Sintered Properties and Sintering Behavior of Mgo–Zro2 Composite Hydrogel Prepared by Coprecipitation Technique

Active precursor powders of MgO–ZrO2 composite system were prepared in three different mole ratio by electrolytic interaction of inorganic salts in aqueous phase. Complete hydrolysis occurred at the critical instability point of the system at pH 9. The amorphous precursor powders exhibited very high specific surface area. The sintered properties of the composite system are dependent on mole ratio of MgO:ZrO2, sintering temperature and the rate of crystallization of constituent phases. The presence of periclase grains and cubic stabilized ZrO2 particles in the sintered body was confirmed by X-ray diffraction analysis while scanning electron micrographs indicated that MgO particles occupy the inter-granular positions between ZrO2 polycrystals.


Introduction
Present day synthesis of precursor powder eyes on preparing tailor made products for specific application.Powders used in electronic and special ceramics are prepared by controlling crystal chemistry to produce specific physical properties.This transformation processing is dependent on micro-morphology and surface chemistry.The transformation processing includes manipulation of various types of sols, gels or organometallic precursors at low temperature and subsequent change in the phase state of these substances at elevated temperature.
Zirconia is one of the most important refractory materials and cubic stabilized zirconia, as solid electrolyte, find application in electrochemical devices, fuel elements, oxygen analyzer sensor, electrolyzer for water vapor, oxygen monitors, oxygen pumps etc [1].Fully stabilized zirconia has cubic structure and the addition of cubic stabilizing oxides viz., CaO, MgO, Y 2 O 3 in varying proportions resulted in the development of many novel and innovative materials.
Coprecipitation technique has been widely used to produce zirconia magnesia composite ceramics [2][3][4].The precursor powders synthesized by this route exhibited large specific surface area, high specific pore volume, better pore connectivity and uniform pore size distribution [5].The oxide powders derived through this route have high sintering capacity and require low concentration of additives to stabilize the cubic phase.A novel meso-porous solid base of MgO-ZrO 2 composite system was prepared via sol-gel technique exhibited high specific surface area and thermal stability [6].
The solubility of MgO in different polymorphs of ZrO 2 varies with temperature.The solubility of MgO in monoclinic ZrO 2 is negligible up to tetragonal transformation; it increases slowly with temperature but still less than 1% at 1300 °C.A cubic solid solution becomes stable at 1400 °C with a eutectoid at 13 mol.%MgO.Homogeneous fluorite solid solution then exists at this composition above 1400 °C.
The most significant feature of MgO-ZrO 2 system is the decomposition of the cubic solid solution into constituent oxides at around 1100 °C [7].As this occurs MgO has a tendency to precipitate at the intra-granular voids.This affects the mechanical properties of the composite.Several researchers investigated the instability of c-ZrO 2 and resulting diffusion controlled reactions [8][9].The diffusion controlled reactions resulting from the instability include precipitation of either t-ZrO 2 or one or more defective fluorite intermediate compounds or eutectoid decomposition of the system (MgO-ZrO 2 ) to m-ZrO 2 and MgO.Colloidal consolidation routes provide required control over the mechanism of particleparticle interaction.Reactive MgO-ZrO 2 composite precursor powder attained close to 100 % theoretical density on sintering above 1600 °C [10].Very high density, low thermal shock resistance and electrical conductivity values are suitable for high temperature oxygen gas detection.E.N.S Muccillo et al. [11] prepared nano-sized 13 mol.%MgO doped ZrO 2 by controlled coprecipitation technique and the linear shrinkage-temperature curves indicated different rates of densification up to 1600 °C.
In the present study, sol-gel derived synthetic precursor powders of MgO-ZrO 2 system were subjected to reaction sintering and the sintered properties were studied in relation to mole ratio.Development of different phases, phase assemblage and particle morphology of the sintered samples were also studied to understand the sintering behavior and develop the mechanism of densification.

Experimental 2.1. Synthesis of the mixed hydroxide hydrogel
The hydroxide hydrogel was synthesized by interaction of Mg(NO 3 ) 2 and ZrOCl 2 in aqueous phase.Ingredient solutions were prepared by dissolving the salts in de-ionized water.Solution mixtures were prepared at three different mole ratio of MgO:ZrO 2 by taking requisite volume of the individual solution.In each case, the solution concentration was adjusted to 5% w/v by adding de-ionized water.Gelation of the sol began on slow addition of 1:1 ammonia solution with continuous stirring.The whole mixture sets into an enblocked gel at pH9.The set gel was allowed to age for 24 hrs whereby syneresis occurred indicating perfect gel formation.The aged gel was dispersed in large bulk of water and allowed to settle.The supernatant clear liquid was carefully siphoned off followed by addition of fresh water.This process was repeated until the settled mass showed the tendency to disperse.It was then filtered through a Buckner funnel, washed with hot deionised water and finally dried under vacuum at 50 °C..

Chemical analysis
About 0.2 g of accurately weighed sample was dissolved in 1:4 H 2 SO 4 solution.Zirconium hydroxide was precipitated from the solution by addition of NH 4 OH-NH 4 Cl buffer at pH 10.2.The solution was filtered, washed with de-ionized water and the residue was ignited to estimate ZrO 2 content gravimetrically.From the filtrate MgO was estimated complexometrically with EDTA in presence of solochrome black-T indicator.

Surface area and particle size distribution measurement
Surface area and particle size distribution of the precursor powders were measured in Carlo Erba Strumentazione of Italy (Model No. 1750 Sorpty) which is based on gas adsorption BET principle (using N 2 adsorption).The temperature of de-gassing was 100 °C.

Fabrication of discs and bars
The precursor powder was pre-calcined at 1000 °C to remove gel water which may cause large shrinkage and cracking.The calcined mass was milled and mixed thoroughly with 10 % uncalcined composite gel which acts as green bond.Discs (φ 25 mm and height 4 mm) and bars (60 mm×6 mm) were fabricated under uniaxial pressure of 100 MPa.The fabricated shapes were dried at 110 °C and subjected to reaction sintering at five different temperatures between 1350 °C to 1550 °C at 50 °C interval for a constant soaking period of 2 hours.Natural cooling was followed for 24 hours.

Study of sintered properties
Sintered properties were ascertained through relevant tests after sintering and subsequent cooling of the samples to ambient temperature.Linear shrinkage was determined by measuring the dimensional change in the bars after sintering and reported in percentage.In apparent porosity measurement, the sintered bars were suspended freely in xylene under vacuum for a period of 2 hrs to fill the open pores completely with the liquid.Apparent porosity was calculated using Archimedes principle.The bulk density of the heat treated samples was calculated using the results of soaked weight and suspended weight obtained during the measurement of apparent porosity.True density was measured in a pycnometer using kerosene (boiling range 180-220 °C) as reference liquid.The sintered sample was powdered, screened through 100 mesh BS sieve and dried at 110 °C for true density determination.Flexural strength of the sintered bars was measured by three point bending method in Instron Universal Testing Machine (1185).The span length selected was 45 mm and cross-head speed was maintained at 0.5 mm/min.
Identification of different crystalline phases present in the heat treated samples was done through X-ray diffraction study of the samples in powder form.The experiment was carried out in D5000 X-ray diffractometer (Siemens) with scanning speed of 2° (2θ)/min.D5000 measures atomic spacing in crystals using diffraction of approximately monochromatic X-radiation (copper k-alpha).
The phase assemblage and particle morphology of the sintered samples were studied through scanning electron micrographs.Fractured surface of the pallet sample was mounted on a standard mount head using silver plate and exposed to gold sputtering under vacuum.The secondary electron images were taken on a standard black & white 200 ASA film by the scanning electron microscope S-530, Hitachi.

Results and discussion
The mixed hydroxide hydrogel prepared by wet interaction process exhibited all the characteristics of a true gel including syneresis.The ingredient solutions didn't show any sign of hydrolysis and during slow addition of ammonia the whole solution set into enblock gel at pH 9.This was common to all the three different solutions used for making three different hydrogels and is related to the critical instability point of the system.The mixed hydroxide hydrogel possesses residual colloidal charge which is responsible for the proper orientation of the polar water molecules in the gel structure.The filtered and washed gel was dried under vacuum to prevent the formation of hard agglomerates.The dried powder was semi-soft, white and homogeneous in nature.The loose bulk density of the precursor powders was sufficiently low, indicating the characteristics of active precursor powder.However, it was found to be a direct function of ZrO 2 content in the composition due to higher atomic weight of ZrO 2 (Tab.I).The purity of the samples was ascertained through chemical analysis.The mole ratio of MgO:ZrO 2 in all three batches didn't show variation from the intended batch composition which in turn indicates that complete hydrolysis occurred at the critical instability point.

Tab
The sol-gel derived precursor powder exhibited significantly high specific surface area which might be due to high internal porosity and poor agglomeration tendency.The chemical reactions and electrical characteristic of oxide-water interface are the major controlling factors for agglomeration during synthesis.In the dried powder, the adhesive of agglomeration varies from weak Van der Waal's forces to solid bridges as strong as primary bonding.Absorbed moisture or even the moisture condensed between the particle contact lead to commonly observed cohesive power.The average particle size in the powders, though found to increase with increase in ZrO 2 content, the difference is too small to be considered as significant.

Fig. 1. Relationship between percent linear shrinkage and sintering temperature
The compacted shapes were sintered at temperatures between 1350 °C to 1550 °C for a fixed soaking period of 2 hours in oxidizing atmosphere.The individual cations in the mixed gel, however, were not susceptible to redox change.No major defects viz., cracking, lamination or sagging were observed in any of the fired samples.
The linear shrinkage of the green sample is related to the expulsion of residual OH groups attached to MgO and ZrO 2 lattice and subsequent crystallization and phase transformation followed by grain growth and elimination of voids.Linear shrinkage follows a direct relationship with sintering temperature.Again, the shrinkage increased with increase in ZrO 2 content in the composition, which is related to the rate of crystallization of individual oxides as well as relative volume change.Similar nature of linear shrinkage-temperature curves (Fig. 1) for all three batches suggests almost identical mechanism of densification.The mechanism of this solid state reaction might have proceeded in a way that the hydrogel lattice structures collapsed during initial heating stage followed by crystallization and some intermediate transformation of ZrO 2 with MgO acting as dopant.In this system the individual oxides will not interact with each other; rather only a portion of MgO will go in solid solution with ZrO 2 causing stabilization of the later phase.

Fig. 2. Relationship between percent apparent porosity and sintering temperature
Continuous decrease of apparent porosity on progressive heat treatment in all three batches indicated higher densification at elevated temperatures with no sintering defects and glass formation.The minimum porosity was obtained in the batch with least ZrO 2 content.The porosity-temperature relationship was not continuous (Fig. 2) but proceeded in steps depending on the crystallization behavior of the individual oxides.At the first inflection point at 1400 °C, the rate becomes slower which slowed down further at 1500 °C due to slower growth rate of periclase crystals.
The decrease in porosity with temperature was clearly manifested in the bulk density results.Identical nature of bulk density-temperature curves (Fig. 3) indicates almost unaltered mechanism of transformation.The initial rate was sharp which slowed down at the first inflection point at 1400 °C.The rate further slowed down after second inflection point at 1450 °C.Batch III exhibited highest bulk density because of the higher percentage of ZrO 2 in the batch composition which clearly outweighed the effect of higher porosity.However, the general observation was that at each temperature the relative density decreased with increase in ZrO 2 content in the composition.

Fig. 3. Relationship between bulk density and sintering temperature
True density of these fired composite systems is an average of the densities of one or more mixed phases of polymorphic forms of ZrO 2 and periclase.True density in all three batches found to increase with sintering temperature (Fig. 4) and also directly related to ZrO 2 content.Excepting the only deviation with batch III at 1500 °C where the change in density was very sharp, slow increase in true density with all other batches is due to slower rate of crystallization of individual phases.Thus true density, an intrinsic property of the composite system, is dependent on the molar ratio of MgO:ZrO 2 as well as the nature of phase transformation during sintering.

Fig. 4. Relationship between true density and sintering temperature
The flexural strength of the sintered MgO-ZrO 2 composite system, measured by three point bending method, exhibited positive correlation with sintering temperature due to enhanced crystallization and interlocking at elevated temperature.Maximum strength was observed at highest sintering temperature (1550 °C) in all the batches.Incorporation of ZrO 2 in MgO matrix imparted positive influence on the flexural strength of the system as evident from Fig. 5.The first inflection at 1400 °C was significant in that the slope was maximum indicating optimum crystal growth at this point.However, from the results of the mechanical strength it may be concluded that 1500-1550 °C is the ideal sintering zone for this synthetic precursor powder of MgO-ZrO 2 system.

Fig. 5. Relationship between flexural strength and sintering temperature
The conversion of amorphous precursor powder to crystalline phases during thermally activated densification process was ascertained through X-ray diffraction analysis (XRD) (Fig. 6a-c).XRD analysis evidenced the crystallization of both MgO and ZrO 2 to the maximum extent in the sintering zone.MgO crystallized in periclase form.In presence of excess MgO, ZrO 2 has been fully stabilized to cubic form.However the peak height difference in different batches was due to relative proportion of ZrO 2 in the composition.The phase composition remained almost same both at 1500 °C and 1550 °C irrespective of the batch composition excepting one with highest ZrO 2 content where monoclinic ZrO 2 was observed at 1500 °C.
The thermo-mechanical properties of the sintered composite are very much dependent on microstructure which is characterized by the relative amount, spatial distribution and the morphology of the different solid and pore phases.Scanning electron micrographs (Fig. 7a-c) of the samples fired at 1550 °C reveal that MgO grains occupy inter-granular positions between polygonal and sub-rounded cubic ZrO 2 grains.There is positive variation in the grain size with change in molar ratio of MgO:ZrO 2 .Coarsening of the grains takes place with increase in sintering temperature.The higher crystal growth in the microstructure may be explained by two factors viz., residual OH content and high interfacial surface area due to residual porosity in the gel.The residual porosity increases the nucleation rate either by increasing the effective diffusion rate or by promoting heterogeneous nucleation at the poregel interface.Microstructural aspects of ZrO 2 are very prominent in this composite system.Microstructure gradually becomes uniform with increase in sintering temperature due to higher order of crystallization and development of cubic ZrO 2 phase.With increase in ZrO 2 content in the batch composition intra-granular ZrO 2 become visible but these were smaller in size than inter-granular ZrO 2 .However, the compositional effects did not influence the pore morphology.