Study of the sintering properties of plasma synthesized ultrafine SiC powders

A study on the sintering of ultrafine SiC powders synthesized from elemental Si and CH₄ using radio frequency (r.f.) induction plasma technology is reported. The powder had a particle size in the range of 40 to 80 nm and was composed of a mixture of α and β-SiC. It was subjected to pressureless sintering in an induction furnace in the presence of different sintering aids. With the addition of B₄C (2.0 wt% B) by mechanical mixing, the powders could only be partially densified, with the highest value of 84.5% of theoretical density being achieved at 2170 °C for 30 min. Through the use of “in-flight” boron doping of the powder during the plasma synthesis step (1.65 wt % B), the ultrafine powder obtained could be densified to above 90% of its theoretical density at 2050 °C for 30 min. The addition of oxide sintering aids (7.0 wt % Al₂O₃ + 3.0 wt % Y₂O₃) by mehanical mixing produced sintered pellets of 95% of theoretical density at 2000 °C for 75 min. The Vicker’s microhardness of the sintered pellets in this case was as high as 31.2 GPa. In order to improve our understanding of the basic phenomena involved, extensive microstructural (scanning electron energy microscopy: SEM), physical (shrinkage, weight loss, porosity, hardness) as well as chemical analysis (prompt gamma neutron activation analysis (PGNAA), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA)) was carried out. This helped establish a relationship between the properties of the as-synthesized powder and their sintering properties. The influences of sintering temperature, sintering time, additive concentration, and powder purity on the densification behaviour of the plasma-synthesized powders was investigated. The results were compared with data obtained using commercial powder. This revised version was published online in November 2006 with corrections to the Cover Date.     

Sintering and microstructure of silicon nitride prepared by plasma CVD

Ultrafine Si₃N₄ powder with average particle size of 30 nm prepared by a thermal plasma CVD was sintered at 1750 °C in nitrogen for 1 h. The sintering behaviour of the powder was characterized by the crystallization of the powder and the resultant sintered bodies were observed with microscopes. It was found that the sinterability depended strongly on the green density and the degree of crystallization. If the powder was homogeneously mixed with sintering additives, it sintered to 98% density at 1750 °C in a nitrogen atmosphere. The microstructure of the sintered bodies observed by SEM indicated that they consist of needle-like grains with an aspect ratio of about 4. The microstructure of a thin film of the sintered body observed by TEM indicated that the grains with crystal habits were wet with liquid phase. TEM also clarified that two kinds of grain boundaries were present; one was wet with liquid phase along a grain boundary and the other was a coincident one without liquid phase. The lattice fringes of liquid phase suggested the presence of Y-apatite which would be generated during cooling.     

Low-temperature sintering of SiC reticulated porous ceramics with MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> additives as sintering aids

SiC reticulated porous ceramics (SiC RPCs) was fabricated with polymer replicas method by using MgO–Al₂O₃–SiO₂ additives as sintering aids at 1,000∼1,450 °C. The MgO–Al₂O₃–SiO₂ additives were from alumina, kaolin and Talc powders. By employing various experimental techniques, zeta potential, viscosity and rheological measurements, the dispersion of mixed powders (SiC, Al₂O₃, talc and kaolin) in aqueous media using silica sol as a binder was studied. The pH value of the optimum dispersion was found to be around pH 10 for the mixtures. The optimum condition of the slurry suitable for impregnating the polymeric sponge was obtained. At the same time, the influence of the sintering temperature and holding time on the properties of SiC RPCs was investigated. According to the properties of SiC RPCs, the optimal sintering temperature was chosen at 1,300 °C, which was lower than that with Al₂O₃–SiO₂ additives as sintering aids.     

Hot pressing of hafnium diboride aided by different sinter additives

Experiments were conducted by hot-pressing to densify HfB₂ commercial powders using HfSi₂ (5 vol%) or B₄C (7 vol%) as sinter additives. The former was very effective as sintering enhancer: 15 min at 1,600 °C was a sufficient condition to achieve near full density. The occurrence of a liquid phase sintering, which substantially enhanced densification during hot-pressing, was found out. The HfB₂–B₄C powder mixture, hot-pressed at 1,900 °C for 40 min, achieved a relative density of 94%. Aside the key role of sintering enhancer, B₄C also allowed the development of a uniform microstructure, preventing an excessive growth of the diboride matrix (2 μm mean size). On the contrary, thanks to the liquid phase that sustained substantial transfer of mass during heating, the HfSi₂-doped composition had an uneven and more obvious grain growth (4 μm mean size), with faceted diboride grains up to 10 μm.     

Densification behaviour of oxide-coated silicon nitride powders

Powder coating has been explored as a method of incorporating sintering additives into a ceramic powder. This procedure has been explored in the case of Si₃N₄ powders coated with thin layers of MgO.The effectiveness of the powder coating technique has been evaluated by comparing the powder properties, densification behaviour, microstructure and mechanical properties of coated Si₃N₄ powders with identical powders in which the additive oxide has been added in particulate form. It is concluded that the powder coating technique is an excellent method of homogeneously incorporating minor amounts of sintering additive into a powder. The coated powder exhibited improved homogeneity, and gave good green compact density, high green strength, and faster densification rate. Moreover, coated powders densified more easily by pressureless sintering and showed a more homogeneous microstructure, higher strength and faster densification rates, compared with materials prepared using mixed oxide powders. Significant improvements in hardness and fracture toughness were observed for the coated powders.     

Densification of ultrafine SiC powders

Recent results on the densification behaviour of ultrafine SiC powders (below 20 nm) are presented and compared with results on the densification of ultrafine silicon-based ceramic powders given in the literature. A study of different powder processing routes and their influence on the pore-size distribution is given. Pressureless sintered green bodies having pore sizes of about 20 nm show extreme coarsening without significant densification. The results indicate a significant influence of green density on shrinkage. Encapsulated hot isostatic pressing (HIPing) led to a reduction of pore size and to considerable density increase at temperatures below 1600 °C. But even then full density without extensive grain growth was difficult to achieve. The applied method to determine grain sizes (X-ray diffraction measurements, XRD, using the Scherrer formula, scanning electron microscopy, SEM, and transmission electron microscopy, TEM) gave similar results for TEM and SEM but lower values for XRD. A possible explanation is presented. Density and grain growth both during pressureless sintering and HIPing showed significant differences between samples with and without sintering additives (B and C). Whether or not the use of sintering agents is favourable in reaching high densities and fine grain sizes, is discussed. HIP densification was modelled assuming diffusion to be the dominant mechanism. Grain growth according to a t ^1/4 dependence and an activation energy of 6.8 eV was introduced into the model. Results on the properties (hardness, also at elevated temperatures, fracture toughness, bending and compression tests, thermal conductivity) of the hot isostatically pressed samples, are presented.     

超细粉的制备方法对其性能影响的TEM分析

运用透射电子显微镜对超细粉的形貌和粒径进行了研究、分析,比较了液相法和气相法民得超细粉的性能,得出了液相和气相法是制备纯度高、粒度小、粒径分布范围窄的超细粉的较佳方法。     

Microstructure and properties of alumina-SiC nanocomposites prepared from ultrafine powders

Alumina-Silicon Carbide nanocomposites were produced and studied under different aspects: characteristics of the starting materials, processing, microstructure and mechanical properties. The raw materials were two kinds of fine SiC powders (30 and 45 nm) and two Al₂O₃ powders (60 and 140 nm). Different compositions (amounts of SiC in the range 0.5–5 vol%) were performed and the characteristics of the resulting materials compared. The oxygen enrichment in SiC nanopowder due to specific powder treatments was controlled, in order to optimize powder processing routes. Densification tests of Al₂O₃-SiC powder mixtures were performed both by pressureless sintering and hot pressing route. The addition of SiC reduced the densification rate and favoured a refinement of the matrix. Improvement of mechanical properties over monolithic alumina was obtained in composites with the 45 nm SiC. The study pointed out that the critical factor for the success of these materials is the choice of the raw SiC powders in terms of grain size and state of agglomeration. The addition of this ultrafine SiC strongly affected the microstructural evolution, even at low volumetric fractions. The results do not substantiate any remarkable effect by dispersoids in the tested nanosize range.     

Fabrication of silicon carbide composites with carbon nanofiber addition and their fracture toughness

The authors have examined the fabrication conditions of SiC composites containing carbon nanofiber, i.e., vapor-grown carbon nanofiber (VGCF), to enhance the fracture toughness. Commercially available ultrafine SiC powder (specific surface area: 47.5 m² g⁻¹) was mixed with VGCF and sintering aid in the Al₄C₃–B₄C system. Approximately 1.5 g of the mixture was uniaxially pressed at 50 MPa to obtain a compact with a diameter of 20 mm and a thickness of approximately 1.5 mm. The resulting compact was hot-pressed at 1800 °C for 1 h in Ar atmosphere under a pressure of 62 MPa. The relative density of hot-pressed SiC composite decreased from 98.0 to 96.3%, whereas the fracture toughness was enhanced from 3.8 to 5.2 MPa m^1/2, as the amount of VGCF increased from 0 to 6 mass%. Furthermore, an acid treatment of VGCF was conducted to enhance its dispersibility within the SiC matrix, owing to the formation of COO⁻ groups on the VGCF surface. As a result of this treatment, the relative density and fracture toughness of hot-pressed SiC composite with 6 mass% acid-treated VGCF addition increased to 99.0% and 5.7 MPa m^1/2, respectively.     

碳化硅超细粉的制备新法

用一种新的方法———双重加热法制备了直径在 5 0～ 6 0nm范围内的SiC超细粉 ,用化学分析方法、X射线衍射、透射电子显微镜等手段对SiC超细粉进行了表征。研究结果表明 ,用双重加热法制备SiC超细粉的最佳温度为135 0℃ ,恒温时间为 6 0min ,SiC超细粉的产率可达 98% (质量分数 )左右     

超细镍粉的制备及还原生长机理研究

以联氨为还原剂,在硫酸镍水溶液中控制液相还原反应条件制备了超细镍粉,并讨论了超细镍粉的还原生长机理.通过实验分析了工艺参数对还原反应的影响,采用X射线衍射(XRD),扫描电镜(SEM),比表面积测定等分析手段对超细镍粉进行表征,结果表明:超细镍粉的形核和生长独立进行;温度、Ni2+浓度和pH值调控着溶液中镍离子的释放并...     

Effects of copper addition on the sintering behavior and mechanical properties of powder processed Al/SiC<Subscript><Emphasis Type="Bold">p</Emphasis></Subscript> composites

The effect of copper addition on powder processed Al-10 vol% SiC composites was studied in regards to their sintering responses. Copper was mixed with aluminum powder either as elemental powders or as the coated layer on SiC particles. After sintering at 600°C for 1 h, Al-SiC composites with no copper addition showed little densification. It also demonstrated very low bend strengths of 49 and 60 MPa, indicating poor bonding between the powders in the sintered composite. The addition of 8% copper to the Al/SiC system effectively improved the sintering response, producing over 95% theoretical density, a bend strength of 231 MPa with the copper coated SiC, and a 90% density with over 200 MPa bend strength with the admixed copper.The as-sintered microstructures of the Al–SiC composites clearly revealed particle boundaries and sharp pores, indicating that only a limited neck growth occurred during sintering. In the case of Al–Cu–SiC composites, however, a liquid phase was formed and spread through particle boundaries filling the interfaces or voids between SiC particles and the matrix powders. The coated copper on SiC particles produced a somewhat better filling of the interface or voids, resulting in a little more densification and better sintered strength. Since the solubility of copper in aluminum is less than 2% at the sintering temperature, the alloying of copper in the aluminum matrix was limited. Most of the copper added was dissolved in the liquid phase during the sintering and precipitated as CuAl₂ phase upon cooling.     

The synthesis of ultrafine SiC powder by the microwave heating technique

Ultrafine powders of silicon carbide synthesized by microwave and conventional heating are described. Silicon carbide powders with diameters in the nanometer range were formed by reducing SiO₂ with various forms of carbon in a nitrogen atmosphere. Ultrafine SiO₂ powder, phenolformaldehyde resin and ultrafine carbon black were used as starting materials. The properties of the powders were determined by means of X-ray diffraction, TEM, etc. The results showed that the technique and conditions for preparing samples, as well as the temperature and type of heating, had obvious effects on the powders characteristics. This revised version was published online in November 2006 with corrections to the Cover Date.     

Sinterability of commercial 8 mol% yttria-stabilized zirconia powders and the effect of sintered density on the ionic conductivity

The sintering behaviour of a number of commercially produced 8 mol% yttria-stabilized zirconia powders has been studied. The effect of different sintering regimes on the density and microstructure of the sintered ceramic was determined using density measurements, scanning electron microscopy (SEM) and dilatometry. The chemical homogeneity, particle size and the morphology of the as-received powder were related to the sintering behaviour of the different commercial powders. Powders prepared via a route which involved a spray-drying step sintered more readily than those prepared without a spray-drying step. Plasma-derived powders did not sinter to as high an apparent density as co-precipitated powders. The effect of sample density on the ionic conductivity of sintered YSZ ceramics was studied using a.c. impedance spectroscopy. This technique allowed separation of the bulk and grain-boundary components, enabling clear intepretation of the effects of sample porosity of the conduction pathways. Ceramics prepared from the three different powders achieved a bulk ionic conductivity of 16 S cm-1 at 1000 °C for sintered densities of 95% or greater. The results obtained are compared to values reported for a variety of other commercial powders. © 1998 Kluwer Academic Publishers     

The sinterability of ultrafine WC powders obtained by a CVD method

The sintering behaviour of ultrafine WC powders produced by a CVD method (particle size <0.3m) and commercial WC powders (particle size ~ 1m) is investigated in hydrogen and in vacuum. It has been found that ultrafine WC powders have an extremely high sinterability and give a sintered body with a relative density of 100% by sintering at a considerably lower temperature than normal, such as 1750° C. Also WC powders with a large particle size and a wide-size distribution have a high sinterability caused by the presence of fine particles and the sinterability of WC powders is influenced significantly by the sintering atmosphere. The atmospheric effect is discussed in connection with the heating behaviour of a surface oxide layer and free carbon.     

Sintering ofβ-sialon with 5mol% Y2O3-ZrO2 additives

The sintering behaviour ofβ-Sialon composition powders with 5 mol% Y₂O₃-ZrO₂ additives at 1750°C for 1.5 h in nitrogen or argon atmospheres was studied.β-Sialon composition powders could be pressureless-sintered to about 93% theoretical density by the addition of 5 wt% 5 mol% Y₂O₃-ZrO₂. By HIPing the pressureless-sintered bodies the density was increased to higher than 98% theoretical density, and uniform submicrometre ZrO₂ particles were homogeneously dispersed in theβ-Sialon matrix, resulting in an increase of fracture toughness,K _1c, from 5.1 to about 5.7 MN m^−1.5. Increasing the amount of tetragonal ZrO₂ transformable to monoclinic phase in theβ-Sialon matrix increasedK _1c.     

SiC—WC复合陶瓷材料的研究

本文对亚微细SiC—WC复合陶瓷粉末的制备及热压烧结性能进行了研究。在探讨烧结体密度、强度随热压条件变化的基础上得出了适合于SiC—WC复合陶瓷的热压工艺条件。在此条件下烧结体的相对密度达99％以上,弯曲强度达1019MN/m~2。研究表明,在SiC中加入5～25Vol.％的WC,能改善材料的烧结性能,加快致密化速率,并能提高烧结体的强度与韧性。     

Production of submicron SiC particles by d.c. thermal plasma: a systematic approach based on injection parameters

Aiming at producing high temperature structural ceramics, ultra-fine SiC powders were synthesized by the gas phase reaction of silicon tetrachloride with methane in a d.c. thermal plasma. The influence of parameters as the SiCl₄ feeding rate, C/Si and H₂/C molar ratios and internal pressure on the powder properties were investigated. The SiC powders were characterized by chemical analysis, Fourier transform infrared spectroscopy, X-ray diffraction and scanning electronic microscopy. The experimental set-up allows the production of β-SiC powders at a rate of 200 g h⁻¹ with particle size around 0.1 μm. The main impurities in the as-produced powder handled at ambient atmosphere are: oxygen (1.8–2.5%) and free carbon (3–4%). Interesting relationships were found between the SiCl₄ feeding rate and the H₂/C molar ratio and between the C/Si molar ratio and the internal pressure. The internal pressure plays a major role in controlling the particle size.     

Sintering behaviour of monodispersed ZnS powders

The sintering behaviour of monodispersed ZnS powders having particle size of 0.1–0.9 μm, which were prepared from aqueous zinc nitrate solution by a homogeneous precipitation method using thioacetamide, was studied. Dry-pressed pellets of ZnS powders were fired at 900–1250 °C for 2 h in a nitrogen atmosphere. Monodispersed ZnS powders showed high sinterability in comparison with agglomerated or aggregated ZnS powders. The sinterability increased with decrease in particle size, and ZnS powders with particle size of 0.1 μm were densified to above 98% theoretical density by conventional sintering at 1000 °C for 2 h. Ultrafine monodispersed, spherical particles gave a uniform and fine-grained microstructure. This revised version was published online in November 2006 with corrections to the Cover Date.     

Preparation of Fe3AI based iron aluminides from elemental and prealloyed povvders

Abstract

Iron aluminides were prepared by a powder metallurgy process from elemental powders, mixtures of prealloyed and elemental powders, and prealloyed powder. The sintering behaviour of various powders was studied using scanning electron microscopy, optical microscopy, and density measurement. It was found that sintering of elemental powder involved two distinct processes, i.e. alloying and densification, but sintering of prealloyed powder involved densification alone. The addition of prealloyed powder to elemental powders was helpful in restraining the swelling of sintered samples, the degree of swelling of sintered samples being reduced as the amount of prealloyed powder increased. For samples made from Fe-25 at.-%Al prealloyed powder, remarkable shrinkage was measured after sintering at 1250°C for 1 h. Within the correct range, their density increased with sintering temperature and time, but prolonged sintering at high temperature resulted in the loss of aluminium and a two phase microstructure. The difference in sintering behaviour between the various powders was discussed on the basis of thermodynamics.