Thomas R. Anthony

Heat treating and melting material with a scanning laser or electron beam

A thermal analysis for laser heating and melting materials is derived for a Gaussian source moving at a constant velocity. The resulting temperature distribution, cooling rate distribution, and depth of melting are related to the laser spot size, velocity, and power level. As the power is increased to heat the liquid above the boiling point, a transition to deep penetration welding is described. Calculations are presented for 304‐stainless steel which are in agreement with experiment.

Surface rippling induced by surface‐tension gradients during laser surface melting and alloying

During laser surface melting and alloying, temperature gradients on the melt surface between the laser‐beam impact point and the intersection line of the solid‐liquid interface with the surface generate surface‐tension gradients that sweep liquid away from beam impact point. The resulting flow of liquid creates a depression of the liquid surface beneath the beam and ridging of the liquid surface elsewhere. As the beam passes to other areas of the surface, this distortion of the liquid surface is frozen in, creating a roughened rippled surface. If the laser‐beam sweep velocity exceeds a critical velocity, the liquid does not have sufficient time to form ripples, and rippling from surface‐tension gradients can be avoided.

Magnetic resonance surgery using heat waves produced with focussed ultrasound

Surgery is performed with pulsed heat means that selectively destroys tissue in a region within a patient. The size of the region destroyed is dependent upon the frequency of the pulses of the pulsed heat means and thermal conductivity of the tissue of the patient. The pulsed heat means can be a coherent optical source that is guided by laser fiber to the tissue to be destroyed. In another embodiment the pulsed heat means is a focussed ultrasound transducer that dissipates ultrasonic energy at a focal point within the region of tissue to be destroyed. A magnetic resonance imaging system employing a temperature sensitive pulse sequence creates an image of the tissue and the region being heated to allow a surgeon to alter the position of the pulsed heat means or vary the pulse frequency.

Anodic bonding of imperfect surfaces

The intimate surface contact necessary for anodic bonding of a metal to a glass is examined in terms of the electrostatic forces which promote this contact and surface imperfections which prevent this contact. Surface imperfections that are considered include foreign particles, isolated hillocks, and periodic curved and angular surfaces. With curved and angular surfaces, a critical voltage exists that causes spontaneous mating of the surfaces by elastic or plastic deformation. Below this voltage, intimate surface contact is achieved by viscous flow. For isolated hillocks and foreign particles, surface conformation generally occurs by viscous flow because of large stress concentrations at these imperfections. The resulting time required for intimate surface contact is a very sensitive function of the imperfection size.

Surface normalization of sensitized stainless steel by laser surface melting

A scanning laser beam was used to melt and normalize the surface layer of sensitized 304 stainless steel. Subsequent Strauss tests indicated a complete resistance to intergranular corrosion. Mechanical testing at strains less than 15% also showed laser surface melting to indefinitely extend specimen life in a stress corrosion environment. At strains greater than 15%, the laser‐scanned protective layer was breached by cracks. A maximum critical laser‐scanning velocity compatible with normalization of the surface layer is calculated. Similarly, a minimum critical laser‐scanning velocity required to avoid resensitization is determined. The stress distribution in a 304 stainless‐steel specimen with a laser‐melted and self‐quenched surface layer is estimated and shown to be compatible with the observed appearance of martensite in the melted surface layer.

Thermal Migration of Liquid Droplets through Solids

A transparent solid and liquid were chosen to study the thermomigration of liquid droplets through solids. Irreversible processes associated with the transfer of atoms between the solid and the liquid phases at the solid‐liquid interface of the droplet were found to have a profound influence on the migration behavior of the liquid droplets. For large droplets, these interface kinetics cause the droplet to disintegrate in a thermal gradient. In smaller droplets where the containing effects of surface tension are relatively greater, the droplets experience a prolonged transient period of thermomigration during which there is a monotonic increase in the velocity of the droplet and a gradual distortion of the droplet shape. Following this transient period, the smaller drops enter a steady‐state period during which both their shape and velocity remain constant. The steady‐state velocity of droplets in this region decreases with decreasing droplet volume and falls to zero below a critical droplet size. This var...

The diamond 13C/12C isotope Raman pressure sensor system for high-temperature/pressure diamond-anvil cells with reactive samples

By using a thin 13C diamond chip together with a 12C diamond chip as sensors, the diamond Raman spectra provide the means to measure pressure precisely (±0.3 GPa) at any temperature (10–1200 K) and simultaneous hydrostatic (or quasihydrostatic) pressure (0–25 GPa) for any sample compatible with an externally heated diamond-anvil cell. Minimum interference between the Raman spectrum from the diamond anvils and those of the pressure sensors is obtained by measuring pressures with the Raman signal from the 13C diamond chip up to 13 GPa, and that from the 12C chip above 10 GPa. The best crystallographic orientation of the diamond anvils is with the [100] direction along the direction of applied force, in order to further minimize the interference. At 298 K, the pressure dependence of the 13C diamond first-order Raman line is given by ν(P)=νRT+aP for 91 at. % 13C diamond, where νRT(13C)=1287.79±0.28 cm−1 and a(13C)=2.83±0.05 cm−1/GPa. Analysis of values from the literature shows that the pressure dependence of...

Dielectric isolation of silicon by anodic bonding

Dielectrically isolated silicon was produced by anodically bonding together a pair of silicon wafers whose surfaces were covered with an electrically nonconductive micron layer of thermally grown oxide. Although anodic bonding normally requires a conductive oxide, anodic bonding works with nonconductive silicon oxide if the total layer of silicon oxide is less than ten microns thick. The time needed for the anodic bonding process decreases monotonically with temperature because the increase in the deformability of silicon oxide overcomes the decrease in the maximum permissible anodic bonding voltage with temperature. However, factors such as silicon degradation and electrode reactions at very high temperatures indicate that a compromise temperature range of 850–950 °C is best for the anodic bonding of silicon oxide. Bonding voltages of 30–50 V for times of about an hour produced the best bonding yields at these temperatures. Anodically bonded silicon wafers were examined with infrared and ultrasonic trans...

Forming electrical interconnections through semiconductor wafers

To fabricate electrical interconnections through a semiconductor wafer, an array of holes is laser drilled in the wafer and then a conductor is formed in the holes. Six techniques, including capillary wetting, wedge extrusion, wire insertion, electroless plating, electroforming, and double‐sided sputtering and through‐hole electroplating were used to form conductors in the array of laser‐drilled holes in silicon‐on‐sapphire (SOS) wafers. These techniques are described, and their respective strengths and weaknesses discussed and compared.

Metastable Synthesis of Diamond

Abstract Diamond can be grown metastably at subatmospheric pressures and moderate temperatures from hydrocarbon gases in the presence of atomic hydrogen. Atomic hydrogen serves a number of critical roles in CVD diamond growth, namely: (1) stabilization of the diamond surface, (2) reduction of the size of the critical nucleus, (3) ‘dissolution’ of carbon in the gas, (4) production of carbon solubility minimum, (5) generation of condensable carbon radicals in the gas, (6) abstraction of hydrogen from hydrocarbons attached to surface, (7) production of vacant surface sites, and (8) etching of graphite. Atomic hydrogen can carry out these functions because of favorable relationships between energies for carbon-carbon, carbon-hydrogen and hydrogen-hydrogen bonds.