John S. Haggerty

Semisolid solidification of high temperature superconducting oxides

Experiments are reported on two techniques for melt‐texture processing Ba2YCu3O6.5 by directional solidification from a semisolid melt containing particles of BaY2CuO5 and a copper‐rich liquid. One of these employs an electric resistance furnace with ambient or oxygen enriched atmosphere; the other is a laser‐heated furnace operating at 1.3 atm oxygen. Solidification interface morphologies and other structural features were examined in quenched specimens. Depending on growth rate and temperature gradient, three different types of growth morphologies of the growing 123 phase were observed: ‘‘faceted plane front,’’ ‘‘cellular dendritic’’ or ‘‘equiaxed blocky.’’ The interface temperature decreased markedly with increasing growth rate for the faceted plane front specimens. In the remaining specimens, solidification took place over a range of temperatures. The temperature of the ‘‘root’’ of the solidification front dropped, but temperature of the solidification front ‘‘tip’’ did not. A solidification model is ...

Sinterable Powders from Laser Driven Reactions

Increasingly, because of their superior properties, ceramic materials are being considered for applications where there are high stress levels and where their failure would cause a major problem. These properties include hardness, high-temperature strength, erosion, oxidation and corrosion resistance, low density, and, for some applications, specific electrical and optical properties. The use of ceramic materials in these applications can only be considered, however, if their reliability is improved. Brittle materials fail catastrophically, and the wide distribution of observed strengths specifically associated with ceramic materials forces engineers to design so conservatively that ceramics lose their intrinsic advantages relative to conventional materials. For instance, it is impossible to design for a load stress that is less than one-tenth the mean strength and retain a superior strength to weight ratio.

Laser-induced vapour-phase syntheses of boron and titanium diboride powders

Small diameter boron and titanium diboride powders were synthesized from vapour phase reactants heated with infrared radiation from a CO2 laser. Boron powders were synthesized from BCI3 + H2 gas mixtures undfrom B2H6. TiCl4 + B2H6 gas mixtures yielded TiB2 powder. BCI3 + H2 + TiCl4 gas mixtures yielded TiCl2 powder but no TiB2. Novel equipment designed to vapourize TiCI4 liquid is described, Detailed characterizations of the product powders are presented.

Laser‐induced chemical vapor deposition of hydrogenated amorphous silicon. I. Gas‐phase process model

In the laser‐induced chemical vapor deposition (LICVD) process, a CO2 laser beam impinges on a gas mixture parallel to the substrate upon which the film is deposited. Since heating of the reactant gases is accomplished only via the absorption of infrared photons, the reaction zone can be controlled precisely. The LICVD technique is a cold‐wall thermal process allowing independent control of both the gas and substrate temperatures. In this paper, we propose a model for LICVD of silane (SiH4) and growth of hydrogenated amorphous silicon (a‐Si:H) thin films in which the film growth is controlled by gas‐phase homogeneous thermal decomposition of the SiH4. The peak gas temperature Tg depends on many process parameters, namely, gas partial pressures, laser power, substrate temperature, and cell geometry. Due to the extreme sensitivity of the growth rate G to the values of the partial pressures and laser power, these parameters must be fixed to within ±1% variation in order to control G to ±50% and prevent powde...

Highly textured and single crystal Bi2CaSr2Cu2Ox prepared by laser heated float zone crystallization

Abstract A CO 2 laser heat source was used to float zone 0.36 cm diameter ceramic rods of Bi 2 CaSr 2 Cu 2 O x . Well behaved, stable molten zones were obtained in air and in O 2 pressures up to 2.6 atm. Highly textured elongated grains ∼ 1 cm long with the c -axis normal to the rod axis were obtained at slow (0.19 cm/h) growth rates. Crystals 3×1.2×0.3 mm were easily cleaved from such specimens. In the best cases (slow growth rates), X-ray powder diffraction indicated ∼ 100% Bi 2 CaSr 2 Cu 2 O x , the superconducting transition temperature was as high as 87 K, and diamagnetism measurements indicated 60–70% superconducting phase. Meissner measurements however, in common with most Meissner measurements on Bi-containing superconductors, indicated substantially less superconducting material. Critical currents were comparable to untextured ceramic and flux grown single crystals. Float zone crystallization shows great promise for preparing single crystals and textured ceramics of incongruently melting superconductors, even when detailed phase diagrams are not known.

Defect‐property correlations in garnet crystals. V. Energy transfer in luminescent yttrium aluminum–yttrium iron garnet solid solutions

The absorption and luminescence spectra of yttrium aluminum garnet single crystals doped with iron have been studied. At low levels of iron doping, the intensity of the defect photoluminesence at 320 nm decreases with increasing iron concentration, as a result of energy transfer to Fe+3 ions which luminesce at 800 nm. Further increases in iron concentrations eliminate luminescence at all wavelengths. Differences between the absorption bands of iron in octahedral and tetrahedral sites, and inter‐ion charge transfer processes are discussed.

A Model for the Growth of Silicon Particles from Laser-Heated Gases

Laser-synthesized silicon powder is spherical and nonagglomerated when produced under proper conditions. Otherwise it consists of agglomerates of very small particles or fused aggregates of larger particles. A synthesis model has been developed that includes silane decomposition kinetics, interdiffusion of reactant and annular gases, particle growth by collision coalescence of Si droplets, and a measured temperature distribution. Predicted particle size distributions agree closely with a measured distribution. Variation in the growth time for different flow streams primarily broadens the mass distribution, whereas intermixing of the silane flow with the annular gas primarily broadens the number distribution.

Laser-induced vapour-phase synthesis of titanium dioxide

Small-diameter titanium dioxide powders were synthesized from vapour-phase reactants that were heated with 10.591μm infrared radiation from a CO2 laser. Two reactants, titanium isopropoxide and titanium butoxide, were evaluated. Anatase powder was generated from titanium isopropoxide in both static gas and flowing gas configurations. The addition of oxygen to the titanium isopropoxide gas stream reduced the percentage of volatile components in the anatase powder. Detailed characterizations of the product powders are presented.

Laser‐induced chemical vapor deposition of hydrogenated amorphous silicon. II. Film properties

Properties of hydrogenated amorphous silicon thin films prepared by the laser‐induced chemical vapor deposition (LICVD) of silane gas are described. We report the results of measurements of hydrogen concentration, infrared absorption, unpaired‐spin density, optical and mechanical properties, electrical conductivity, and photoconductivity experiments. We conclude that the film properties are controlled primarily by the substrate temperature Ts. LICVD films are superior to those produced by conventional CVD because of the permissibly low values of Ts. This results in an increased hydrogen content (up to 30 at. %) and a reduced defect density (∼1016 spins/cm3). The hydrogen concentration is determined by the surface chemistry for Ts 20 at. %) and by H2 evolution for Ts>300 °C([H]<20 at. %). The hydrogen is incorporated primarily in the SiH2 configuration and for Ts<300 °C, the films contain some polysilane (SiH2)n regions. All the physical properties of the films are discussed in conjunction with...

Doped hydrogenated amorphous silicon films by laser-induced chemical vapor deposition

We report the growth and characterization of both n‐type and p‐type doped hydrogenated amorphous silicon films prepared by laser‐induced chemical vapor deposition. For both doping types, the activation energy for electrical conduction has been reduced to below 0.2 eV and controlled doping has been achieved. Phosphine lowers the growth rate, while diborane has essentially no effect on the laser‐induced growth but enhances thermal growth. Diborane also decreases the hydrogen concentration of the films, resulting in reduced optical gaps.