Densification of Precursor-Derived Si-C-N Ceramics by High-Pressure Hot Isostatic Pressing

Densification behavior of precursor-derived Si-C-N ceramics by hot isostatic pressing (HIP) has been investigated to obtain dense ceramics derived from polymer precursor. An as-pyrolyzed ceramic monolith, which had a porosity of about 17%, could be deformed up to a strain of 8% in preliminary uniaxial compression tests. The flow stress of the material was much higher than 200 MPa at 1600°C; thus high stress was necessary for densification by HIP. The density of the monolith increased from 1.9 to 2.4 g/cm³ by HIP at 1600°C and 980 MPa. Although the number of pores decreased, large pores were formed in the hot isostatically pressed monolith. On the other hand, denser ceramics, in which pores were not observed by optical microscopy, were obtained by hot isostatically pressing the pyrolyzed powder compact.     

粉体热压法制备As2Se3硫系玻璃研究

本研究尝试将As₂Se₃红外硫系透镜生产过程中产生的块体玻璃废料进行回收利用,首先将清洗后的块体玻璃废料球磨成粉体,然后采用粉体热压技术实现高光学质量As₂Se₃玻璃片的制备。研究了粉体粒度、热压参数对制备的As₂Se₃玻璃光学性能的影响,对比了粉体热压法和熔融淬冷法制备的As₂Se₃玻璃的性能,评估了通过粉体热压途径制备红外硫系玻璃的可行性。结果表明:随着球磨时间的延长,As₂Se₃玻璃粉体的平均颗粒尺寸逐渐减小,且颗粒尺寸的分布趋于更加均匀;使用平均颗粒尺寸为9.7 μm的粉体(球磨10 min),在压力为40 MPa、热压温度为250 ℃、热压时间为10 min的条件下获得的热压玻璃的致密度达到99.8%,其折射率与熔融淬冷法制备的玻璃的折射率接近(在10 μm波长的折射率差仅为0.003),在10 μm波长的透过率达61%(理论透过率为63.7%)。通过进一步提高玻璃粉体的纯度和尺寸均匀性,有望制备出与熔融淬冷法制备的玻璃性能相当的热压玻璃。     

Interface-Reaction-Controlled Kinetics in the Hot Isostatic Pressing of Submicrometer Alumina Powder

The kinetics of densification of alumina powder compacts at 1000° to 1200°C were studied by using a dilatometer to monitor volumetric changes during hot isostatic pressing. The densification rate is proportional to the square of the applied pressure, and densification does not occur below a threshold pressure. Results are interpreted in terms of an interface-controlled reaction mechanism involving the movement of grain-boundary dislocations.     

Microstructure and Crystal Phases of Praseodymium Oxides in Zinc Oxide Varistor Ceramics

The effect of sintering temperature on the microstructure and crystal phases of the intergranular praseodymium oxides in ZnO varistor ceramics was investigated using transmission electron microscopy and high-resolution electron microscopy. The ZnO grains were three-dimensionally separated from the intergranular praseodymium oxides. On the basis of microdiffraction analyses of the intergranular layer, the phase transformation from fcc-Pr₆O₁₁ into hcp-Pr₂O₃ was found when the sintering temperature increased from 1300° to 1350°C. The defect reaction equation and the decrease of donor concentration with increasing sintering temperature can verify the certainty of phase transition during the liquid-phase sintering observed by transmission elecron microscopy. Additionally, on the basis of the small variations of the breakdown voltage per grain boundary, the number of active grain boundaries is not a dominant factor for the donor concentration dependence on the sintering temperature.     

Hot Isostatic Pressing of Alumina and Examination of the Hot Isostatic Pressing Map

The hot isostatic pressing (HIP) of four alumina powders is studied in the temperature range 1100° to 1400°C, at 5- to 200-MPa applied pressure, and for times ranging from 0.5 to 4 h. Density increases with increasing HIP temperature, pressure, and time; decreasing grain size results in increased density after HIP. An empirical relation is derived for grain growth during HIP, and the HIP map proposed by Helle et al. is found applicable to the present results. Densification is governed by the grain-boundary diffusion of aluminum ions; with the transport coefficient and the grain-growth values found in the present study, the map can be used to express experimental results to within a factor of 4 for all densification stages except near full density.     

Hot Isostatic Pressing of Reactive SnO2 Powder

Tin(IV) oxide (SnO₂) crystallizes at room temperature by adding hydrazine monohydrate ((NH₂)₂· H₂O) to a hydrochloric acid solution of tin, followed by washing and drying. Well-densified SnO₂ ceramics (99.8% of theoretical) with an average grain size of 0.9 μm have been fabricated by hot isostatic pressing for 2 h at 900°C and 196 MPa. Their Vickers hardness and bending strength are 14.4 GPa and 200 MPa, respectively. They exhibit an electrical conductivity of 2 × 10⁻³−9 × 10⁻³ S·cm⁻¹ at room temperature.     

Microstructure and Crystal Structure of Bismuth Oxide Phases in Zinc Oxide Varistor Ceramics

The microstructure and crystal structure of bismuth oxide phases of as-sintered and additionally heat-treated ZnO varistor ceramics was examined by means of transmission electron microscopy and X-ray powder diffraction. Grains of β-Bi₂O₃ present in as-sintered samples were found to consist of two types of domains which have the same basic tetragonal lattice. They can, however, be distinguished by the contrast between local strain centers due to microdefects present in one domain. Some multiple-grain junctions contained both α- and β-Bi₂O₃ grains. Heat treatment induces a transformation of the α- and β-Bi₂O₃ phases into γ-Bi₂O₃. A clear crystallographic relationship between the transforming phases β to γ is shown. The β-Bi₂O₃ phase was also found to transform into the non-stoichiometric crystalline Bi₂O_2.33 phase.     

原位热压TiC/Ti/Al合成Ti2AlC的研究

以TiC／Ti／Al为原料，采用热压工艺在1400℃原位合成和烧结了含少量第二相Ti3AlC2的Ti2AlC材料。通过不同温度和不同热压时间下合成试样的：XRD分析探讨了Ti2AlC的合成过程。结果表明，高温下Ti与Al反应生成中间相TiAl金属间化合物，然后TiC与TiAl金属间化合物反应生成Ti2AlC。初期反应非常迅速，大部分Ti2AlC在此阶段生成。反应后期反应物减少，速度变慢，同时生成少量第二相Ti3AlC2。不同温度下合成的Ti2AlC颗粒具有不同的形貌特征。     

Evaluation of Hot Pressing and Hot Isostastic Pressing Parameters on the Optical Properties of Spinel

The effect of different hot pressing and hot isostatic pressing (HIP) temperatures and pressures on the optical properties of spinel was studied. Extinction coefficients of spinel samples were estimated by comparing the measured transmittance with the theoretical transmittance as calculated via a Sellmeier model. Results showed that the relative size of the scattering sites was large compared with the wavelengths of light (0.35–5.5 μm). Overall, increasing HIP temperature and pressure resulted in decreasing the optical extinction. The lower of two hot pressing temperatures (1620° vs 1650°C) prior to HIPing resulted in lower scatter coefficients after HIPing; this effect was most significant in the infrared.     

Growth of Dense Single Crystals of Potassium Sodium Niobate by a Combination of Solid-State Crystal Growth and Hot Pressing

(K_0.5Na_0.5)NbO₃ (KNN) is a potential lead-free replacement for Pb(Zr, Ti)O₃ (PZT) piezoceramics, but its piezoelectric properties are inferior to those of PZT. By growing single crystals of KNN, it may be possible to improve the piezoelectric properties. Recently, single crystals of KNN were grown by the solid-state crystal growth (SSCG) method, but the crystals were very porous. In this paper, a method of growing dense single crystals by SSCG in a hot press will be described. (110)-oriented single seed crystals of KTaO₃ were buried in KNN powder, with 0.5 mol% of K₄CuNb₈O₂₃ added as a sintering aid. After consolidation by uniaxial and isostatic pressing, the tablets were hot pressed in a two-stage treatment. During hot pressing, dense single crystals of KNN grew using the KTaO₃ seed crystal as a template. The effect of hot pressing on single crystal growth will be discussed.     

Yttrium α-Sialon Ceramics by Hot Isostatic Pressing and Post-Hot Isostatic Pressing

Dense α-sialon materials were produced by hot isostatic pressing (HIP) and post-hot isostatic pressing (post-HIP) using compositions with the formula Y _x (Si_{12–4.5 x} , Al_{4.5 x} )-(O_{1.5 x} ,N_{16–1.5 x} ) with 0.1 ≤ x ≤ 0.9 and with the same compositions with extra additions of yttria and aluminum nitride. X-ray diffraction analyses show how the phase content changes from large amounts of β-sialon ( x = 0.1) to large amounts of α-sialon ( x = 0.4) and increasing amounts of mellilite and sialon polytypoids ( x = 0.8). Samples HIPed at 1600°C for 2 h contained unreacted α-silicon nitride, while those HIPed at 1750°C for 1 h did not. This could be due to the fact that the time is to short to achieve equilibrium or that the high pressure (200 MPa) prohibits α-sialon formation. Sintering at atmospheric pressure leads to open porosity for all compositions except those with excess yttria. Therefore, only samples with excess yttria were post-HIPed. Microstructrual analyses showed that the post-HIPed samples had the highest α-sialon content. A higher amount of α-sialon and subsequently a lower amount of intergranular phase were detected at x = 0.3 and x = 0.4 in the post-HIPed samples in comparison to the HIPed. The hardness (HV10) and fracture toughness ( K _IC) did not differ significantly between HIPed and post-HIPed materials but vary with different x values due to different phase contents. Measurements of cell parameters for all compositions show a continuous increase with increasing x value which is enhanced by high pressure at high x values.     

Hot Pressing of Aluminum Oxide

The hot-pressing characteristics of two aluminum oxides were studied in order to acquire a better understanding of the densification process and to develop a method of predicting the effects of experimental conditions other than those actually tested. A rapid hot-pressing technique was developed which greatly reduced the time element. The rate equation proposed by Murray, Livey, and Williams for hot-pressing ceramics was found to hold for the two aluminas, thus making it possible to predict the effects of changes in hotpressing conditions. The equation used was     . Viscosity, η, was calculated as a function of temperature using this equation and the experimental hot-pressing results. Values varied from 1.2 × 10¹² poises at 1300°C. to 4.6 × 10¹⁰ at 1600°C.     

Porous Ceramic Filters Produced by Hot Isostatic Pressing

Porous TiO₂ ceramics were produced by capsule-free hot isostatic pressing (HIPing) and were evaluated by measuring gas permeability and pore size distribution. In comparison to conventionally sintered TiO₂, the HIPed TiO₂ has a higher permeability and narrower pore size distribution. The differences are caused by the effects of high-pressure gas during sintering.     

Hot Pressing of Potassium-Sodium Niobates

Compositions in the system KNbO₃-NaNbO₂ have been hot-pressed to yield ceramics with relative densities greater than 99%. Because these materials lack any degree of pyroplastic behavior, temperatures approaching those required for air sintering are needed. These hot-pressed specimens exhibit a finer microstructure than that obtainable by conventional processing. Enhanced piezoelectric coefficients are observed, and in certain compositional regions radial coupling coefficients have been increased from 32 to approximately 48%. Dielectric constants increase with densification and ac losses remain relatively high. The fine structure and improved piezoelectric activity, especially near the equimolar composition, make these materials desirable for electromechanical transducers, particularly in the 10 to 20 Mc per second range. The rate equation of Murray, Live, and Williams for hot-pressing ceramics was found to apply in this system.     

Preparation of High-Strength and Translucent Alumina by Hot Isostatic Pressing

A vacuum-pressure slip-casting technique and hot isostatic pressing (HIPing) were used to prepare high-strength and translucent alumina ceramics. A low-viscosity and high-solids-content slurry (46 vol% solids) was prepared, and a dense green compact was formed. The samples were sintered and subjected to capsule-free HIPing. Extremely high-density (99.9%) and fine-grained (0.7 to 15 μm in diameter) alumina ceramics were obtained. The HIPed samples showed high bend strength and translucency with in-line transmittance of 30% to 46% (1 mm thick).     

Hot Pressing of Fused Silica

The hot pressing of powdered fused silica at temperatures below the melting and devitrification regions is described. The fabrication of high-density shapes of fused silica is possible by this process. A relation expressing densification as a function of viscosity, time, applied pressure, and initial compaction is essentially substantiated.     

Effect of Coarse-Powder Portion on Abnormal Grain Growth during Hot Pressing of Commercial-Purity Alumina Powder

By classification, two powder portions, one consisting of coarse particles and the other consisting of fine particles, were separated from a MgO-doped (1000 ppm) commercial-purity Al₂O₃ powder. Examinations of microstructure evolution during hot pressing showed that extensive abnormal grain growth occurred for the coarse portion. For the fine portion, although there was an indication that grain-size distribution deviated from normal distribution on prolonged hot pressing, such extensive abnormal grain growth did not occur. Extensive abnormal grain growth also occurred when the coarse portion was mixed into a high-purity powder that exhibited no abnormal grain growth alone. Chemical analyses revealed that the coarse portion contained the higher concentration of impurities but lower concentration of magnesium than the fine portion. It was discussed that particle aggregates in the coarse portion might have been responsible for the higher concentration of impurities but lower concentration of magnesium and, thus, for the extensive abnormal grain growth.     

Sintering and Hot Pressing of Uranium Tetrafluoride

The basic sintering and hot-pressing conditions have been established for UF₄. Either sintering of cold compacted material in argon or direct hot pressing in graphite dies can be used satisfactorily to obtain high-bulk-density compacts. Vacuum sintering is ruled out because of the volatility of the UF₄. The occurrence of black localized areas in both sintered and hot-pressed material is attributed to the internal distortion of fluorine atoms into lattice voids resulting in the formation of strained areas.     

Densification of Lead Selenide and Lead Sulfide by Hot Isostatic Pressing

PbSe and PbS are both volatile and difficult to densify by normal sintering. We tried to sinter these compounds in a closed system, that is, by hot isostatic pressing in a glass capsule. Hot isostatic pressing was carried out for these compounds on the basis of their thermal behaviors in air as studied by TG-DTA. The maximum bulk densities obtained were 8.10 and 7.50 g/cm³, respectively, corresponding to their theoretical densities. The microstructures of the isostatically hot-pressed bodies were also observed by SEM. The present work shows that ceramic compounds which are normally volatile can be densified by encapsulation in glass followed by hot isostatic pressing.     

Translucent α-Sialon Ceramics by Hot Pressing

The single-phase α-sialon ceramics with high optical transmittance have been prepared by hot pressing. The maximum transmittance reached 65.2% and 52.2% in the infrared wavelength region, 58.5% and 40% in the visible region for the samples 1.0 and 1.5 mm thickness, respectively. The material also exhibited good mechanical properties of high hardness (20 GPa) and better fracture toughness (5.1 MPa·m^1/2). Both high optical transmittance and improved toughness of α-sialon ceramics were attributed to the less-grain-boundary glassy phase and the homogeneous microstructure, which was obtained by a proper process and confirmed by SEM and TEM observation, compared to that prepared by ordinary sintering. It is, therefore, expected that the translucent α-sialon ceramics could be a promising optical window material.