纳米铜锰复合氧化物前驱物晶粒生长动力学研究

通过对低热固一固化学反应法制备的纳米铜锰复合氧化物前驱物晶粒生长动力学的研究,得到其晶粒生长变化的规律,及晶粒生长激活能和动力学指数的平均值,分别是:E≈33.5 kJ/mol,n≈2.80,为其工业化生产提供理论参考依据.     

A Diffusion-Compensation Relation for Normal Grain Growth and Grain-Boundary Diffusion of Oxygen in Oxides

Values of D₀/δ and Q for grain-boundary diffusion, derived from published studies of normal grain growth in Al₂O₃, BeO, CaO, MgO, SiO₂, and CaSiO₃, are fit by the linear compensation equation log D₀/δ=0.03170Q -7.6792 (r²=0.9384). Comparison of grain-boundary diffusion coefficients derived from grain growth in oxides with those obtained by direct experimental measurement suggests that the kinetics of normal grain growth are controlled by grain-boundary diffusion of oxygen.     

Effect of Dislocations on Grain Growth in Strontium Titanate

Two series of experiments were done to investigate the effect of dislocations on grain growth in SrTiO₃. In the first series, we observed the growth behavior of a single-crystal plate toward a TiO₂-excess SrTiO₃ powder compact, containing different dislocation densities on two equivalent {100} surfaces. The surface with a higher dislocation density exhibited faster growth, showing interface mobility enhancement by dislocation. In the second series, a polycrystalline SrTiO₃ sample which had been plastically deformed by hot pressing was embedded in a TiO₂-excess SrTiO₃ powder compact, and its growth behavior toward the powder compact was compared with that of a sample without hot pressing. As with a single crystal, the grains with the higher dislocation density in the plastically deformed sample grew faster. In addition, some grains in the plastically deformed sample showed the characteristic of abnormal grain growth. The transmission electron microscopy (TEM) observations on a sintered TiO₂-excess SrTiO₃ showed that the abnormally large grains contained many dislocations while the fine matrix grains contained practically no dislocations. This result suggests that the uneven distribution of dislocations between grains is possibly one of the causes of the abnormal grain growth in polycrystalline SrTiO₃.     

Effect of Second-Phase Particles on Grain Growth in Calcite

Grain growth in polycrystalline caicite (CaCO₃) at 800°C and 300-MPa confining pressure is inhibited by the addition of Al₂O₃ particles; volume fractions of second phase (f) ranged from 0.02 to 10.0 vol%, and several powders with different average particle size were used. The stable grain size reached during heat treatment was inversely proportional to l/f^m where m varied from 0.3 to 0.55 — agreeing with results from other grain growth experiments in ceramics and metals, and with models developed for particles located at grain boundaries and grain corners, but not agreeing with models developed for random dispersions. The dependence of the stable grain size on second-phase particle size in these experiments was less systematic, possibly because of variations in the particle size distributions used, or because of particle agglomeration effects. In the single-phase aggregates, the growth kinetics were consistent with a normal grain growth equation with n=3, although uncertainties were large.     

Effect of Matrix Grain Growth Kinetics on Composite Densification

The effect of matrix grain growth kinetics on the densification behavior of a particulate composite has been investigated. Large alumina inclusions were added to two types of zirconia matrices having different grain growth kinetics, i.e., Zr(8Y)O₂ and Zr(3Y)O₂. Despite the evolution of different grain sizes, both matrices reached an end-point relative density of 0.92 and 0.94, respectively, even though they contained 0.25 volume fraction of inclusions (based on total solid volume). However, the morphology and spatial distribution of the residual pores were distinct. The Zr(8Y)O₂ matrix, which developed very large grains, had a dual microstructure with porous regions surrounded by a connective, denser region, as previously reported for an alumina matrix containing zirconia inclusions. Conversely, the Zr(3Y)O₂ matrix was fully dense and uniform, but the composite contained large cracklike voids at inclusion/matrix interfaces. The large opening displacement of these cracklike voids locally relaxed the constraint to matrix shrinkage imposed by the inclusion network.     

Effect of Sintering Atmosphere on Grain Shape and Grain Growth in Liquid-Phase-Sintered Silicon Carbide

We investigated the effects of the sintering atmosphere on the interface structure and grain-growth behavior in 10-vol%-YAG-added SiC. When α-SiC was liquid-phase-sintered in an Ar atmosphere, the grain/matrix interface was faceted, and abnormal grain growth occurred, regardless of the presence of α-seed grains. In contrast, when the same sample was sintered in N₂, the grain interface was defaceted (rough), and no abnormal grain growth occurred, even with an addition of α-seed grains. X-ray diffraction analysis of this sample showed the formation of a 3C (β-SiC) phase, together with a 6H (α-SiC) phase. These results suggest that the nitrogen dissolved in the liquid matrix made the grain interface rough and induced normal grain growth by an α→β reverse phase transformation. Apparently, the growth behavior of SiC grains in a liquid matrix depends on the structure of the grain interface: abnormal growth for a faceted interface and normal growth for a rough interface.     

Effect of Powder Parameters on Grain Growth in Manganese-Zinc Ferrite

The effect of calcination and ball milling on the grain growth in Mn-Zn ferrite is presented. Rates of grain growth and the effect of ball milling on the growth behavior were observed for ferrite powders calcined above and below the recrystallization temperature. It is shown that in addition to particle size and distribution, calcination temperature was a critical factor responsible for the growth behavior of ferrite.     

Effect of Pore–Grain-Boundary Interactions on Discontinuous Grain Growth

Pore behavior as influenced by migration of an abnormal grain boundary was studied. The pore behavior during the discontinuous growth of hot-pressed spinel (MgAl₂O₄) — whether the pore can move attached to the grain boundary — is particularly governed by the pore size and the matrix grain size. These phenomena can be understood from the interaction between the pore and the abnormal grain boundary.     

Effect of Grain Growth on Hot-Pressed Optical Magnesium Fluoride Ceramics

An optical MgF₂ ceramic was hot-pressed at 838 to 983 K with 241-MPa pressure. The maximum mechanical strength (>180 MPa) developed in the resulting ceramic within the grain-growth range μ/μ₀= 7 to 10, where μ is the grain size of the hot-pressed article and μ₀ that of the starting powder. In the early stages of grain growth (μ/μ₀<7), however, mechanical strength was directly proportional to grain size, which appeared to contradict the expectations of the Petch and Knudsen relations. Optical transmission was inversely proportional to grain size, especially in the visible wavelength. This study thus demonstrates a particular case in which increasing grain size promotes higher mechanical strength but lower optical transmission.     

Abnormal Grain Growth of Alumina: CaO Effect

The critical concentration of Ca required for the onset of abnormal grain growth in alumina was determined by controlled doping of Ca in ultrapure alumina (>99.999%), by sintering under clean contamination-free conditions, and by microstructural characterization. As in the case of Si, the excess concentration of Ca beyond its solubility limit was inversely related to the average grain size at the moment of first appearance of abnormal grains, which corresponds to the moment of sufficient enrichment of Ca in grain boundaries to form stable intergranular liquid films. However, the critical concentration of Ca was found to be in the range of only a few tens of ppm, which is lower than that of Si by almost 2 orders of magnitude. The equivalent silica concentration to form such a stable intergranular calcium aluminate glass film and its minimum thickness were estimated from the inverse relationship with the assumption that the glass composition is close to calcium hexaluminate.     

Effect of Liquid Content on the Abnormal Grain Growth of Alumina

Alumina specimens with small amounts of CaO and TiO₂ were prepared and their microstructural evolution during sintering was investigated. Because of the appearance of a liquid phase during sintering, a duplex microstructure of a few abnormal grains and fine matrix grains was obtained when the CaO + TiO₂ content was small (≤0.04 wt%). When the CaO + TiO₂ content was relatively high (≥0.1 wt%), many grains grew and impinged upon each other. As a result, a rather uniform and homogeneous microstructure was observed.     

Effect of Sintering Atmosphere on the Densification and Abnormal Grain Growth of ZnO

When a ZnO powder compact is sintered at 1450°C in air, the grains at the surface region of the specimen are observed to grow abnormally. Such abnormal grain growth does not occur when the specimen is sealed in a small cylindrical quartz tube and sintered at the same temperature. The higher rate of densification at the near-surface region than in the interior is suggested to trigger the abnormal grain growth observed.     

Effect of a Liquid Phase on the Morphology of Grain Growth in Alumina

In this investigation we have studied how the presence of a liquid phase affects the grain morphology and grain growth kinetics in Al₂O₃ at 1800°C using the growth of both matrix grains and large spherical single-crystal seeds growing into the matrix. The growth rates of the matrix grains were found to decrease in the following order: undoped Al₂O₃, AI₂O₃ with anorthite, AI₂O₃ with anorthite and MgO, and Al₂O₃ with MgO. Except for the samples doped with MgO alone, the matrix grains were faceted and appeared tabular in polished sections. In samples containing anorthite both with and without MgO, the single-crystal seeds exhibit basal facets with continuous liquid films and slow growth in the 〈0001〉 relative to all other crystallographic directions. When only MgO is added, the growth of the single-crystal seeds was not isotropic; however, no faceting was observed. We discuss how anisotropic growth rates caused by the anorthite additions can stimulate discontinuous grain growth in Al₂O₃.     

Structure of High-Angle Grain Boundaries in Metals and Ceramic Oxides

The current knowledge of the structure of high-angle grain boundaries in metals and ceramic oxides is critically reviewed. Particular attention is given to effects due to differences in the bonding and crystal structure in these solid types. The results of recent experimental work and efforts to model grain-boundary structure using computer simulation methods are described. Important characteristic features of boundaries in these materials are discussed. Difficulties which are presently being encountered in efforts to determine their structure are pointed out.     

The Effect of MgO Addition on Grain Growth in PMN–35PT

When Pb(Mg_1/2Nb_2/3)O₃–35PbTiO₃ (mol%) (PMN–35PT) is sintered at 1200°C after packing in PbZrO₃ powder, the grains show normal growth with time invariant normalized grain size distributions. If 0.5 wt% MgO is added to PMN–35PT, abnormal grain growth occurs with the large abnormal grains developing nearly cubic shapes. The interfaces between grains and PbO-rich liquid at grain triple junctions are flat, indicating that they are singular. Many central segments of the liquid films and possibly grain boundaries between the abnormal grains and the small neighboring grains are also flat along the {100} planes of the abnormal grains. The abnormal grain growth in the MgO-doped specimens is likely to be caused by the presence of these singular interfaces. Most of the large abnormal grains do not contain any Σ=3 penetration twin boundaries unlike the previous observations in PbO-excess PMN–35PT.     

A Huge Effect of Weak dc Electrical Fields on Grain Growth in Zirconia

We show that the application of a modest dc electrical field, about 4 V/cm, can significantly reduce grain growth in yttria-stabilized polycrystalline zirconia. These measurements were made by annealing samples, for 10 h at 1300°C, with and without an electrical field. The finding adds a new dimension to the role of applied electrical fields in sintering and superplasticity, phenomena that are critical to the net-shape processing of ceramics. Grain-growth retardation will considerably enhance the rates of sintering and superplasticity, leading to significant energy efficiencies in the processing of ceramics.     

Grain Growth in Sintered Uranium Dioxide: II, Columnar Grain Growth

Evidence is presented which shows that columnar grains can be induced to grow in high-density sintered uranium dioxide specimens by applying a steep temperature gradient at temperatures above 1700°C but below the melting point of 2800°C. Columnar growth apparently is a result of the migration of large transverse voids, whose individual widths define the cross sections of the grains, up a temperature gradient by a sublimation process. The grains grown by this process have a (111) preferred orientation along their columnar axis. A consequence of such void migration in operating fuel elements containing solid UO₂ pellets is the formation of a central void bounded by a region of oxide exhibiting columnar growth.