Small Thermal Resistance Research Articles

INVESTIGATIVE STUDY OF MANIFOLD MICROCHANNEL HEAT SINKS FOR ELECTRONIC COOLING DESIGN

Advances in microfabrication technology have allowed the use of microchannels in ultra compact, very efficient heat exchangers, which capitalize on the channels large surface area to volume ratio to transport high heat fluxes with small thermal resistances. One example is the cooling of microchips. However, the research into microscale flow and heat transfer phenomena conducted by various researchers provided substantial experimental data and considerable evidence that the behaviour of fluid flow and heat transfer in microchannels without phase change may be different than that which normally occurs in larger more conventional sized channels. This paper serves to perform a numerical analysis on the flow and heat transfer in manifold microchannels. A numerical model for 16 sets of parametric conditions is presented here using a CFD package, ANSYS/FLOTRAN. Pressure, temperature and velocity contour plots were obtained and analyzed. The results obtained were then compared with a derived analytical model. The...

Impact of FEM simulation on reliability improvement of packaging

Abstract An important role of packages is the protection of the chip to environmental influences like moisture, pollutants and other chemical active species. If the chip is exposed these influences for instance corrosion is resulting and it may come to early failures. The reliability of the package is affected by thermo-mechanical stress. Due to this, an essential aspect of the package design is the careful combination of materials to avoid mechanical stress and to improve the thermal resistance of the package, which is significant for example in power or automotive applications. For a reduction of the mechanical stress, a good heat conductivity into the ambient and by this a small thermal resistance of the package should be aspired. Finite element analysis. (FEM) can give a fast and reliable solution to investigate the influence of mechanical stress as well as the thermal behaviour of the package, depending on the material properties and on the package geometry. For the finite element analysis special boundary conditions like convection and radiation and the influence of the board have to be taken into account.

3D thermal wave scattering on buried inhomogeneities in ac thermal microscopy

The scattering of the thermal waves on buried small thermal resistances is theoretically studied in the framework of conductive thermal modelling of the coupling between a SThM probe and a sample. Lateral resolutions for the amplitude and phase of the probe's ac thermal response are obtained. The modelling, using an iterative method, goes beyond the usual first Born approximation results. Technically, the ac temperature field in the entire geometry is expressed in terms of two-dimensional plane wave spectra which permit direct use of fast Fourier transform algorithms. The various analytical results are easily obtained via a multilayer treatment of the probe and of the sample.

Continuous-wave operation of InGaN/GaN/AlGaN-based laser diodes grown on GaN substrates

InGaN multi-quantum-well-structure laser diodes (LDs) grown on GaN substrates were demonstrated. The LDs showed a small thermal resistance of 30 °C/W and a lifetime longer than 780 h despite a large threshold current density of 7 kA/cm2. In contrast, the LDs grown on a sapphire substrate exhibited a high thermal resistance of 60 °C/W and a short lifetime of 200 h under room-temperature continuous-wave operation.

Heat transport in thin dielectric films

Heat transport in 20–300 nm thick dielectric films is characterized in the temperature range of 78–400 K using the 3ω method. SiO2 and SiNx films are deposited on Si substrates at 300 °C using plasma enhanced chemical vapor deposition (PECVD). For films >100 nm thick, the thermal conductivity shows little dependence on film thickness: the thermal conductivity of PECVD SiO2 films is only ∼10% smaller than the conductivity of SiO2 grown by thermal oxidation. The thermal conductivity of PECVD SiNx films is approximately a factor of 2 smaller than SiNx deposited by atmospheric pressure CVD at 900 °C. For films <50 nm thick, the apparent thermal conductivity of both SiO2 and SiNx films decreases with film thickness. The thickness dependent thermal conductivity is interpreted in terms of a small interface thermal resistance RI. At room temperature, RI∼2×10−8 K m2 W−1 and is equivalent to the thermal resistance of a ∼20 nm thick layer of SiO2 .

Analysis of microchannels for integrated cooling

Recent advances in the precision machining of small channels have made possible the use of such micro-channels in ultra compact, very efficient heat exchangers, which capitalize on the channels' large surface area to volume ratio to transport high heat fluxes with small thermal resistances. For instance, such micro-channels can be fabricated right into the back of electronic chips to effectuate the cooling of these chips. In this paper, we analyze such heat exchangers by solving numerically a conjugate heat transfer problem consisting of the simultaneous determination of the temperature fields in both the solid substrate and the fluid. Additionally, we present a design algorithm for the selection of the heat exchanger's dimensions.

Application of Three ASTM Test Methods to Measure Thermal Resistance of Clothing

This paper describes the application of three ASTM methods de veloped for testing thermal insulating materials. To permit testing fabrics with small thermal resistance, a standard air layer was incorporated as a part of the test assembly. This air layer, placed on the cold side of tested fabric, is used for two reasons: (1) to reduce experimental errors, and (2) to permit testing radiative properties of the fabric surface.

Nuclear Spin-Lattice Relaxation in Metals at Low Temperatures

In NMR experiments on metals at low temperatures, the analysis to obtain the nuclear spin-lattice relaxation time τ1 can be complicated because the lattice thermal capacity becomes small and other thermal resistances to the heat flow to the thermal bath can become significant. A theoretical model is developed which gives consistent results for κ=τ1T when applied to experimental data obtained for Pt powder over a wide range of experimental conditions.

Etude des facteurs d'intensification de l'échange de chaleur dans le refroidissement des fruits et légumes

Increasing the rate of fruit and vegetable cooling is of economic significance for the following reasons: biochemical and microbiological processes are quickly suppressed; surface mass-exchange with ambient air is slowed down; power consumption is reduced, and the freezing output of cooling equipment and installations is increased. It was assumed that the fruits and vegetables in crates were arranged regularly and covered with thin paper of a negligible small thermal resistance in order to eliminate the infiltration of cold air within the layers. Crates were arranged on a pallet so that they touched horizontally. Thus, the packages form plates above and below the cooling air flow. Heat-exchange with a one-dimensional temperature field is established. It was found that intensification of cooling can be achieved by increasing the speed of cooling air as well as by stopping the process at a higher final temperature. Increasing the speed up to 5 m s −1 shortens the time of the process by a factor of two compared with the low speeds (0.3–0.5 m s −1) which are normally used. The product can be cooled until the temperature in the layer centre reaches 5°C, and the surface temperature about 2°C, which is low enough. Afterwards the cooled product is transferred to a freezing chamber or to a van-freezer. This factor results in a further halving of the cooling time. The combined effect of the two factors shortens the duration of the process by about 75% and increases the output of the cooling equipment from 3.5 to 4 times.

Theory of helium under heat flow near the ? point. I. Interface of He I and He II

A He I-He II interface is shown to exist under heat flow and is studied near the λ point. The temperature in the superfluid region is found to be of the form Tλ −T ∝ Q3/4 if dynamic scaling is assumed, where Tλ is the critical temperature andQ is the heat flow. In the normal region the temperature has a finite gradient. Here we are neglecting a small thermal resistance due to vortices in the superfluid region. In a finite system with size much greater than the correlation length a first-order phase transition occurs as an inverted bifurcation as the temperature at the cooler boundary is lowered slightly below Tλ. Namely, the system will jump from the disordered state to a state in which the two phases are separated by an interface. The theory can be constructed analogously to the theory of superconductors in a magnetic field.

High-frequency high-power static induction transistor

The principal operating mechanism of the static induction transistor (SIT) that shows exponential rather than the saturated I-V characteristics, is based on the static induction of both gate and drain voltages. It is known that the SIT has low noise, low distortion, and high audio-frequency power capability. The SIT is also a very promising device for high-frequency and high-power operation because of its short channel length, low gate series resistance, small gate-source capacitance, and small thermal resistance. Si SIT's which generate a 40-W output power at 200 MHz and 10 W at 1 GHz, with a cutoff frequency higher than 2.5 GHz, have been fabricated. This is the first step toward the realization of a power microwave SIT. Future developments of a higher power higher frequency SIT can be realized by employing a distributed electrode structure and traveling-wave operation.

Measurement of heat flow by means of the Nernst effect

A comparison is made between the Seebeck and Nernst effects in semiconductors for the measurement of heat flow. The latter effect would seem to be superior since the Nernst EMF is proportional to temperature gradient rather than temperature difference. This means that devices for measuring the flow of heat based on the Nernst effect may have a very small thermal resistance. A device, incorporating InSb-NiSb eutectic as the active material, has been tested successfully.

Surface area of powdered cerous magnesium nitrate

The anomalously small thermal boundary resistance between liquid He3 and powdered cerous magnesium nitrate (CMN) crystals must be due either to an unexpectedly large surface area, to a porous surface layer, or to a new coupling mechanism between the thermal excitations on either side of the interface. Measurement of the surface area of powdered CMN by the BET adsorption method shows that the first two possibilities can be ruled out.