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1.
Grain growth in a high-purity ZnO and for the same ZnO with Bi2O3 additions from 0.5 to 4 wt% was studied for sintering from 900° to 1400°C in air. The results are discussed and compared with previous studies in terms of the phenomenological kinetic grain growth expression: G n— G n0= K 0 t exp(— Q/RT ). For the pure ZnO, the grain growth exponent or n value was observed to be 3 while the apparent activation energy was 224 ± 16 kJ/mol. These parameters substantiate the Gupta and Coble conclusion of a Zn2+ lattice diffusion mechanism. Additions of Bi2O3 to promote liquidphase sintering increased the ZnO grain size and the grain growth exponent to about 5, but reduced the apparent activation energy to about 150 kJ/mol, independent of Bi2O3 content. The preexponential term K 0 was also independent of Bi2O3 content. It is concluded that the grain growth of ZnO in liquid-phase-sintered ZnO-Bi2O3 ceramics is controlled by the phase boundary reaction of the solid ZnO grains and the Bi2O3-rich liquid phase.  相似文献   

2.
Calcia-stabilized zirconia, CSZ (7.5 wt% CaO), with impurities of A12O3, SiO2, MgO, Fe2O3, and TiO2, was sintered in air at 1783 K for times (t) up to 230 h. The microstructure consisted of grains of CSZ with small amounts of pores and a CaO-Al2O3-SiO2 eutectic. Grain diameters grew in proportion to t1/3. The results are consistent with a mechanism in which grain growth is limited by diffusion of Zr4+ ions through the liquid eutectic.  相似文献   

3.
The effects of the dopants, Mg2+, Sr2+, Sc3+, Yb3+, Gd3+, La3+, Ti4+, Zr4+, Ce4+, and Nb5+, on the grain boundary mobility of dense Y2O3 have been investigated from 1500° to 1650°C. Parabolic grain growth has been observed in all cases over a grain size from 0.31 to 12.5 μm. Together with atmospheric effects, the results suggest that interstitial transport is the rate-limiting step for diffusive processes in Y2O3, which is also the case in CeO2. The effect of solute drag cannot be ascertained but the anomalous effect of undersized dopants (Ti and Nb) on diffusion enhancement, previously reported in CeO2, is again confirmed. Indications of very large binding energies between aliovalent dopants and oxygen defects are also observed. Overall, the most effective grain growth inhibitor is Zr4+, while the most potent grain growth promoter is Sr2+, both at 1.0% concentration.  相似文献   

4.
Grain growth of ZnO during liquid-phase sintering of a ZnO-6 wt% Bi2O3 ceramic was investigated for A12O3 additions from 0.10 to 0.80 wt%. Sintering in air for 0.5 to 4 h at 900° to 1400°C was studied. The AI2O3 reacted with the ZnO to form ZnAl2O4 spinel, which reduced the rate of ZnO grain growth. The ZnO grain-growth exponent was determined to be 4 and the activation energy for ZnO grain growth was estimated to be 400 kJ/mol. These values were compared with the activation parameters for ZnO grain growth in other ceramic systems. It was confirmed that the reduced ZnO grain growth was a result of ZnAl2O4 spinel particles pinning the ZnO grain boundaries and reducing their mobility, which explained the grain-growth exponent of 4. It was concluded that the 400 kJ/mol activation energy was related to the transport of the ZnAl2O4 spinel particles, most probably controlled by the diffusion of O2- in the ZnAl2O4 spinel structure.  相似文献   

5.
The effects of the dopants, Mg2+, Ca2+, Sr2+, Sc3+, Yb3+, Y3+, Gd3+, La3+, Ti4+, Zr4+, and Nb5+, on the grain boundary mobility of dense CeO2 have been investigated from 1270° to 1420°C. Parabolic grain growth has been observed in all instances. Together with atmospheric effects, the results support the mechanism of cation interstitial transport being the rate-limiting step. A strong solute drag effect has been demonstrated for diffusion-enhancing dopants such as Mg2+ and Ca2+, which, at high concentrations, can nevertheless suppress grain boundary mobility. Severely undersized dopants (Mg, Sc, Ti, and Nb) have a tendency to markedly enhance grain boundary mobility, probably due to the large distortion of the surrounding lattice that apparently facilitates defect migration. Overall, the most effective grain growth inhibitor at 1.0 % doping is Y3+, while the most potent grain growth promoter is either Mg2+ (e.g., 0.1%) or Sc3+ at high concentration (greater than 1.0%).  相似文献   

6.
When sintered 85Al2O3–15Fe2O3 (in wt%) specimens consisting of corundum grains and spinel particles were annealed at temperature where only a corundum phase was stable, phase transformation of spinel into metastable FeAIO3 and subsequently complete dissolution of the metastable phase occurred together with the migration of grain boundaries at the surface of the specimens. Since the grain boundary migration was induced by grain boundary diffusion of Fe2O3 from the transforming and dissolving particles, the boundary migration by temperature decrease corresponds to a discontinuous dissolution of the spinel particles and a chemically induced grain boundary migration by temperature change. Inside the specimens, however, the transformation—dissolution and the grain boundary migration were suppressed because of unavailable accommodation of the volume expansion due to the transformation.  相似文献   

7.
The control of the microstructure of Ce-doped Al2O3/ZrO2 componsites by the valence change of cerium ion has been demonstrated. Two distinctively different types of microstructure, large Al2O3 grains with intragranular ZrO2 particles and small Al2O3 grains with intergranular ZrO2 particles, can be obtained under identical presintering processing conditions. At doping levels greater than ∼ 3 mol% with respect to ZrO2, Ce3+ raises the alumina grain-boundary to zirconia particle mobility ratio. This causes the breakaway of grain boundary from particles and the first type of microstructure. On the other hand, Ce4+ causes no breakaway and produces a normal intergranular ZrO2 distribution. The dramatic effect of Ce3+ on the relative mobility ratio is found to be associated with fluxing of the glassy boundary phase and is likewise observed for other large trivalent cation dopants. The ZrO2 second phase acts as a scavenger for these trivalent cations, provided their solubility limit in ZrO2 is not exceeded.  相似文献   

8.
When sintered 95Al2O3-5Fe2O3 (wt%) specimens constituting corundum grains and iron aluminate spinel precipitates were annealed under high oxygen partial pressure (Po2) where only a corundum phase is stable, fast dissolution of particulate spinel precipitates occurred, together with the migration of corundum grain boundaries. Behind the migrating boundaries, a corundum solid solution enriched with Fe2O3 formed. Discontinuous dissolution (DD) of particulate spinel precipitates thus occurred by Po2 increase. In contrast, when 95Al2O3-5Fe2O3 specimens constituting only corundum grains were annealed under low Po2 where both corundum and spinel phases are stable, grain boundaries migrated without spinel precipitation, leaving behind a corundum phase depleted of Fe2O3, similar to chemically induced grain-boundary migration (CIGM) observed during solute depletion. The volatilization of Fe2O3 appeared to cause the boundary migration without precipitation. The observed CIGM and DD would suggest various possibilities of microstructure control in other oxide systems through oxygen partial pressure change.  相似文献   

9.
The average grain size of ZrO2(+Y, o,) materials sintered at 1400°C was observed to depend significantly on the Y2O3 content. The average grain size decreased by a factor of 4 to 5 for Y2O3 contents between 0.8 and 1.4 mol% and increased at Y2O3 contents of 6.6 mol%. Grain growth control by a second phase is the concept used to interpret these data; compositions with a small grain size lie within the two-phase tetragonal + cubic phase field, and the size of the tetragonal grains is believed to be controlled by the cubic grains. This interpretation suggests that the Y2O3-rich boundary of the two-phase field lies between 0.8 and 1.4 mol% Y2O3. Transformation toughened materials fabricated in this binary system must have a composition that lies within the two-phase field to obtain the small grain size required, in part, to retain the tetragonal toughening agent.  相似文献   

10.
The sintering and grain-growth kinetics of finely divided MgAl2O4 were determined from 1300° to 1600°C for densities up to 96% of theoretical. These results show that sintering is governed by volume diffusion and that the temperature dependence of the diffusivities is 157 exp [(-118 kcal/mol)/ RT ] cm2/s. Grain growth follows the expression ( G 2- G 02)= Kt , where K =51.3 exp [(-110 kcal/mol)/ RT ] cm2/s.  相似文献   

11.
The thermal expansion of the skeletal framework was essentially zero for NaZr2(PO4)3-type compounds; the interstitialion, e.g., Na+, was primarily responsible for the total thermal expansion. The composition dependence of the thermal expansion is discussed in terms of the amounts, crystallographic sites, and ionic radii of the interstitial ions. The mechanism which results in low thermal expansion was clarified, particularly for KZr2(PO4)3, in which a larger ion is substituted for Na+, and NbZr(PO4)3, which does not contain Na+. Polycrystalline ceramics formed from these crystals might be useful as thermal-shock-resistant materials.  相似文献   

12.
Lead-free (K0.5Na0.5)NbO3 (KNN) thin films were prepared on Pt/Ti/SiO2/Si substrates by a sol–gel processing method, and titanium diffusion from the substrates into the KNN films under different thermal treatment conditions were investigated by the secondary ion mass spectroscopy depth profile and X-ray photoelectron spectroscopy surface analysis. Titanium diffusion was evident in all the KNN thin films, which was further aggravated not only by increasing the annealing temperature, but also surprisingly by higher ramping rate attributed to the resulting larger grain boundaries. The pronounced effects of the titanium diffusion and the resulting substitution of Ti4+ for Nb5+ with different valence states on the composition, structure, and electrical properties of the KNN thin films are analyzed and discussed. The results showed that the Ti diffusion from the substrate played a crucial role in affecting the structure and electrical properties of the ferroelectric KNN thin films deposited on Pt/Ti/SiO2/Si substrate.  相似文献   

13.
The precursor [NH4]2[Ti(catecholate)3] · 2H2O is known to react with Ba(OH)2· 8H2O in an acid/base process that generates Ba[Ti(catecholate)3] · 3H2O, a compound which undergoes low-temperatue calcination to produce BaTiO3 powder. Attempts to develop similar routes to PbTiO3 have been frustrated, since lead(II) hydroxide does not exist. The amphoteric yellow PbO and the basic oxide, Pb6O(OH)64+, are both insufficiently basic to react with [NH4]2[Ti(catecholate)3] · 2H2O. Based on the large sizes of both the [Ti(catecholate)3]2- anion and the Pb2+ cation, a precipitation method has been developed in which lead nitrate and [NH4]2[Ti(catecholate)3] · 2H2O are added together in an aqueous medium causing precipitation and leaving only NH4NO3 in solution. The lead-titanium-catecholate complex that forms in this manner undergoes low-temperature pyrolysis to produce PbTiO3. SEM indicates a submicrometer ultimate crystallite size.  相似文献   

14.
The effect of Al2O3 inclusions with a greater average size (0.6 μm) than the average particle size of the major phase powder (<0.1 μm) on grain gowth was examined by sintering ZrO2/Al2O3 composites (0,3,5,10, and 20 vol%) at 1400°C and then heat-treating at temperatures up to 1700°C. Normal grain growth was observed for all conditions. The inclusions appeared to have no effect on grain growth until the ZrO2 grain size was ∼1.5 times the average inclusion size. Grain growth inhibition increased with volume fraction of the Al2O3 inclusion phase. At temperatures 1600°C, the inclusions were relatively immobile and most were located within the ZrO2 grains for volume fractions <0.20; at higher temperatures, the inclusions could move with the grain boundary to coalesce. Grain growth was less inhilited when the inclusions could move with the boundaries, resulting in a larger increase in grain size than observed at lower temperatures. Analogies between mobile voids, entrapped within grain at lower temperature due to abnormal grain growth during the last state of sintering, and the observations concerning the mobile inclusions are made suggesting that grain-boundary movement can "sweep" voids to grain boundaries and eventually of four-grain junctions, where they are more likely to disappear by mass transport.  相似文献   

15.
Crystalline TiO2 powders were prepared by the homogeneous precipitation method simply by heating and stirring an aqueous TiOCl2 solution with a Ti4+ concentration of 0.5 M at room temperature to 100°C under a pressure of 1 atm. TiO2 precipitates with pure rutile phase having spherical shapes 200-400 nm in diameter formed between room temperature and 65°C, whereas TiO2 precipitates with anatase phase started to form at temperatures >65°C. Precipitates with pure anatase phase having irregular shapes 2-5 µm in size formed at 100°C. Possibly because of the crystallization of an unstable intermediate product, TiO(OH)2, to TiO2 x H2O during precipitation, crystalline and ultrafine TiO2 precipitates were formed in aqueous TiOCl2 solution without hydrolyzing directly to Ti(OH)4. Also, formation of a stable TiO2 rutile phase between room temperature and 65°C was likely to occur slowly under these conditions, although TiO2 with rutile phase formed thermodynamically at higher temperatures.  相似文献   

16.
Sodium-rich NASOCON-type ceramics, the NaM2(PO4)3+yNa2O (M = Ge, Ti, Hf, Zr) system, were investigated in order to obtain a material having a high Na+ conductivity and high density. The ionic conductivity and the sinterability were greately improved by an increase in the valve of y for all of the system examined. Added Na2O was not souble in teh NASICON-type skeletton, sice the lattice constants and teh X-ray diffraction patterns were not changed by the Na2O addintion in all of the samples. Na2O acts as a flux for obtaining highly dense ceramics and highly conductive grain boundaries. Partial A2 site insertion by Na+ ions is effective for the enhancement of conductivity, because the conductivity for Na1.5M(III)0.5Zr1.5(PO4)3 (M = In or Y) is about 1 order of magnitude higher than the maximum conductivity of the NaZr2(PO4)3+yNa2O system.  相似文献   

17.
Surface, grain-boundary, and volume inter diffusion coefficients for the NiO-Al2O3 system were measured concurrently by using a diffusion couple consisting of an A12O3 bicrystal and an NiO single crystal. The A12O3 bicrystals having various tilt angles were fabricated by firing 2 single crystals to be joined in an H2 atmosphere at 1800°C for 30 h. Diffusion profiles over the surface, along the grain boundary, and in the bulk of the bicrystal were determined with an electron probe microanalyzer. Mathematical analysis of the diffusion profiles gives D s = 7.41×10-2 exp (-35,200/ RT ), D gb = 2.14×10-1 exp (-63,100/ RT ) (tilt angle =30°), and D v = 1.26×104 exp (-104,000/ RT ). The grain-boundary diffusion coefficient increases with the mismatch at the boundary.  相似文献   

18.
Annealing of ZrO2-toughened Al2O3 (ZTA) at elevated temperatures causes growth of both the intergranular ZrO2 particles and the Al2O3"matrix" grains. Exaggerated ("breakaway") grain growth occurs in some, but not all, specimens. Analytical electron microscopy of two ZTA's, both of which contained a continuous amorphous (glassy) grain-boundary phase, but only one of which showed breakaway grain growth, revealed that the occurrence of breakaway grain growth could be correlated with the chemistry of the ubiquitous glassy grain-boundary phase.  相似文献   

19.
The characterization and properties of ceramic composites containing the phases Al2TiO5, ZrTiO4, and ZrO2 are described. The range of compositions investigated gives very low average thermal expansions (α24–1000°C as low as −2.0 × 10−6°C−1) and excellent high-temperature stability. The low thermal expansions are apparently due to a combination of microcracking by the titanate phases and a contractive phase transformation by the ZrO2. The crystal chemistry and microstructure of the product are processing dependent. Although the composites represent a complex microcracking system, the low thermal expansions and high-temperature stability make them potential candidates for commercial applications requiring thermal shock resistance.  相似文献   

20.
An experimental technique employing localized laser heating has been developed to seed abnormal grain growth in Mg-doped Al2O3 permitting the migration behavior of individual grain boundaries to be investigated. Nonuniform growth of both naturally nucleated and laser-nucleated abnormal grains was observed. Calculated boundary mobilities exceeded those characterizing grain growth in both doped and undoped porous Al2O3).  相似文献   

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