P. Ravindranathan

Diamond synthesis via a low-pressure solid-state-source process

Abstract We describe herein a process which, also operating below one atmosphere pressure, transforms virtually any form of finely divided solid carbon into crystalline diamond in the reaction chamber. The key to the process is the creation of a two (or more) phase micro (or nano) composite of the carbon solid form (graphite, carbon black, charcoal, etc.) with a few percent of very fine (typically in the micrometer range) diamond (or SiC or Si or BN, etc.). The mixed powders as pastes or gels are shaped into various morphologies and reacted in a pure H 2 microwave plasma or in a tungsten filament assisted reactor at temperatures of 500°–1000°C. No carbon is introduced externally via any gaseous species. The mixtures are transformed wholly into a mass of diamond crystals often 5–10μm in size at rates of several microns an hour. The conversion is total in smaller and partial in larger shapes depending on the density of the starting material. Raman, scanning electron microscopy (SEM), and x-ray diffraction (XRD) evidence confirm the essentially complete transformation irrespective of other variables such as substrate materials, form of carbon, or second phase. The mechanism of conversion has not been studied and may well involve mass transfer over nanometer and micrometer distances via the vapor phase.

Evidence for hydrothermal growth of diamond in the C–H–O and C–H–O halogen system

Powder x-ray diffraction (XRD) and Raman evidence are presented for the formation of crystalline diamond in the “hydrothermal” pressure-temperature regime 1–5 kbars, 2 O at, say, 800 °C and 1 kbar. The other includes pyrolysis of highly selected organic solid/liquid precursors (halogenated aliphatics such as iodoform) onto similar diamond seeds. In all the cases, powder x-ray diffraction evidence shows a marked increase of the diamond XRD peaks, likewise the Raman spectrum shows a strong increase of the 1331 cm −1 line. However, the crystals apparently are too small to be seen in the SEM. TEM diffraction data, on the other hand, seem to lend support to the possibility of all the grown diamonds being very small.

Processing of Pb(Zn1/3Nb2/3)O3 ceramics at high pressures

Abstract Lead zinc niobate (PZN) with 1 mole% of BaTiO3 (BT) and pure PZN, both of pyrochlore structure were synthesized by solid state and sol-gel routes respectively. The pyrochlore ceramics were subjected to high pressures (20–35 Kbars) to convert them to perovskite phases. About 80% perovskite resulted from conventionally processed PZN with 1 mole% BaTiO3 additive at 20 Kbar, 2 hr. A maximum dielectric constant value of 11200 was obtained at 1 kHz for the PZN ceramics processed with 20 Kbar pressure.

Crystallization of diamond below 1 atm from carbon–metal mixtures

We report the result that various forms of solid carbon can be converted, essentially completely, to well crystallized diamond below atmospheric pressure if mixed with small amounts (ca. 1–10 wt.%) of metallic Mo (or Cu or Ni) and heated in a pure H2[or H2–(1%)CH4] plasma, excited typically in a 2.45 GHz microwave field at 600–1000 °C.

Dielectric properties of sol-gel derived lead magnesium niobate ceramics

Abstract Lead magnesium niobate (PMN) was prepared by a sol-gel technique. The gel calcined at 850°C for 2hr resulted in > 98% of perovskite phase. About 98% of the theoretical density was achieved for pellets sintered at 1200°C for 4hr. The room temperature dielectric constant of the pellets sintered at 1250°C for 4hr showed a maximum value of 11686 at 1 KHz. The dielectric constant increased with increasing grain size of the samples sintered at various temperatures.

Survival of diamond at 2200 °C in hydrogen

Abstract A modified strip furnace was used to heat diamond crystals of various sizes (1–10 μm) to 2200 °C in a hydrogen atmospheres (1 atm total pressure), and their thermal stability and growth were investigated. Graphite fibers coated with 1, 5 and 10 μm diamond seeds were used as the resistive element in the modified strip furnace and heated in the hydrogen atmosphere. It was found that diamond seeds of these sizes survive in a hydrogen atmosphere at temperatures as high as 2200 °C. Considerably smaller crystals (compared with the original diamond seeds) are also observed on the surface of the graphite fibers, suggesting new diamond growth under these conditins.

Preparation of metal supported montmorillonite catalyst : a new approach

A new approach for the preparation of metal supported montmorillonite catalyst is reported here for the first time. The polyol process in which ethylene glycol is used as reducing agent was employed for the preparation of uniformly distributed copper metal particles on and within the montmorillonite matrix. A hydroxy copper acetate intercalated form of montmorillonite was refluxed at 195 °C with liquid ethylene glycol for 6 h. The reduction of copper acetate to copper metal was apparent from the red color of the reduced product. An X-ray powder diffraction pattern showed the deposition of copper metal on the external surface of montmorillonite and scanning electron micrographs exhibited uniformly distributed spherical copper metal particles of about 0.5 μm size. This process may also be extended for the preparation of other fine metal particles supported on montmorillonite catalysts.

Synthesis and sintering of amorphous cordierite powders by a combustion method

[31. Recently, the synthesis of oz-Al203 and metal aluminates in a very short time using exothermic combustion processes initiated at lower temperature (500 °C) has been reported [4]. This has provided an opportunity to produce amorphous cordierite powders in a short time. This study reports the synthesis and sintering of amorphous cordierite powders by the combustion of metal nitrate-amorphous silica-urea mixtures. Stoichiometric amounts of aluminium nitrate nonahydrate, magnesium nitrate tetrahydrate, amorphous fumed silica, urea and ammonium nitrate were mixed and a homogeneous slurry was prepared by adding a small amount of deionized water and grinding the mixture in a mortar. The slurry was transferred to a Pyrex beaker/dish and heated in a furnace held at 500 °C. In a few minutes the mixture decomposed and ignited, resulting in an incandescent combustion. The products were handground in an agate mortar and used for analysis and sintering. The X-ray diffraction (XRD) patterns of asprepared powders usually showed amorphous material. In some cases, the peaks of spinel and #-cordierite could be detected by adding large amounts of urea and ammonium nitrate. Therefore, it is assumed that crystalline o~-cordierite powder may be prepared by adjusting the amount of urea and ammonium nitrate, thus increasing the heat input. XRD patterns of amorphous cordierite powders after calcination at different temperatures (1000 °C, 1100 °C, 1200 °C and 1250 °C) for 12 h are shown in Fig. 1. A schematic crystallization model for a mixture of fumed silica, Al-nitrate, Mg-nitrate and urea solution is shown in Fig. 2. After calcining at 1000 °C for 12 h, spinel phase appears on the surface of amorphous fumed silica. At 1100 °C for 12 h, #-cordierite forms from the solid state reaction between spinel and amorphous fumed silica. The amorphous fumed silica present in the precursor

Reduction of copper acetate hydroxide hydrate interlayers in montmorillonite by a polyol process. A new approach in the preparation of metal-supported catalysts

Copper metal clusters of 4–5 A have been intercalated in the interlayers of montmorillonite by in situ reduction of bulky copper acetate hydroxide species with ethylene glycol (polyol process) at 195 °C. Various amounts of CuII as copper acetate hydroxide hydrate in excess of ion-exchange capacity were introduced into the interlayers by titrating copper acetate solution with sodium hydroxide in the presence of montmorillonite. The exchange of fully developed copper acetate hydroxide hydrate interlayers in the montmorillonite gave a basal spacing of 19.6 A. The reduced samples exhibited a basal spacing of 14.4 A indicating the presence of 4.8 A copper metal clusters after subtracting the thickness of the silicate layer (9.6 A). Further evidence for formation of copper metal clusters in the interlayers was obtained from transmission electron microscopy (TEM) and UV-VIS-NIR spectroscopy. Both water sorption isotherms and nitrogen B.E.T. surface-area measurements indicated the porous nature of Cu metal intercalated phases similar to pillared clays. During the process of reduction, large amounts of copper metal particles were expelled from the interlayers which grew to ca. 0.1-0.5 µm on the external surfaces of montmorillonite.

Modified mixed oxide perovskites 0.7Sr(Al1/2B1/2) O3.0.3LaAlO3 and 0.7Sr(Al1/2B1/2) O3.0.3NdGaO3 (B=Ta5+ or Nb5+) for high-Tc superconductor substrate applications

Single-crystal fibres of modified strontium aluminium tantalum oxide (1-x)Sr(Al1/2Ta1/2) O3·xLaAlO3(SAT-LA) and (1-x)Sr(Al1/2Ta1/2)O3·xNdGaO3 (SAT·NG), and modified strontium aluminum niobium oxide (1-x)Sr(Al1/2Nb1/2)O3·xNdGaO3(SAT·NG) and (1-x)Sr (Al1/2Nb1/2)O3·xLaAlO3 (SAN·LA) were grown using a laser-heated pedestal growth technique. 0.7SAT·0.3LA grows congruently and retains a twin free simple cubic perovskite structure (as the SAT) when cooled down to room temperature. 0.7SAT·0.3LA crystals have a moderate dielectric constant (κ = 21.7) and low dielectric loss (tan δ = 7.5 × 10−5) at 10 kHz and 90 K. The reduction problem of Ta5+ is eliminated (which is common in the case of SAT growth). 0.7SAT·0.3NG and 0.7SAN·0.3NG have lower melting temperatures and crystal growth is easier. NdGaO3 addition to the SAT and SAN enhances the potential of SAT and SAN as large-area substrates for high-Tc superconductor growth. However, the dielectric constants increased from κ∼-12 to κ∼-16(0.7SAT·0.3NG) and from κ∼18 to κ∼23 (0.7SAN·0.3NG) as a result of NdGaO3 incorporation.