Microwave plasma enhanced chemical vapor deposition diamond synthesis from high carbon content source

To synthesize diamond films by microwave plasma enhanced chemical vapor deposition (MPECVD), the methane concentration (CH₄/H₂)plays a crucial role. It is well-known that there always exists a critical methane concentration (≤0.6%) only below which a good quality diamond film can be obtained. In this study, however, the phenomena of diamond synthesis resulting from high carbon concentration conditions were observed. The molten metals, e.g., Ag, Cu, were used as the deposition substrates on which crystalline diamonds can be achieved from a methane content of CH₄/H₂≥6% or even from solid carbon sources. These results suggest that there may exist a low methane content boundary layer (<0.6%) in the proximity of molten metal surface on which suitable species, CH, CH⁺, H_α and H_β are composed for the diamond nucleation and growth similar to the condition as in the conventional low methane contents. The molten metal inclines to dissolve other forms of carbonaceous materials other than diamond, and thus keeps a much higher steady supply of carbon atoms that enhances the quality as well as the growth rate of the forming diamonds. Received: 23 June 2001 / Accepted: 23 July 2001     

In-situ hot pressing/solid-liquid reaction synthesis of dense titanium silicon carbide bulk ceramics

 An in-situ hot pressing/solid-liquid reaction process was developed for the synthesis of dense polycrystalline Ti₃SiC₂ ceramics using Ti, Si, and graphite powders as starting materials. The present work demonstrated that this process was one of the most effective and simple methods for the preparation of dense bulk Ti₃SiC₂ materials. Lattice constants of a=3.068 and c=17.645 are calculated for Ti₃SiC₂ made through this process. The synthesis temperature influenced the phase composition, microstructure and mechanical properties of Ti₃SiC₂ prepared at different temperatures. And bulk materials with flexural strength of 480 MPa and fracture toughness of 7.88 MPa.m^1/2 were obtained at 1600°C. The high fracture toughness and strength are discussed based on microstructure analysis. Received: 31 July 1998 / Accepted: 28 August 1998     

New rapid diamond synthesis technique; using multiplexed pulsed lasers in laboratory ambients

 QQC, Inc. has developed a revolutionary diamond deposition technique which does not require a vacuum, or H₂, or ”chemical” or physical pre-treatment of surfaces. The process utilizes a combination of various pulsed lasers and is performed at the laboratory p−t conditions: with CO₂ and N₂ as shrouding gases. Crystalline diamond films and DLC or, more accurately, tetrahedral non-crystalline carbon (TNC) have been formed on various substrates. On WC cutting tools the rate of deposition of crystalline diamond is approximately 1 μm/sec. In the context of fabricating a diamond coating on a substrate, the entire pre-surface treatment and material synthesis process including a coating regime, is performed in one step. Received: 10 September 1997/Accepted: 25 September 1997     

A novel method for the preparation of sulfides and selenides and its application for phosphor synthesis

 A new method for the synthesis of binary and tertiary sulphides and selenides from a solution of sulphur in hydrazine hydrate is described. The preparation of zinc sulphide such as SrGa₂S₄ is described in detail. Precipitates formed by this reaction are generally amorphous, but yield crystalline powders on further heating. Initial work has focussed on the preparation of sulphide phosphors such as ZnS:Ag. The new method has yielded phosphors with very promising characteristics, in some cases better than those made by conventional techniques (in relation to their luminescent properties). The new method does not require the elaborate precautions needed to remove the toxic gasses generated by the presently used industrial process (H₂S and CS₂). The new general synthetic method promises a range of possibilities for the preparation of materials with enhanced properties. Received: 22 July 1998 / Accepted: 4 September 1998     

Textural stability as a characterization tool of high temperature catalysts

 Factors, both physical and chemical, influencing the pore-structure stability of high temperature catalysts and/or catalyst supports were discussed with a view to formulate strategies to design and develop high temperature catalyst supports. Factors influencing the sintering behavior and techniques to manipulate them are discussed. Chemical ways of modifying porosity reduction behavior can lead to changes in catalytic behavior. Therefore several physical techniques like control of packing and coordination by post precipitation treatment and nanocomposite formation, changing particle morphology and introducing rigid constraints were discussed. Received: 20 June 1998 / Accepted: 30 July 1998     

Fluctuation synthesis and characterization of Ti3SiC2 powders

 A novel fluctuation method for the synthesis of Ti₃SiC₂ powders was developed. The raw materials used in this process are Ti, Si, and graphite powders. Fluctuation synthesis utilized Si as in-situ liquid forming phase (additive), which was formed by heating the powder mixtures to 1300°C and using the heat released from the exothermic reaction for Ti₃SiC₂ formation. The result demonstrated that the reaction time for the formation of Ti₃SiC₂ was dramatically shortened using fluctuation method and the powders produced using this method contained more than twice amount of Ti₃SiC₂ compared to the solid reaction synthesized powders. The powders prepared by fluctuation method are fiber-like in morphology with dimensions of 0.8–2 μm in width and 5–10 μm in length. The growth direction of the fiber-like Ti₃SiC₂ particulate is {101^–1}*. The lattice parameters for Ti₃SiC₂ were determined by a trial-and-error method and are a=3.067 ? and c=17.645 ?. Received: 28 September 1998 / Reviewed and accepted: 1 October 1998     

Non-hydrothermal synthesis of copper-, zinc- and copper-zinc hydrosilicates

Cu/SiO₂, Zn/SiO₂ and Cu-Zn/SiO₂ samples have been prepared by the homogeneous deposition-precipitation method. The samples were analyzed by thermal analysis, X-ray diffraction and infrared spectroscopy after various heat treatments and compared with data obtained for several minerals. It has been shown that interaction between the components occurs through formation of hydrosilicates. Copper-silica system at a Cu:Si ratio ≤ 1, gives rise to a hydrosilicate stable up to a calcination temperature of 930 K resembling the mineral Chrisocolla; at higher ratios a hydroxonitrate (gerhardite type) is also formed. Zinc-silica interaction produces two hydrosilicates such as a well crystallized Hemimorphite at Zn:Si = 2 and highly dispersed Zincsilite at Zn:Si ≤ 0.75, both stable up to 1073 K. The Zincsilite structure consists of three layered sheets (an octahedral layer sandwiched by two tetrahedral ones) like the Stevensite mineral group. For the copper-zinc-silica system no copper hydrosilicate is formed. Copper merely enters the Zincsilite structure independenly of the applied (Cu + Zn):Si ratio. Resulting layered copper-zinc hydrosilicate may be described by formula Zn_x-yCu_y(Zn_3-x–zCu_z–y▪_x)[Si₄O₁₀](OH)₂.nH₂O, where Zn_3-x-zCu_z-y– ions are located in octahedral sites, Zn_x-yCu_y–ions in the interlayer; ▪_x are vacancies in the layers. Copper and zinc in excess of the Zincsilite ratio of Me:Si = 0.75, gives rise to copper and copper-zinc hydroxonitrates. Received: 7 November 2000 / Reviewed: 23 January 20001 / Accepted: 23 January 2001     

A new synthesis reaction of Ti3SiC2 from Ti/TiSi2/TiC powder mixtures through pulse discharge sintering (PDS) technique

Ti/TiSi₂/TiC powder mixtures with molar ratios of 1:1:4 (M1) and 1:1:3 (M2) were first employed for the synthesis of Ti₃SiC₂ through pulse discharge sintering (PDS) technique in a temperature range of 1100–1325 °C. It was found that Ti₃SiC₂ phase began to form at the temperature above 1200 °C and its purity did not show obvious dependence on the sintering temperature at 1225–1325 °C. The TiC contents in M2 samples is always lower than that of the M1 samples, and the lowest TiC contents in the M1 and M2 samples were calculated to be about 7 wt% and 5 wt% when the sintering was conducted at the temperature near 1300 °C for 15 minutes. The relative density of the M1 samples is always higher than 99% at sintering temperature above 1225 °C, indicating a good densification effect produced by the PDS technique. A solid-liquid reaction mechanism between Ti-Si liquid phase and TiC particles was proposed to explain the rapid formation of Ti₃SiC₂. Furthermore, it is suggested that Ti/TiSi₂/TiC powder can be regarded as a new mixture to fabricate ternary carbide Ti₃SiC₂. Received: 5 September 2001 / Accepted: 11 September 2001