Growth Kinetics and Thermal Stress in AlN Bulk Crystal Growth

Group III nitrides, such as GaN, AlN and InGaN, have attracted a lot of attention due to the development of blue-green and ultraviolet light emitting diodes (LEDs) and lasers. In this paper, an integrated model has developed based on the conservation of momentum, mass, chemical species and energy together with necessary boundary conditions that account for heterogeneous chemical reactions both at the source and seed surfaces. The simulation results have been compared with temperature measurements for different power levels and flow rates in a reactor specially designed for nitride crystal growth at NCSU. It is evident that the heat power level affects the entire temperature distribution greatly while the flow rate has minor effect on the temperature distribution. The results also show that the overall thermal stress level is higher than the critical resolved shear stress, which means thermal elastic stress can be a major source of dislocation density in the as-grown crystal. The stress level is strongly dependent on the temperature gradient in the as-grown crystal. Results are correlated well with defects showing in an X-ray topograph for the AlN wafer.

Anisotropic Thermal Diffusivity of Aligned Multiwalled Carbon Nanotube Arrays

This work employs a photothermoelectric technique to measure the anisotropic thermal diffusivity of an aligned multiwalled carbon nanotube array. A modulated laser beam incident to the front surface of the sample creates a thermal wave which is detected by a fast responding thermocouple formed between the back surface of the sample and the tip of a sharp metallic probe. The anisotropic thermal diffusivity values are obtained by fitting the radial and frequency dependent thermal signals with an anisotropic heat conduction model. The room temperature thermal diffusivity measured perpendicular to the alignment direction is 0.246×10−5m2/s, an order of magnitude smaller than thermal diffusivity along the CNTs alignment direction 4.4×10−5m2/s. However, the thermal diffusivity of the aligned multiwalled CNT is two orders of magnitude smaller than expected for an individual multiwalled CNT.

Kinetic Theory Treatment for Heat Transfer in Plasma Spraying

In plasma spraying process thermal plasma is used as a heat source to heat and melt metallic or ceramic particles. In this paper, heat transfer from a thermal plasma to a solid spherical particle has been analyzed using a kinetic theory approach. We have considered a solid particle introduced in an ionized gas made up of electrons, ions, and neutrals. Two-sided electron velocity and temperature distributions and two-sided ion velocity distributions are used. Maxwell’s transport equations are obtained by taking moments of the Boltzmann equation. The transport equations are solved with the Poisson’s equation for the self-consistent electric field. The ion and the electron number density distributions, temperature distribution, and the electric potential variation are obtained. The charged species flux to the particle surface is evaluated. Heat transport to the surface is calculated by accounting for all the modes of energy transfer including the energy deposited during electron and ion recombination at the surface. Results indicate that contribution to heat transfer from charged species recombination is substantial at high plasma temperatures.

Modeling of Macrosegregation and Secondary Phase Precipitation During Solidification of Binary Alloys

A multiphase volume-averaged model describing macrosegregation in an alloy that solidifies by the formation and growth of a primary and a secondary solid phase has been formulated. The model is based on the work presented by Ni and Beckermann (Metall. Trans. 22B, 1991, p. 349), but is extended to account for secondary phase precipitation. A CFD model has been developed by implementation of the mathematical model in a commercial CFD code. Macrosegregation due to thermo-solutal convection in binary alloys with a stationary solid phase (primary and secondary solids) has been simulated. The predictions compare fairly well to experimental results and simulations previously reported in the literature.

Numerical Investigation on the Effect of Marangoni Convection in an Industrial Czochralski Crucible

The quasi direct numerical simulations (DNS) of the flow and the thermal fields in an industrial Czochralski crucible have been carried out in order to investigate the effect of thermocapillary or Marangoni convection employing an optimised parallel-vector block-structured Navier-Stokes equations solver. The simulations have been performed without and with the Marangoni effect at a specified rotation of the crucible during the synthesis of mono-crystalline Silicon (Pr=0.011). The time-averaged flow field reveals that the inward radial velocity at the free surface of the melt is quite high for the case with Marangoni convection. The flow is directed towards the solid crystal due to the presence of significant surface tension gradients at the free surface. A stronger downward flow has been observed at the center of the crucible owing to this strong radial velocity. Due to the superposition of the Marangoni convection, temperature fluctuations are reduced under the free surface and at the crystal interface. Thus the fluctuations in the growth rate are reduced. The turbulent kinetic energy, k is smaller below the crystal at different depths in the melt for the cases without any effect of the Marangoni convection as compared to the cases with Marangoni convection. The temperature along the free surface of the melt is increased when the thermocapillary effect is included.

Transient Thermal Investigation for a SmartMOS Based Airbag Circuit Driver

A numerical study was conducted to model the transient thermal behavior of an airbag squib driver using commercially available software. The squib driver is part of an airbag deployment IC. The simulations were primarily used to predict the thermal gradient across the die for determining the optimal sensor location for thermal shutdown that would protect the device from destruction. The temperature sensor should be placed such that it gets hot enough for any electrical pulses that heat up the device close to the destruction point. The overall purpose is to provide a thermal detection circuit for disabling current prior to reaching a thermally destructive level. A preliminary wafer level study correlates the simulated and measured values and indicates that the junction temperature is lower for the case with thicker die and adiabatic boundary conditions; an opposite trend is observed for the cases with fixed temperature boundary condition attached to the domain bottom side. The study of the high IC side dissipating 80W for 5 ms indicates that the bottom and top center monitor points reach temperatures of 188.2°C and 130.5°C at the end of the 5 ms timeframe, corresponding to a peak source temperature of 294.6°C. A similar study with 30W uniform dissipation for 20 ms indicates that the peak junction temperature is lower than before (220°C vs. 294°C). The study of the low IC side reveals higher peak temperatures compared to the high side, due to the larger power density for these cases. The peak temperatures are 368.7°C for 50W/5 ms, and 301.8°C for 25W/20 ms. The left monitor point temperature ranges between 210°C–260°C while the right monitor point temperature ranges between 140°C–160°C. The thermal investigation of the package after the thermal shutdown predicted the time needed for the FETs to reach predetermined temperatures for different scenarios. The temperatures of the low side FETs drop by almost 50% within the 30 ms following the 20 ms of constant powering at 50W. When the high-side FETs are powered at 80W for 5 ms then cooled, the temperature rises then decays within 0.1 s.

Interaction of Solidification Fronts With Embedded Particles and Biological Cells

The interaction of solidification fronts with embedded heterogeneities, such as particles and living cells, is an important physical ingredient underlying the successful solidification processing of some materials. An example in the first category is the processing of metal-matrix composites (MMCs) where solidifying metal dendrites interact with ceramic reinforcements [1–3]. Processing of biological material, such as cells and tissue for cryopreservation [4], falls into the second category. In each case, the evolving solidification microstructure interacts in complex ways with the embedded particle.

Quasi-Steady State Natural Convection in Laser Chemical Vapor Deposition With a Moving Laser Beam

Numerical analysis of laser chemical vapor deposition (LCVD) of Titanium Nitride by a moving laser beam is investigated numerically. The effect of natural convection due to temperature and concentration differences in the gases mixture is modeled and implemented into the thermal model of LCVD by a moving laser beam. The problem is formulated in the coordinate system that moves with the laser beam. The results show that the effect of natural convection on the shape of deposited film is very insignificant for cases with lower laser power but it becomes important when the laser power is increased.

Flow and Heat Transfer Study in an Autoclave for Hydrothermal Crystal Growth With a Three-Dimensional Conjugate Heat Transfer Model

Numerical modeling becomes an important technique to study hydrothermal crystal growth since experimental measurements in hydrothermal autoclaves are extremely difficult due to the high pressure and high temperature growth conditions. In all existing models for hydrothermal growth, isothermal boundary conditions are assumed, although electric heaters are employed around the outside surface of the thick autoclave wall in practice. In this paper, a conjugate heat transfer model based on an industry size autoclave is developed to investigate the validity of such an assumption. The model includes not only turbulent fluid flow and heat transfer of the solution but also the heat conduction in the thick wall. The outside surfaces of the wall are under constant heat flux conditions, simulating electric resistance heating used in practice. Non-uniformity of the heat flux in the circumferential direction is also introduced in the model. The results indicate that the temperature at the solution/wall interface is far away from uniform. The isothermal wall boundary condition in previous efforts is questionable. Predictions of the isothermal wall model are analyzed. Parametric studies with the conjugate model show that total heat supply rate does not affect vertical uniformity dramatically. Heat loss can be lowered without affecting the flow and temperature fields if heaters are put half diameter or further away from the middle height (baffle) plane.

Nanostructuring With Scanning Probe Microscope Tip Irradiated With Femtosecond Laser

Nanostructures, which have characteristic dimensions that are difficult to achieve by conventional optical lithography techniques, are finding ever-increasing applications in a variety of fields. High resolution, reliability and throughput fabrication of these nanostructures is essential if applications incorporating nanodevices are to gain widespread acceptance. Owing to the minimal thermal and mechanical damage, ultra-short pulsed laser radiation has been shown to be effective for precision material processing and surface micro-modification. In this work, nanostructuring based on local field enhancement in the near field of a Scanning Probe Microscope (SPM) probe tip irradiated with femtosecond laser pulses has been studied. High spatial resolution (~10–12nm), flexibility in the choice of the substrate material and possibility of massive integration of the tips make this method highly attractive for nanomodification. We report results of nanostructuring of gold thin film utilizing an 800nm femtosecond laser system in conjunction with a commercial SPM in ambient air. Further, Finite Difference Time Domain (FDTD) simulation results for the spatial distribution of the laser field intensity beneath the tip are presented. Potential applications of this method include nanolithography, nanodeposition, high-density data storage, as well as various biotechnology related applications.