Synthesis of Fine-Grained Polycrystalline Diamond Compact and Its Microstructure

Sintering of a fine-grained polycrystalline diamond compact with grains less than 1 μm in size was successfully carried out by making a laminate on a WC/Co powder compact under sintering conditions of 5.8 GPa and 1430° to 1480°C for 30 min, in which small amounts of poly(ethylene glycol) and fine powder of cubic boron nitride were added to the starting diamond powder. The former played the role of preventing agglomeration of the diamond powder and the latter of suppressing abnormal grain growth during sintering. Microstructural observation of the polycrystalline diamond showed that in the regions near the WC/Co layer a comparatively large amount of Co metal was present between diamond grains, but in other regions the amount of Co decreased, and the diamond grains were seen to be bonded strongly. The Vickers hardness of the polycrystalline diamond was 55 ± 5 GPa with 19.6 N load.     

Direct Characterization of Grain-Boundary Electrical Activity in Doped (Ba0.6Sr0.4)TiO3 by Combined Imaging of Electron-Beam-Induced Current and Electron-Backscattered Diffraction

Simultaneous measurements of remote electron beam induced current (REBIC) and orientation imaging microscopy (OIM) in a scanning electron microscope (SEM) have been applied to a polycrystalline (Ba_0.6Sr_0.4)TiO₃ with a positive temperature coefficient of resistivity (PTCR) to elucidate a grain-boundary character dependence of the potential barrier formation. The absence of electrical activity in a coherent Σ3 twin boundary is clearly imaged. The resistivity of individual grain boundaries estimated from a resistive contrast image is interpreted in terms of geometrical coherency, which is defined by the degree of coincidence in the reciprocal lattice points.     

Comparison between SEM and TEM Imaging Techniques to Determine Grain-Boundary Wetting in Ceramic Polycrystals

Two different non-oxide ceramics, Si₃N₄ and SiC, were characterized with respect to their grain-boundary structure employing both scanning and transmission electron microscopy. The latter method, which enables one to gain direct insight of the atomistic interface structure, was utilized to verify whether grain-boundary wetting occurred. SEM imaging of plasma-etched surfaces revealed a characteristic bright contrast along interfaces for both ceramics, Si₃N₄ as well as SiC, suggesting the presence of an intergranular glass film. High-resolution TEM studies of the Si₃N₄ sample confirmed that these fine bright lines along grain boundaries represent intergranular glass films separating Si₃N₄ matrix grains. However, when high-resolution TEM was employed on SiC samples, which showed a similar contrast variation across SiC grain boundaries in the SEM, the presence of residual glass films was not detected. The SiC materials showed clean grain boundaries with no indication of residual glass even at triple pockets. Chemical analysis monitored yttrium and aluminum segregation at interfaces, which creates a potential barrier (space charges) and therefore affects both the inner mean potential at the interface (Fresnel fringes) and the plasma-etching response. Although SEM imaging showed a similar interface contrast for both Si₃N₄ and SiC ceramics, HRTEM studies clearly revealed grain-boundary wetting in the former and clean interfaces in the latter material, respectively. Hence, SEM imaging and Fresnel fringe TEM imaging alone are not conclusive when characterizing interface wetting in ceramic polycrystals.     

Origin and Growth Kinetics of Platelike Abnormal Grains in Liquid-Phase-Sintered Alumina

The grain-growth behavior of Al₂O₃ compacts with small contents (≤10 wt%) of various liquid-forming dopants was studied. Equiaxed and/or elongated grains were observed for the following dopants: MgO, CaO, SiO₂, or CaO + TiO₂. The platelike grains, defined as the abnormal grains larger than 100 μm with an aspect ratio ≥5 and with flat boundaries along the long axis, were observed when the boundaries were wet with the liquid phase and the codoping satisfied two conditions of size and valence. These dopings were Na₂O + SiO₂, CaO + SiO₂, SrO + SiO₂, or BaO + SiO₂. However, an addition of MgO to the Al₂O₃ doped with CaO + SiO₂ resulted in the change of grain shape from platelike to equiaxial. Equiaxed grains were also observed for the MgO + SiO₂ doping, indicating that two conditions were necessary but not sufficient to develop the platelike grains. The fast growth rate of the platelike grains was explained by an increased interfacial reaction rate due to the codopants. AT the same time the codopants made the basal plane, which appeared as the flat boundaries, the lowest energy plane. The appearance of the platelike grains was favored in compacts with a small grain size and with a narrow size distribution at the onset of abnormal grain growth. Accordingly, the use of starting powders with a small particle size and narrow size distribution, smaller amounts of dopings, and high sintering temperature resulted in an increased number of the platelike grains.     

Low-Frequency Measurements on the Grain-Boundary Internal Friction Peak in Chlorine-Doped Silicon Nitride

A procedure is shown to quantitatively analyze the morphology of the internal friction peak resulting from grain-boundary sliding. A Si₃N₄ polycrystal containing chlorine-doped SiO₂ at grain boundaries is selected as a model system for discussing chemical (e.g., anion) gradients at glassy grain boundaries. In this model material, grain boundaries lodging Cl⁻ anions show nonuniform thickness characteristics, which suggests a non-negligible dependence of the intergranular SiO₂-network structure upon grain misorientation. Both chemical and microstructural inhomogeneities existing in a polycrystalline ceramic body can result in peak broadening. The key for separating broadening contributions of chemical gradients from grain-size/morphology distributions resides in analyzing the peak-width change upon damping frequency. Groups of grain boundaries with different chemical characteristics may produce broadening because different peak components are generated that obey a spectrum of activation energies. On the other hand, microstructural inhomogeneities obey a single activation energy, but they generate a distribution of relaxation times. As a result, when a chemical gradient is present at grain boundaries, the peak may shift upon changing damping frequency with obeying a true activation energy, but its width increases with decreasing damping frequency. When peak broadening results only from microstructural inhomogeneities, the peak width is independent of damping frequency.     

Nondestructive characterization of grain boundary Z-directional lateral surface in ZnO using nanorobot combined with SEM

Different from focusing on grain boundary upper surface in plane X–Y, a unique approach of nanorobot-based nondestructive characterization of grain boundary Z-directional lateral surface within bulk ZnO ceramic can be creatively developed under scanning electron microscope (SEM). By rolling-over bulk ZnO, two-dimensional profiles and grain boundaries in Z-directional lateral surfaces have been imaged in plane Y–Z and individually electrically characterized nondestructively. Experiments demonstrate that it is feasible to realize nondestructive characterization of grain boundary Z-directional lateral surface structures and electrical properties using nanorobot combined with SEM. Relative height differences between grain boundaries within Z-directional lateral surface can characterize the relative position relationships. Z-directional lateral surface structures can further extend irregular grain boundary lengths in plane Y–Z to interpret surface effects of nonlinear electrical properties. Relative minor electrical reactive effects in grain indicate grain boundary dominate in nonlinear macroscopic electrical properties. Furtherly, it can be advanced to promote a nondestructive characterization of grain boundary.     

Oxygen Grain-Boundary Diffusion in Polycrystalline Mullite Ceramics

Oxygen tracer diffusivities of low- and high-alumina mullite ceramics (72 wt% Al₂O₃, 28 wt% SiO₂ and 78 wt% Al₂O₃, 22 wt% SiO₂, respectively) were determined. Gas/solid exchange experiments were conducted in an atmosphere enriched in the rare stable isotope ¹⁸O, and the resulting ¹⁸O isotope depth distributions were analyzed using SIMS depth profiling. The investigation showed that grain-boundary diffusivities for both mullite ceramics were several orders of magnitude higher than mullite volume diffusivity. Activation enthalpies of oxygen diffusion were 363 ± 25 kJ/mol for the low-alumina and 548 ± 46 kJ/mol for the high-alumina materials. Because the glassy grain-boundary films were not identified, the differences between the low- and high-alumina materials might be explained by different impurity concentrations in the grain boundaries of the two materials.     

Determining Crystal Habits from Observations of Planar Sections

A method is described for reconstructing the habits of crystals embedded in a second phase from observations of random planar sections of known orientation. We have generated simulated observations based on five assumed crystal habits and found that it is possible to reconstruct the shape from 100-1000 planar sections, depending on the relative area of each facet. Habit planes comprising as little as 3% of the crystal's area have been detected and axial ratios accurately determined.     

Mechanical Properties of Polycrystalline Lithium Orthosilicate

Room-temperature mechanical properties and high-temperature creep deformation of lithium orthosilicate (Li₄SiO₄) were studied. Elastic constants, flexural strength, and fracture toughness were determined for specimens with densities between 68% and 98% of theoretical. Critical quenching temperature and thermal-shock resistance parameters for 90% dense specimens were also measured. High-temperature creep deformation was investigated by a constant-strain-rate test in an argon atmosphere at temperatures between 750° and 1025°C and strain rates ranging from about 10⁻⁶ to 10⁻³ s⁻¹. At 950°C and above, the stress exponent, n , was determined to be 3.6, with a creep activation energy of 715 kJ/mol. Selected results obtained for Li₄SiO₄ are compared with results obtained for other Li-containing ceramics that are under consideration as candidates for fusion reactor breeder blankets.     

Morphology of Platelike Abnormal Grains in Liquid-Phase-Sintered Alumina

The microstructural features of platelike grains in liquid-phase-sintered Al₂O₃ were investigated. The flat boundaries of platelike grains wetted with a liquid phase were basal planes. After impingement of platelike grains, flat boundaries progressively changed to curved boundaries which consisted of both basal and rhombohedral planes as microscopic facets. Second-phase particles were observed for most doped samples. Small-angle subgrain boundaries inside platelike grains were also observed.     

Electrical Properties of Polycrystalline Nickel Zinc Ferrites

The electrical properties of three ferrites of composition (Ni_0.36Zn_0.64)_{1– x} Fe_{2+ x} O₄, x =−0.03, 0, 0.13, were investigated by impedance spectroscopy. The observed impedance responses corresponded to that of a highly resistive grain-boundary component in series with a less resistive bulk component. Bulk resistances gave linear Arrhenius conductivity plots ( E _a ≤0.25 eV) that were independent of thermal cycling. Grain-boundary resistances changed greatly on cycling between room temperature and 200°C, probably because of changes in the extent of oxidation at the grain boundaries.     

吸波多晶铁纤维制备工艺研究

介绍了吸波多晶铁纤维的制备装置和制备工艺,研究了反应温度、励磁电流、羰基铁流速及1次进样总量对多晶铁纤维的影响。通过实验,确定了多晶铁纤维的制备工艺为:反应温度400℃,励磁电流6A,羰基铁滴加速度1.0ml/min,1次进样总量为10ml。     

Impurity Distribution in Polycrystalline Aluminum Nitride Ceramics

Convergent-beam electron diffraction and diffuse darkfield imaging of transmission electron microscopy were used to obtain qualitative information regarding the distribution of impurities in polycrystalline AIN. Impurities are distributed homogeneously within the grains of a given ceramic, but an amorphous grain-boundary phase on the order of 1 to 2 nm in thickness is observed between the AIN matrix grains.     

硅粉密相气力输送技术在多晶硅生产中的应用

介绍某多晶硅厂对硅粉加料采用密相气力输送技术的输送方式和工艺流程,分析了该气力输送系统中主要设备和管道、阀门选型及设计时的考虑因素。     

Creep Mechanism of Polycrystalline Yttrium Aluminum Garnet

The high-temperature deformation behavior of a fine-grained polycrystalline yttrium aluminum garnet (YAG) was studied in the temperature range of 1400° to 1610°C using constant strain rate compression tests under strain rates ranging from 10⁻⁵/s to 10⁻³/s. The stress exponent of the creep rate, the activation energy in comparison with that for single-crystal YAG, and the grain size dependence suggest that Nabarro–Herring creep rate limited by the bulk diffusion of one of the cations (Y or Al) is the operative mechanism.     

Sintering and Characterization of Polycrystalline Monoclinic, Tetragonal, and Cubic Zirconia

Polycrystalline monoclinic ( m ), tetragonal ( t ), and cubic ( c ) ZrO₂, sintered at 1500°C, were annealed in the cubic stability field and rapidly cooled to permit the displacive c → t ' transformation to occur in compositions containing 0–6 mol% Y₂O₃. The bulk fracture toughness of coarse-grained (> 25 μm) m , t ', and c zirconias were compared with conventionally sintered, fine-grained (typically less than 1 μm) materials. The ferroelastic monoclinic and tetragonal zirconias were more than twice as tough as paraelastic cubic zirconia.     

Temperature Dependence of Electrical Condutivity in Polycrystalline Tin Oxide

The electrical conductivity of polycrystalline SnO₂ in air was measured between 400° and 1400°C using samples prepared by hot isostatic pressing and normal sintering. The apparent activation energy for conduction above 800°C was 1.3 to 1.4 eV in the hot isostatically pressed sample and 1.9 to 2.0 eV in the normally sintered sample. This difference can be interpreted as a result of the pore contributing an additional blocking effect to conduction. Also we found that the electrical conductivity above 1150°C bent down slightly from the linear regime when using a cumulative slope model.     

Dynamic Grain Growth During Superplastic Deformation of Yttria-Stabilized Tetragonal Zirconia Polycrystals

Both static and dynamic grain growth were studied during superplastic deformation of fine-grained yttria-stabilized tetragonal zirconia. It was found that significant grain growth does not take place below 1300°C. Both static and dynamic growth were found to obey a similar equation of the form D³−D³₀=kt, where D and D₀ are the instantaneous and initial grain sizes, respectively, t is the annealing time, and k is the kinetic constant for either static or dynamic grain growth. The activation energies were approximately 580 and 520 kJ/mol for static and dynamic grain growth, respectively.     

Effect of Porosity on the Electrical Properties of Polycrystalline Sodium Niobate: II, Dielectric Behavior

The dielectric behavior of dense and porous NaNbO₃ ceramic samples, synthesized by a suitable chemical route, was investigated by impedance spectroscopy between room temperature and 800°C in dry air. The dielectric behavior and thermal stability of the samples were evaluated as a function of several thermal cycles. The dielectric constant was calculated from the relaxation frequency, and from an alternative approach based on the variation of the opposite of the imaginary part of impedance as a function of reciprocal angular frequency. The values obtained using both relations were in a good agreement. After the porosity was corrected, the porous and dense samples presented the same dielectric constant. All samples evaluated displayed a broad dielectric anomaly between 300°–400°C. Neither the orthorhombic-tetragonal- nor the tetragonal-cubic-phase transitions were detected by dielectric measurements. The Curie-Weiss law was found to be valid above the transition temperature, whereas the corresponding phase transition presented a diffuse nature. The origin of the related thermal hysteresis is discussed herein.     

Farbication of Optically Transparent Lead Magnesium Niobate Polycrystalline Ceramics Using Hot Isostatic Pressing

The physical, dielectric, and optical properties of hot isostatically pressed lead magnesium niobate polycrystalline ceramics modified with 1/2 mol% La₂O₃, Pb_{1–3/2 x} La _x □ _{x /2}-(Mg_1/3Nb_2/3)O₃, have been investigated. Methods used to characterize the ceramics included determination of the dielectric permittivity, optical transmittance, and refractive index dispersion. The materials exhibited relaxor ferroelectric type behavior with a peak dielectric constant K > 14000 and average T _c ∼−35°C. Various sintering, hot isostatic pressing, and annealing conditions were examined to produce highly dense and optically transparent materials. Through the use of hot isostatic pressing, densities more than 99.5% theoretical and transmittance greater than 50% at 633-nm wavelength were obtained. Hot isostatic pressing technique appears to be a good alternative to hot uniaxial pressing without the associated problem of PbO volatility, reactivity with the pressure vessel, and geometrical constraints.