Oxidation Behavior of SiC-Whisker-Reinforced Alumina-Zirconia Composites

During high-temperature oxidation in air, SiC-whisker-reinforced Al₂O₃—ZrO₂ composites degrade by the formation of a whisker-depleted mullite-zirconia scale. The reaction kinetics have been studied as a function of time and temperature for composites with whiskers preoxidized for different times. The evolution of the microstructure has been investigated by optical, scanning and transmission electron microscopy. Possible reaction mechanisms have been discussed. A model compatible with our observations on Al₂O₃—ZrO₂—SiC and the results reported in the literature for Al₂O₃—SiC whisker composites is proposed: The oxidation occurs at an internal reaction front. Oxygen diffuses along dislocations and grain boundaries through the mullite scale to react at this front with silicon carbide, thereby forming amorphous silica and graphite. Silica penetrates grain boundaries and further reacts with alumina and zirconia to form mullite and zircon, while the second reaction product, graphite, is oxidized into carbon monoxide when the reaction front moves deeper into the sample.     

Experimental Evaluation of Toughening Mechanisms in Alumina-Zirconia Composites

The contributions of transformation toughening and microcrack toughening in an 85Al₂O₃15ZrO₂ (vol%) composite were quantitatively evaluated. Three types of hot-pressed samples with similar size (∼0.5 µm) and size distribution of ZrO₂ grains, but with different contents (vol%) of monoclinic- ( m -) ZrO₂ (0 (AZS), 45 (AZT), and 67 (AZM)) were prepared using Y₂O₃ and MoO₂ dopants. Therefore, the possible effect of ZrO₂ grain size on each toughening mechanism was eliminated. When the measured fracture toughnesses of m -ZrO₂ volume fractions before and after fracture were compared, the transformation-toughening constant was estimated as 3.0 MPam^1/2 and the microcrack-toughening constant 0.2 MPam^1/2 for a 100% monoclinic transformation of ZrO₂ grains. This result indicates that transformation toughening was the dominant toughening mechanism in the studied composite.     

Formation of Nanocrystalline Silicon Carbide Powder from Chlorine-Containing Polycarbosilane Precursors

Nanocrystalline ß-SiC particulates with a grain-size range of 5-20 nm were prepared by heating a prepyrolyzed, chlorine-containing polysilane/polycarbosilane (PS/PCS) to 1600°C. The transformation from the prepyrolyzed PS/PCS to nanocrystalline SiC was investigated by differential thermal analysis, thermogravimetric analysis, X-ray diffractometry, mass spectrometry, and infrared spectroscopy. The results indicated that the nanocrystalline ß-SiC was formed by the crystallization of the PS/PCS random network and the crosslinking of Si-Si, Si-Cl, and Si-CH₂-Si bonds. Transmission electron microscopy observation showed that SiC particulates consisted of equiaxed, randomly oriented, ultrafine grains.     

Sol–Gel Combustion Synthesis of Nanocrystalline YAG Powder from Metal-Organic Precursors

Yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) nanocrystalline powders were synthesized by a novel sol–gel combustion process. Yttrium acetate and aluminum sec-butoxide were used as the precursors and triethanolamine was used as the chelating agent and fuel. Thermal and crystallization behaviors of the YAG precursor powders were investigated by thermal gravimetric differential thermal analysis (DTA), Fourier transform infrared spectrum, and X-ray diffraction. The combustion-synthesized powders are amorphous and transform to a pure YAG crystalline phase at 900°C. The crystallization activation energy of amorphous YAG precursor was investigated by variable heating rate DTA. The calculated activation energy is 58.9 KJ/mol. The average crystalline size of heat-treated YAG powders at 900°C is ∼20 nm.     

Computer Simulation of Grain Growth and Ostwald Ripening in Alumina-Zirconia Two-Phase Composites

The kinetics of grain growth and Ostwald ripening in Al₂O₃–ZrO₂ two-phase composites was systematically investigated using two-dimensional (2-D) computer simulations, based on a diffuse-interface field model. Using average values for the experimentally measured ratios of the grain boundary energies to the interphase boundary energy as the input, the predicted 2-D microstructural features and their evolution are in excellent qualitative agreement with experimental observations on 2-D cross sections of 3-D Al₂O₃–ZrO₂ two-phase composite microstructures. It was found that the coupled grain growth in Al₂O₃–ZrO₂ composites is controlled by long-range diffusion and the average size ( Rt ) as a function of time ( t ) follows the power-growth law, R ^m _t − R ^m ₀= kt with m = 3, which is independent of the initial microstructures and volume fractions of the two phases. The predicted variation of the kinetic coefficient ( k ) on the volume fraction follows a trend similar to that experimentally measured through the entire range of volume fractions. The scaling of grain size distributions is observed at a given volume fraction, i.e., they are time-invariant in the steady state. However, the characteristics of size distributions vary with the initial microstructures and the volume fractions. The relationship between matrix grain size and second-phase grain size is discussed.     

Analytical Modeling of Chemical Vapor Infiltration in Fabrication of Ceramic Composites

A model for chemical vapor infiltration is applied to the study of the growth of alumina from the chemical reaction among AlCl₃, H₂, and CO₂ within a SiC-fiber bundle which is situated in an isothermal hot-wall reactor. The pore space between the fibers is simulated by cylindrical capillary tubes. The model considers binary diffusion of CO₂ and H₂, chemical reaction on the inner surface of the tube, and deposition film growth. Furthermore, diffusion-controiled and chemical-reaction-controlled processes are taken into account to determine the dominating process in chemical vapor infiltration. Both molecular diffusion and Knudsen diffusion are considered sequentially in this model during the infiltration process. Based upon this model, the optimum processing conditions required for chemical vapor infiltration to form a SiC/Al₂O₃ composite can be predicted for different fiber preform systems.     

Internal Stresses and the Martensite Start Temperature in Alumina-Zirconia Composites: Effects of Composition and Microstructure

n alumina-based composites containing ceria-stabilized tetragonal zirconia, the martensite start temperature ( M_s ) of the tetragonal-to-monoclinic zirconia phase transformation exhibits a grain size dependence that becomes increasingly pronounced as the zirconia content decreases. Neutron diffraction experiments confirm earlier dilatometry measurements of M _s in composites containing ≥20 vol% ZrO₂ and were instrumental in obtaining M _s values in lower zirconia content (i.e., 10 vol%) composites. The dependence of M _s on zirconia content is related to the internal stresses that arise from differences in thermal expansion coefficients between the two phases. Neutron diffraction measurements show that the internal tensile stresses in the zirconia grains increase with decreasing zirconia content. The measured internal stresses are in quantitative agreement with predictions based on models assuming isolated ZrO₂ particles at low zirconia contents and a continuous ZrO₂"matrix" phase at higher zirconia contents. This assumption is consistent with the observed microstructural development in which the low zirconia contents result in isolated zirconia grains, whereas higher zirconia contents result in more interconnected zirconia grains.     

Nanocrystalline Yttria-Stabilized Zirconia Fibers from Plant Fibers and their Polymer Composites

Nanocrystalline zirconia–8-mol% yttria (yttria-stabilized zirconia (YSZ): ZrO₂–8-m% Y₂O₃) fibers have been prepared from aqueous poly vinyl alcohol (PVA)–zirconium oxy nitrate solution and jute (plant fiber). Soluble Zr and Y ions in PVA solution formed a uniform coating on the surface of jute once it dried completely. Slow hydrolysis of zirconium ion with ammonium hydroxide deposited zirconium hydroxide on the jute surface. Decomposition of the dried zirconium hydroxide-coated jute at high temperature (1200°C/2 h) resulted in the formation of single-phase, nanocrystalline cubic-YSZ with the corresponding average X-ray crystallite size 30–35 nm. Heat-treated fibers have been characterized by X-ray diffraction and scanning electron microscopy. We also prepared polymer composites by incorporating chopped, ground YSZ fibers into epoxy matrix and investigated polymer/fiber interface by transmission electron microscopy analysis.     

Modeling of an improved Chemical Vapor infiltration Process for Ceramic Composites Fabrication

A quasi-steady-state approach is applied to model the pressure-driven, temperature-gradient chemical vapor infiltration (improved CVI process) for ceramic matrix composites fabrication. The deposited matrix in this study is SiC which is converted from the thermal decomposition of methyltrichlorosilane gas under excess hydrogen. A three-dimensional unit cell is adopted to simulate the spatial arrangements of reinforcements in discontinuous fiber mats and three-dimensionally woven fabrics. The objectives of this paper are to predict (1) the temperature and density distributions in a fibrous preform during processing, (2) the advancement of the solidified front, (3) the total fabrication period, and (4) the vapor inlet pressure variation for maintaining a constant flow rate. Furthermore, the effect of boundary temperature and inlet pressure variations on the total proassing period is also studied. The fabrication temperature examined in this paper is in the range between 873 and 1473 K, and the pressure is from 1.0001 to 2.0000 atm (1.0134 × 10⁵ to 2.0265 × 10⁵ Pa). Based upon this analysis, the influence of the reactor condition on the density of the final product in a CVI process can be quantified.     

Nanocrystalline Seeding Effect on the Crystallization of Two Leucite Precursors

The effect of nanocrystalline leucite seeding with leucite precursors prepared by sol–gel and hydrothermal methods on the leucite crystallization process and the microstructure of its sintered porcelain was studied. The introduced seeds lowered the crystallization temperature of leucite by 100° and 50°C for the precursor prepared by hydrothermal and sol–gel methods, respectively. The crystallization process was changed after the seeds were introduced. As the transition phase during leucite crystallization, kalsilite did not appear after the seeds were added. When the seeded hydrothermally derived precursor was treated at 650° and 700°C, part of the cubic leucite was stabilized to room temperature. This stabilization was due to the crystallization of nanocrystalline leucite on the seeds at a low temperature. The leucite synthesized by the hydrothermal method with seeding at 800°C had an average particle size of 0.4 μm that grew to about 0.6 μm in the sintered porcelain.     

Fabrication of Net-Shape Functionally Graded Composites by Electrophoretic Deposition and Sintering: Modeling and Experimentation

It is shown that electrophoretic deposition (EPD) sintering is a technological sequence that is capable of producing net-shape bulk functionally graded materials (FGM). By controlling the shape of the deposition electrode, components of complex shapes can be obtained. To enable sintering net-shape capabilities, a novel optimization algorithm and procedure for the fabrication of net-shape functionally graded composites by EPD and sintering has been developed. The initial shape of the green specimen produced by EPD is designed in such a way that the required final shape is achieved after sintering-imposed distortions. The optimization is based on a special innovative iteration procedure that is derived from the solution of the inverse sintering problem: the sintering process is modeled in the "backward movie" regime using the continuum theory of sintering incorporated into a finite-element code. The experiments verifying the modeling approach include the synthesis by EPD of Al₂O₃/ZrO₂ 3-D (FGM) structures. In order to consolidate green parts shaped by EPD, post-EPD sintering is used. The fabricated deposits are characterized by optical and scanning electron microscopy. The experimentally observed shape change of the FGM specimen obtained by EPD and sintering is compared with theoretical predictions.     

Comparison of Two Processes for Manufacturing Ceramic Matrix Composites from Organometallic Precursors

A commercial polysilazane is used as a silicon carbonitride matrix precursor for the manufacture of ceramic matrix composites using bi-directional SiC Nicalon fabrics as reinforcing material. The objective is to develop a simple and fast process leading to materials able to compete with SiC/C/SiC composites obtained by the Chemical Vapour Infiltration (CVI) route. Two processes are investigated: (1) a ‘conventional’ process using the densification of a SiC fibre preform by several cycles of impregnation of the preform with the polymer followed by pyrolysis and (2) a ‘modified’ process consisting in a powder filling of the fibre preform prior to the precursor impregnation and pyrolysis. This paper describes the different steps of both processes. The materials obtained are characterised in terms of their porosity, microstructure and mechanical properties. ©     

Microstructural and Microchemical Characterization of Silicon Carbide and Silicon Carbonitride Ceramic Fibers Produced from Polymer Precursors

Several continuous SiC and SiC/N-based ceramic fibers prepared from different polymer precursors have been characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and high-resolution electron microscopy (HREM). Methods to prepare longitudinal as well as cross-sectional thin specimens from brittle ceramic fibers were developed to facilitate HREM and EELS studies. Lattice images clearly showed nanometer-sized crystallites, as well as amorphous regions. Microchemical analysis using EELS permitted study of the form and distribution of the various chemical species within the fibers.     

环氧树脂多孔复合材料的制备及性能

以环氧树脂为基体、蓖麻油酸(RA)或二聚蓖麻油酸(DRA)改性的四乙烯五胺(TEPA)(RATEPA/DRATEPA)作为固化剂、水为致孔剂、釉粉为无机填料,通过树脂-水-填料悬浮乳液复合体系聚合法,在室温下合成了环氧树脂多孔材料,采用SEM、压汞仪、电子万能试验机、TGA对多孔材料的形貌、孔径分布、孔隙率、机械性能及热性能进行了表征和测试。结果表明:随着水相质量分数和填料粒径的增大,多孔材料的孔径和孔隙率增大,压缩强度减小;随着固化剂分子量的增大,多孔材料的孔径和孔隙率减小,压缩强度增大。当填料粒径为40μm,固化剂为RATEPA,m(水相)∶m(树脂相)=2∶1时,多孔材料的综合性能最佳,其最可几孔径为3.449μm,孔隙率为21.8%,压缩强度为26.89 MPa。TGA和DTG测试结果表明:环氧树脂多孔材料的热稳定良好,具有耐高温性能,可以在高温条件下应用。     

纳米金刚石薄膜的制备及场发射研究进展

纳米金刚石薄膜因具有纳米材料和金刚石的双重性质且易于制备,作为场发射材料在平板显示领域拥有潜在的应用价值,而引起人们的极大兴趣。本文对近年来国内外有关纳米金刚石薄膜的制备、场发射的研究现状进行了综述。     

Fabrication of Porous Nanocrystalline TiO2 Materials by SPS

A dense microcellular ceramic structure is fabricated using micron-sized Al₉₀Mn₉Ce₁ alloy powder clad with 8 wt% TiO₂ nanopowder by filling the ceramic with the Al₉₀Mn₉Ce₁ metal alloy composite and subjecting the mixture to a temperature of 520 °C. SEM imaging shows that the thickness of the ceramic cell wall is 1.0–2.0 μm; and, the inner dimension of the alloy is 15–40 μm. The ceramic filled with the metal alloy composite is subjected to deep corrosion with 10% hydrochloric acid solution; and, a porous nanocrystalline TiO₂ structure is obtained. Results show that this novel preparation procedure provides a pathway with which to produce porous nano-materials for a wide range of applications. Based on the experimental results, a transient interface sintering mechanism of the spark plasma sintering (SPS) nano/micro mixing powder is proposed.     

Preparation of C/C‐SiC Composites from All‐Cellulose Precursors

All‐cellulose composites (ACCs) are manufactured from high‐performance cellulose fibers and a cellulose‐containing ionic liquid (IL) as matrix‐forming dope via wet‐winding processes, using different concentrations of cellulose in the IL. ACCs are carbonized at 1650 °C and then infiltrated with liquid silicon. Application of a carbonization aid (ammonium dihydrogenphosphate, ADHP) substantially improves the carbon yield after carbonization but also results in the depletion of the mechanical properties of the final carbon/carbon silicon carbide (C/C‐SiC) material. The microstructure of the porous carbon/carbon preforms strongly depends on both the concentration of cellulose in the IL and the concentration of ADHP. A C/C‐SiC composite manufactured from 6 wt% cellulose in the matrix‐forming dope, in the absence of ADHP, has a maximum flexural strength of 60 MPa. New C/C‐SiC composites with different shapes including Z‐profiles and tubes are successfully manufactured from pre‐shaped ACC precursors. These composites keep their shape during carbonization and the final siliconization process step.     

纳米晶PVC在PVC／CaCO3复合材料中的作用

研究了不同粒径的纳米晶PVC的增韧、增强作用及对纳米CaCO3改性时偶联剂对材料力学性能的影响。结果表明：两种粒径的纳米晶PVC均能起到显著的增韧和增强作用，且粒径小的纳米晶PVC作用更明显。材料拉伸强度、冲击强度随偶联剂含量的增加而提高。纳米晶PVC和纳米CaCO3使复合材料达到工程材料的标准。     

Strength, Structure, and Fracture Properties of Ceramic Fibers Produced from Polymeric Precursors: I, Base-Line Studies

Classical brittle failure has been observed for ceramic fibers produced by pyrolysis from three different polymer precursors. Characterization by SEM fractography methods has shown typical flaw-mirror-mist-hackle morphology and such study has provided observations for understanding the fracture mechanisms. The gage length dependence of tensile strength combined with the fractography study showed the flaw-controlled failure mechanism of all the ceramic fibers. Systematic failure types have been identified for specific strength ranges. Complementary study of the microstructure and chemical structure by TEM, X-ray diffraction, and electron spectroscopy for chemical analysis (ESCA) has shown the nature of the bulk ceramic to be predominantly random and glassy even in the case of a fiber with microcrystalline β-SiC. Fracture mechanics calculations have shown the fibers all behave as glasses.     

电刷镀纳米镍镀层制备及其形成机理分析

在碳钢表面,使用电刷镀技术制备纳米镍镀层。采用X-射线衍射仪、扫描电子显微镜、场发射电子显微镜和原子力显微镜对镀层进行表征。分析了镍镀层形成机理和主要影响因素。使用阳离子活性剂十六烷基溴化铵来改善镀层表面形貌。随着镀液中十六烷基溴化铵添加量的增加,刷镀层由纳米尺度颗粒组成的微尺度团簇的尺度随之减少、形成(111)织构,且能制备出完全由单一纳米尺度颗粒构成的表面。