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.

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.

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.

Validation of Twin-Screw Polymer Extruder Modeling

The mathematical and numerical modeling of twin-screw polymer extruders is examined with respect to accuracy of results and validity of the simulation. A numerical model is developed incorporating the translation region, which is similar to a single-screw extruder channel, and the intermeshing, or nip, region. The numerical modeling is carried out for steady and time-dependent operation, considering various polymeric materials like polyethylene and corn meal. A range of design parameters and operating conditions are considered. The results are evaluated in terms of the expected physical behavior of the system and compared with experimental results available in the literature to determine the accuracy of the predictions. In many cases, only qualitative comparisons are possible since the operating conditions and design parameters are not explicitly known. However, the basic trends are as expected and good quantitative comparisons with experimental data is used to validate the model. Validated numerical models can extend the domain of relevant inputs for process design and optimization.

Convection Effects in Three-Dimensional Dendritic Growth

A phase-field model is developed to simulate free dendritic growth coupled with fluid flow for a pure material in three dimensions. The preliminary results presented here illustrate the strong influence of convection on the three-dimensional (3D) dendrite growth morphology. The detailed knowledge of the flow and temperature fields in the melt around the dendrite from the simulations allows for a detailed understanding of the convection effects on dendritic growth.

Manufacturing of Electrically Conductive Microstructures by Dropwise Printing and Laser Curing of Nanoparticle-Suspensions

A novel method for the manufacturing of electric microconductors for semiconductor and other devices is presented. The method brings together three technologies: controlled (on demand) printing, laser curing, and the employment of nanoparticles of matter, possessing markedly different properties (here, melting point) than their bulk counterparts. A suspension of gold particles in toluene solvent is employed to print electrically conducting line patterns utilizing a modified on demand ink jet printing process. To this end, microdroplets of 80–100 μm diameters are deposited on a moving substrate such that the droplets form continuous lines. Focused laser irradiation is utilized in order to evaporate the solvent, melt the metal nanoparticles in the suspension, and sinter the suspended particles to form continuous, electrically conducting gold microlines on a substrate. The ultra fine particles in the suspension have a diameter size range of 2 – 5 nm. Due to curvature effects of such small particles, the melting point is markedly lower (400°C) than that of bulk gold (1063°C). Thermodynamic aspects of the effect of particle size on the melting and evaporation temperatures of gold and toluene, respectively, are discussed in the paper. The structure and line width of the cured line as a function of the laser irradiation power and stage velocity are reported in detail. Preliminary measurements of the electrical conductivity are represented.

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.