Synthesis and characterization of polycrystalline sintered compacts of cubic boron nitride

Synthetic cubic boron nitride (CBN) compacts are an important tool material used extensively for the machining of hardened steels. The paper describes work on the synthesis and characterization of CBN compacts. A 200 tonne cubic press has been used for the generation of high pressures. Solid solutions of TiN and TiC have been used as the binder material. The CBN powder and the binder are homogenously mixed and the mixed powder is pressed in a steel die under a pressure of about 3 kbars (300 M Pa). The pellets so formed are the starting material for synthesizing the compacts. Compacts both with and without a tungsten carbide substrate have been synthesized. The best compacts are formed at 58kbar (5.8 G Pa) and 1450° C. The sintered compacts after grinding and polishing are characterized by using powder X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray analysis. Knoop hardness measurements made on the best CBN compacts give a hardness of 3400HK _0.5.     

Structure and mechanical properties of Al–B composite powder

Al-B composite powder has been obtained by crushing pieces of composite material presenting industrial waste. Structural peculiarities and microhardness of separate powder particles (d1 mm) have been investigated. Original design of high precision microhardness tester made it possible to detect the properties of powder both in near-surface layer and below it. The powder represents a new structurally non-homogenous material with the increased microhardness (1.5 GPa) which grows up to 4 GPa in near-surface layers. Stable oxide compounds are formed on internal surfaces and defects of the aluminium alloy. Powder compacts were obtained. Adhesion on Al–B and Al–Al interfaces at various temperatures and pressures were investigated. The applications of the powder compacts were considered.     

离心沉积成膜法在AFM表征纳米粉体中的应用

研究了原子力显微镜在表征纳米粉体微观形貌中的应用,介绍了离心沉积成膜制备纳米粉末待测试样的方法。结果表明,与普通的沉淀成膜法制样相比,离心沉积成膜法以云母片为基底材料,在分散液中加入了凝聚剂,AFM表征结果表明,离心沉积成膜法使纳米粉体的微观形貌在测试中能更好地反映。该方法拓宽了原子力显微镜在表征粉末状样品微观形貌上的应用。     

The role of hydrogen in the nitridation of silicon powder compacts

High-purity silicon powder compacts have been nitrided in nitrogen atmospheres containing varying partial pressures of hydrogen. The accelerated nitridation rates observed are interpreted in terms of the interaction of the hydrogen with the natural oxide film on the surface of the silicon particles. A model is presented for reactions taking place during the nitridation of these compacts in the presence of furnace atmospheres contaminated by low partial pressures of water vapour.     

Effect of compact density on phase transition kinetics from anatase phase to rutile phase during sintering of ultrafine titania powder compacts

The effect of compact density on the phase transition from anatase phase to rutile phase during sintering of ultrafine TiO₂ powder compacts has been investigated. The compact densities are varied using different compaction pressures. The shrinkage behaviors of compacts are monitored in situ at the condition of uniform heating rate by dilatometric measurement and subsequently related to the phase transition kinetics in the respective compact densities. A phase transition behavior of the compacts (59.3% of theoretical density (TD)) made with 65 MPa pressure is compared either with that of the compacts (86.9% TD) made with 4.5 GPa pressure or with that of loose powder pack without compaction. The phase transition kinetics is proved to be affected strongly by the density of green compacts. The compact with high green density exhibits lower transition temperature than those with low green density. The occurrence of anatase → rutile phase transition at the lower temperature in the high density compacts is attributed to the relatively large coordination number compared with the low density compacts.     

Sinter and sinter-HIP of silicon nitride ceramics with yttria and alumina additions

The sintering behaviour of silicon nitride ceramics has been investigated by preparing powder mixtures containing controlled amounts of yttria and alumina. The fired density of the compacts, having a fixed quantity of yttria, has been observed to increase with increasing amounts of alumina added to the powder. There exists, however, a maximum limit to the beneficial effect of Al₂O₃, because an excess of alumina in the powder mixture does not produce a further increase in density. Sintering of green compacts, obtained by uniaxial pressing, has been carried out at various temperatures and pressures in order to determine the relationship between fired density and the processing variables. The results obtained, after subjecting a selection of specimens to different sinter-HIP cycles, clearly show that the final density is very sensitive to the applied pressure and the time at which it is applied. The range of microstructures produced by different processing routes has also been followed by electron microscopy. From the results obtained by X-ray diffractometry it is shown that the additions of yttria and alumina not only produce denser specimens than pure silicon nitride, but also promote the α-β transformation.     

Hot-press sintering of Ge-Si alloys

Germanium-silicon alloys of the composition Ge₃₀Si₇₀ have been prepared by hot pressing powder compacts at temperatures between 1300 and 1493 K (the solidus temperature) at pressures up to 40 MPa. The densification of the compacts was observed as a function of time. For each temperature and pressure an end-point density less than the full density was found. The time-dependence of the densification and the pressuredependence of the end-point density can be described empirically in terms of a plastic flow mechanism.     

溅射气压对Ge/Si纳米点表面形貌的影响

利用磁控溅射技术在Si(100)衬底上直接外延生长一系列不同压强下的Ge纳米点样品,并利用AFM、Raman和XRF对Ge纳米点样品形貌和结构进行了研究。结果表明Ge薄膜表面粗糙度在某一临界压强下发生突变,高能粒子热化的临界值与这种转变密切相联;分析讨论了Ge岛在不同溅射气压下的生长过程,在一定范围随着压强的增大会显示典型生长阶段的特征。     

Atomic force microscope in a contactless mode: Peculiarities of force interactions

Forces of interaction between the atomic force microscope (AFM) probe and the surface of a solid are calculated with an allowance for the induced cantilever oscillations. A continuous approximation used in this work does not take into account discreteness of the sample and probe structures. Calculations have been performed for various AFM point shapes. It is theoretically demonstrated that the cantilever oscillations increase the interaction force.     

The break point in the pressure-density curve of magnesia prepared by the sea water magnesia process

Uniaxial compacting behaviour of MgO powders calcined at a temperature between 900 and 1200°C in air was investigated; the break point which appeared in the pressure-density curve of MgO powder was effective in elucidating the agglomeration state in the powder. The packing density of MgO powder as a function of calcination temperature was measured before and after milling operations. The plots of relative density versus logarithmic pressure also exhibited a break point indicating the pressure at which the contact points in porous agglomerates began to be destroyed. The agglomerate strength of MgO powders calcined at low and high temperatures (900 and 1200°C) only were measured. The microstructural differences between agglomerates in MgO powder and the surface of a compressed powder (applied pressure 150 MPa) were examined by scanning electron microscopy.     

Aluminothermic Reduction of Silica for the Synthesis of Alumina-Aluminum-Silicon Composite

Under the powder metallurgy route, the aluminothermic reduction of silica was studied for the preparation of alumina particle-reinforced aluminum-silicon composite. Experimental results show that the reduction process, the solidification, and the final structure of the in situ composite are greatly impacted by the heating rate of the green compacts. DSC curves of the studied Al-SiO₂ system exhibit a different in situ process at varied scan rates in which the slowest scan rate gives more accurate and detailed information on the reaction process during heating and on cooling. The solidification process of the in situ composite was also dominated by the sweeping speed, which turned the nominal composition of bulk melt from hypereutectic into hypoeutectic on cooling. X-ray and EDX analysis evidenced that the reduction of silicon from silica by aluminum takes place below the melting of aluminum. The temperature of the exothermic reaction has reached beyond the melting point of pure silicon in the area covered by original silica powders. The above results are also discussed in the paper.     

不同放电能量下铜粉的电磁压实特性与烧结体性能研究

目的 研究放电能量对铜粉电磁压实特性与烧结体性能的影响,提升铜材料的制备效率与质量.方法 通过电磁粉末压实技术在不同放电能量下制备铜压坯,研究压坯的相对致密度与微观形貌随放电能量的变化趋势.随后对其进行真空烧结,对烧结体的拉伸性能与断口形貌进行测试,评估放电能量对烧结体性能的影响.结果 随着放电能量的增加,铜压坯的相对...     

Mechanochemistry: targeted delivery of single molecules

The use of scanning probe microscopy-based techniques to manipulate single molecules and deliver them in a precisely controlled manner to a specific target represents a significant nanotechnological challenge. The ultimate physical limit in the design and fabrication of organic surfaces can be reached using this approach. Here we show that the atomic force microscope (AFM), which has been used extensively to investigate the stretching of individual molecules, can deliver and immobilize single molecules, one at a time, on a surface. Reactive polymer molecules, attached at one end to an AFM tip, are brought into contact with a modified silicon substrate to which they become linked by a chemical reaction. When the AFM tip is pulled away from the surface, the resulting mechanical force causes the weakest bond - the one between the tip and polymer - to break. This process transfers the polymer molecule to the substrate where it can be modified by further chemical reactions.     

Carbon nanotube/silica composites obtained by sol-gel and high-pressure techniques

The successful incorporation of functionalized single-walled carbon nanotubes (f-SWCNTs) into a silica matrix prepared by the sol-gel method is reported herein. SWCNTs produced through catalytic chemical-vapor deposition (CCVD) have been purified and functionalized with sulfuric, nitric and hydrochloric acids to ensure a good dispersion in an aqueous solution. The nanotube composites are prepared using three concentrations of f-SWCNTs (0.025, 0.050 and 0.075?wt%.) in a silica matrix, resulting in translucent monoliths after gelation. Dense, crack-free and hard compacts are obtained by high-pressure processing at 7.7?GPa and room temperature. Compared to the pure silica compact, compacts containing 0.025 and 0.050?wt% f-SWCNTs show an increased toughness of about 54% and 69%, respectively. The influence of f-SWCNTs on some microstructural aspects of the silica matrix has been studied using nitrogen adsorption/desorption isotherms. Raman spectroscopy has been applied to analyze the effect of the silica matrix and high-pressure compaction on the f-SWCNTs incorporated into the silica matrix. These measurements showed that f-SWCNTs remained in the silica matrix under pressure, suggesting an important interaction with the matrix.     

Evaluation of Defatted Soybean Flakes as a Tablet Excipient Part I. As a Disintegrant

Abstract

Oriental people have been using soybeans as a protein foodstuff for centuries. At present soybeans have become a major source of edible oil, and the meal provides an important source of protein for animal feeds. In the present study, the dehulled, and defatted soybean flakes were investigated as a possible tablet excipient.

Four different samples (sieve fraction 150-180 um), namely samples A, B, C, and D were prepared from dehulled, defatted soybean flakes and their physical characteristics were determined subsequently. Compacts of these four substances and their binary mixtures with dicalcium phosphate dihydrate in four different ratios were also prepared at seven different compression pressures.

The changes in density of the compacts under compression were 1 2 interpreted using the Heckel plots ^{1, 2} The crushing strength and disintegration time of the subsequent compacts were also determined. Great differences in the disintegrating properties between the four soybean samples were noticed.

Samples A, B, C, and D, corn starch, microcrystalline cellulose and starch 1500 were added to dicalcium phosphate dihydrate either as intragranular, extragranular or both intra- and extragranular disintegrants respectively; the compacts of these substances were prepared at two compression pressures and the disintegration time of the compacts determined. In general the disintegrating efficiencies are in the rank order corn starch>starch 1500>sample Osample D>sample B>sample A>microcrystalline cellulose.     

Measurement of Surface Topography,Packing Density,and Surface Charge Distribution of an Electrostatically Deposited Powder Layer by Image Analysis of Fluorescent Latex Spheres

A novel method of nonintrusive measurement of surface profile, packing density, and surface charge distributions of a powder layer deposited on a substrate is reported. The method employs the deposition of electrostatically charged monodispersed fluorescent latex spheres (FLS), approximately 2 m in diameter, on the surface of: (1) the substrate before deposition, (2) the powder layer after deposition, and (3) the film formed by curing the powder layer. The surface topography in all cases was mapped using an epi-fluorescent microscope with a vertical resolution of - 2 m in the z axis and - 10 m in the x and y axes. An area of 1 cm 2 1 cm is scanned in 1 mm segments, providing approximately 100 data points per cm 2 for the surface topography. For each measurement of surface topography, the substrate was positioned on the microscope stage in a manner such that the reference points (x, y, and z) remained the same for all measurements of the substrate. The surface profiles, with respect to the same reference points, were plotted using Origin 6.0 software for 3D presentation of the topography. The method was also applied to map the surface charge density distribution of electrostatically charged surfaces. The FLS imaging method provides a new tool for examination of surface profiles, packing density, and charge distribution of powder layers on a microscopic scale not provided by optical or atomic force/electrostatic force microscopy (AFM/EFM). While AFM and EFM are very effective in providing similar information with nanometer resolution, they cannot be directly applied on a larger macroscopic scale to study powder layers and for a larger surface area (up to 1 cm 2 or greater) involving deposited particles in the range of 1-50 m in diameter. For AFM, the range in the z-axis is limited to - 3 m and the x-y scan area is limited to 100 m 2 100 m. The FLS method has a much wider range but it is operated manually; an automated scanning process is required for rapid measurement. A comparison of the FLS and EFM techniques as they apply to analyzing charge distribution on coal surfaces is presented.     

Comparative study on the densification process of different titanium powders

In the powder metallurgy of titanium and titanium alloys, titanium powders produced through hydrogenation/dehydrogenation (HDH) approach and titanium hydride powder are extensively used. The choice of initial powder greatly influences the properties and performance of as-sintered materials. In the present study, comparative experiments were performed on two powders of various sizes to elucidate the peculiarities of their densification process and the characteristics (as-sintered density, impurity content, and tensile properties) of the processed materials. As expected, the sintering performance of both powder-type compacts were greatly affected by the specific surface and contact areas, so finer powders and higher compaction pressures were used to achieve higher densities upon sintering. However, the systematic results clearly indicated the advantage of using titanium hydride powder as a starting material in titanium powder metallurgy. At equal size, compaction, and sintering parameters, materials processed from titanium hydride powder had higher density and lower impurity content, thereby providing better balance of tensile properties compared with materials processed from HDH titanium powder. This advantage is explained by the higher relative density of green compacts made of brittle titanium hydride powder and by the higher sintering ability of such compacts activated by powder-released hydrogen.     

Fabrication of discrete nanoscaled force sensors based on single-walled carbon nanotubes

We present a fabrication technique for discrete, released carbon-nanotube-based nanomechanical force sensors. The fabrication technique uses prepatterned coordinate markers to align the device design to predeposited single-walled carbon nanotubes (SWNTs): Atomic force microscope (AFM) images are recorded to determine spatial orientation and location of each discrete nanotube to be integrated in a nanoscaled force sensor. Electron beam lithography is subsequently used to pattern the metallic electrodes for the nanoscale structures. Diluted hydrofluoric acid etching followed by critical point drying completes the nanosized device fabrication. We use discrete, highly purified, and chemically stable carbon nanotubes as active elements. We show AFM and scanning electron microscope images of the successfully realized SWNTs embedded nanoelectromechanical systems (NEMS). Finally, we present electromechanical measurements of the suspended SWNT NEMS structures.     

Influence of ceramic powders of different characteristics on particle packing structure and sintering behaviour

Ytteria-stabilized zirconia powders from different sources were used to investigate powder packing characteristics and subsequent sintering behaviour. Powders of different levels of agglomeration were controlled by varying degrees of ball-milling, followed by shaping through colloidal casting. Experimental findings revealed that the average pore diameter of the powder compacts appears to be a good representative of the whole pore structure of the green compacts. This pore parameter exhibited a well-defined correlation with respect to the initial mean powder size for each type of powder, irrespective of the difference in particle size distribution and degree of agglomeration, and also correlated soundly with the sintering behaviour of the powder compacts. Critical pore diameter/particle size ratios were experimentally determined, which were shown to be strongly related to the initial particle size by a power-law dependence. This dependence offers an excellent prediction of the critical ratio in alumina powder compacts reported by Zheng and Reed [27], which further supports the feasibility of this relationship. This revised version was published online in September 2006 with corrections to the Cover Date.     

Compacting characteristics of aluminium-fly ash powder mixtures

Aluminium-fly ash powder mixtures containing 5–20 wt% fly ash were prepared using both precipitator (solid) and cenosphere (hollow) fly ash particles. These were compacted at pressures ranging from 138–414 MPa. Spring back, densification parameter, green density, green hardness and green strength of the compacts were determined as a function of compacting pressure and weight per cent of fly ash. It was observed that the spring back, green density and green strength of the compacts increased with increasing compacting pressure. Green density, green strength and green hardness of the compacts decreased, and the spring back increased, with increasing weight per cent of fly ash particles, the cenosphere particles leading to a higher spring back as compared to solid precipitator particles. At compacting pressures of the order of 345 MPa, the solid fly ash particles generally remained intact, and well distributed in the matrix, but the hollow cenosphere particles collapsed, and outer shells were present in the matrix in layered forms.