primary particles, approximately 18 nm in size. The irregular

distribution.

Fig. 6. Cold crushing strength of nano-composite fired at different temperatures.

Fig. 7. Modulus of rupture of nano-composite fired at different temperatures.

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bulky shapes can be predicted to be of ceria as it has larger ionicradii. Pure nano-crystalline a-cordierite called indialite (stablehexagonal form) in the micrograph shows an open microstructure.This open microstructure appearance is due to sample swellingduring the phase change from l to a without further densificationwith sintering temperature. EDS microanalysis shows that the cal-cined powder retained a near stoichiometric composition ofcordierite.

Figs. 9 and 10 illustrate the structure of the obtained nano-com-posites observed using field emissive scanning electron micros-copy of the fractured surfaces revealing sub-micron size grains of5–20 weight% CeO2–ZrO2 accompanied by continuous predomi-nant indialite glassy grain boundary phase. The morphology ofthe investigated material did not differed significantly with the ap-plied sintering temperature, so only samples sintered at 1300 �Care shown here. The nano-size powder prepared by above men-tioned wet chemical routes was very effective in improving densi-fication. Cubic structured CeO2–ZrO2 particles can be observed

homogeneously dispersed on the surface of cordierite matrix inFigs. 9 and 10. The black areas correspond to pores in the micro-structure, which seems to be a result of the b to a modificationof cordierite. Hexagonal grains with hint of pseudo-reticulatedshape are seen in the all the composites due to the liquid phase sin-tering, this interlocking of crystalline grains could be termedresponsible for such high mechanical strengths. Fig. 11 displaysglassy indialite matrix in which small crystals of CeSZ can be envis-aged. Significantly greater densification was observed when com-position moved towards higher CeSZ addition. In samples CZ15and CZ20 where ceria is added in relatively high volumes, theproduct becomes glassier in nature with the cerium oxide actingas a fluxing agent. Increasing content of zirconia and ceria can beclearly observed through the EDS pattern which also provides acomplete compositional chart. In addition, the average composi-tion calculated from three data points by EDS analysis showed thatthe powder had a uniform spatial distribution of constituent phaseon a nano-size scale.

Fig. 8. FE-SEM and EDS plot of ceria stabilized zirconia and pure cordierite prepared.

Fig. 9. FE-SEM and EDS plot of cordierite composite containing 5 and 10 weight% CeSZ.

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Fig. 10. FE-SEM and EDS plot of cordierite composite containing 15 and 20 weight% CeSZ.

Fig. 11. CZ20 at 1300 C.

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4. Conclusions

Nanostructured cordierite and CeSZ were synthesized success-fully by two novel processing techniques. Literatures suggest thatcordierite formation is always supported by a spinel phase andhence increasing calcinations temperature with addition of a sin-tering aid was done successfully to remove any residual phases.Ceria acts as a stabilizer in zirconia ceramics and it was found tohelp in improving crystallinity of a-cordierite formed by actingas a sintering aid. CeO2 also improves compressibility of nano-composites which is evident by the physical property measure-ments. The process limited the formation of zircon and mullite,apparently due to formation of stable cubic cerium zirconium

oxide. It had been reported that making a dense cordierite bodyis very difficult as it has an open structure but present work suc-ceeds in achieving bulk densities near to its theoretical density.The obtained mixture of two high-strength materials displays,however, interesting combination of mechanical properties cou-pled with desired physico-thermal stability.

The present study revealed that the compressive strength aswell as the flexural strength of the obtained nano-composites de-pends on the weight percent of cubic ceria–zirconia present.Unfortunately, from the technological point of view, the observedgradual deterioration of both the properties with increasing sinter-ing temperature above 1300 �C fails short from required character-istics but still it is far better than previous reported values. The

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present authors claim that this drawback can be improved byproper selection of the fabrication conditions, since there are sev-eral toughening mechanisms available in zirconia system.

The liquid and amorphous phase formations at high tempera-ture inhibit their uses for structural applications above 1400 �C.It is worth emphasis that the presented results are consistent,i.e., confirmed through numerous repeating of the measurementand the structural properties show a major improvement com-pared to previous reported values. The obtained material throughnovel processing routes appears to be an interesting candidatefor thermal insulation, and its production involves the phenomenathat need further clarification to be applied in a controlled manner.

Acknowledgments

The authors gratefully acknowledge the financial support ofDST [(TDT Division), Reference No. DST/SSTP/UP/197(G) 2012],Ministry of Science & Technology, New Delhi, India. We gratefullyacknowledge the valuable assistance of Mr. Bhagmal, Mr. AshishTripathi and Mr. Mansharam with the XRD, SEM and MORmeasurements.

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