Improved Heat Conduction Research Articles

Numerical Investigation of Thermal Conductivity of Particle Dispersive Composites Based on Fractal Method

Heat conduction performance of particle dispersive composites has been numerically investigated by using Finite volume method (FVM) and fractal theory. The effects of the thermal conductivity of particle and polymer matrix, the volume fraction of conductive particle, the dispersion and reunion form of particles on the effective thermal conductivity of composites are analyzed in detail. Results from the research indicate that thermal conductivities of composites will increase nonlinearly along with the increase of filling fraction of particles. Keeping the filling fraction constant, the dispersion and reunion form and direction of particles has significant effect on heat conduction performance of composite. Simple use of high thermal conductivity particles has limited effect on thermal performance of composites. Enhancing the contacts of particle in the direction of heat exchange and forming high heat transfer channels are the main and economical ways to improve heat conduction performance of particulate filled polymer matrix composites.

Experimental Study of Heat Transfer in Micro Glass Tubes Mediated by Surface Phonon Polaritons

We report a temperature measurement of the surface of micro structured glass tube, affected by surface phonon polaritons' (SPPs) energy enhancement. The glass tubes were designed to guide monochromatic thermal energy to improve heat conduction or serve as radiative source. The simulation results show strong enhancement of the Free Path when a wave guide radius is between 2.0 and 10 μm. Absorption of enhanced SPPs energy on the waveguide areas with high absorptivity has some possibility to cause local rise in temperature. Various shapes of micro glass tubes are simply fabricated, and heated by using a Pt wire in a vacuum chamber. Surface temperatures of glass tubes are measured by infrared digital thermo microscope as well as by using a micro-thermocouple to investigate the temperature distribution on both tapered and thin micro glass tubes.

Pressure-Tolerant Power Electronics for Deep and Ultradeep Water

Summary This paper presents results from an ongoing research project (SINTEF 2011) on pressure-tolerant power electronics. The main goal of the research is to provide and demonstrate solutions that enable power-electronic converters to operate in pressurized environments. Oil companies have plans for subsea processing of oil and gas. Today's concept considers power-electronic converters in 1-bar vessels. As the depth and the converter power rating increase, these vessels become increasingly bulky and heavy. Pressure-balanced converters would allow lower vessel-wall thickness, thereby giving lower weight, and simpler cooling owing to improved heat conduction through the vessel walls. The new concept considers power-electronic converters placed in vessels completely filled with appropriate dielectric liquid able to operate at pressures from 1 bar up to several hundred bar. Dielectric fluids are required to prove their properties, especially those related to insulation, incompressibility, and heat transport. The methodology presented here considers a mechanical adaptation of components followed by various laboratory experiments for verifying or correcting the proposed solutions (Pittini et al. 2010; Hernes and Pittini 2009; Petterteig et al. 2009). Standard off-the-shelf components and special pressure-adapted components have been subjected to various provocative pressurization tests up to 300 bar. Tests clarified the need for special adaptation of some components, while others could be used without any modification. Subsequently, a full converter phase leg has been built, submerged in dielectric liquid, and tested in full operation up to 300 bar. This phase leg is based on a press-pack IGBT modified for operation at high pressure. Measurements performed at different pressures demonstrate that there is no relevant difference in terms of electrical parameters between this modified IGBT and a standard IGBT. Long-term tests proved the concept's validity. The next step will be to test bonded IGBT technology in a high-pressure environment, and finally, we will realize a test demonstration to be located at a suitable subsea site.

Natural Convection Investigations in Porous Phase Change Materials

Metal foams with high thermal conductivities are generally considered to have high potential to enhance the heat transfer performance for Phase Change Materials (PCMs). In this parametrical study, the effects of metal foams on heat transfer in PCMs are examined, and the investigation is conducted in the aspects of both heat conduction and natural convection. The findings of this paper are that metal foams significantly improve heat conduction in solid region, whilst suppress natural convection in liquid region. Buoyancy-driven velocities are too weak to produce dominant convection, which is due to high viscosity and low thermal expansion ratio of the PCM, as well as the large flow resistance of metal foams.

Liquid-immersion laser micromachining of GaN grown on sapphire

Liquid-immersion nanosecond-pulsed laser micromachining is introduced as an efficient way for device isolation and rapid prototyping on GaN-on-sapphire wafer. Using deionized water as an ambient medium for laser micromachining, smooth trenches that are free from redeposition can be formed in the GaN layer. Coupled with the large difference between the ablation thresholds and ultraviolet absorption coefficients of GaN and sapphire, the GaN/sapphire interface can be left undamaged after the ablation process. This technique overcomes the limitation of heat accumulation in nanosecond-pulse regime, and offers a cost-effective alternative to ultrashort-pulse laser micromachining. In this report, the advantages offered by liquid immersion are elucidated in terms of improved heat conduction, increased plasma-induced recoil pressure due to water confinement, weakened plasma shielding effect in water, and the collapse of cavitation bubbles. Simulation results show that the reduced fluctuation of temperature profile over time in water could be correlated with the reduced redeposition of Ga from thermal decomposition at the trench sidewalls.

Suppression of flux avalanches in superconducting films by electromagnetic braking

Magnetic fields perpendicular to superconducting films often trigger vortex avalanches, which always are very harmful for electronic devices and other applications. Such avalanches can be suppressed by a metal layer placed in contact with the superconductor surface, an effect that up to now has been thought to be a consequence of improved heat conduction. Here we show experimentally that the role of the metal layer is not that of a heat-sink, but rather that of an electromagnetic drag due to eddy currents induced in the metal layer during the abrupt onset of the flux avalanches. The effect is demonstrated for films of MgB2 and Nb.

Design and fabrication of a microfluidic device for near-single cell mRNA isolation using a copper hot embossing master

We describe investigations toward a disposable polymer-based chip for the isolation of eukaryotic mRNA. This work focuses here on the improvement of the fabrication methods for rapid prototyping and the actual application at lowest RNA concentrations with total channel volumes of 3.5 μL. Messenger RNA isolation was achieved using paramagnetic oligo (dT)25 beads within a microfluidic channel which incorporated a sawtooth microstructured design to aid in mixing. The structures were shown to facilitate mixing beteen two fluids in parallel flow when compared to a channel without structures. The chip was fabricated by means of hot embossing poly(methyl methacrylate) (PMMA) using a copper master. Copper was used as the master material due to its excellent thermal, mechanical, and electroplating properties. Fabrication of the master consisted of the structuring of a polished copper plate using KMPR 1050 as an electroplating mold for forming the microchannel structures. The copper master was found to be much more robust than traditional silicon masters used for prototyping. The use of KMPR enabled the generation of high straight walls in contrast to SU-8 masters. In addition, embossing times were able to be decreased by a factor of 3 due to improved heat conduction and avoidance of a lengthy and delicate de-embossing step.

Lattice thermal conductivity of nanoscale AlN∕GaN∕AlN heterostructures: Effects of partial phonon spatial confinement

We report a detailed theoretical modeling of lattice thermal conductivity in nanoscale AlN∕GaN∕AlN heterostructures. Thermal conductivity in such heterostructures is derived based on the solution of the phonon Boltzmann equation in the relaxation-time approximation. Phonon dispersion relations are obtained in the elastic continuum approximation using the finite-difference numerical method. Quasi-two-dimensional (2D) phonon density of states is derived using the actual phonon dispersion. To investigate the effects of partial phonon spatial confinement, numerical simulations are performed for both the three-layer heterostructures and the single GaN thin films. The dependence of the thermal conductivity on the core or cladding layer thickness in an AlN∕GaN∕AlN heterostructure is also discussed. We have demonstrated that partial phonon confinement leads to a higher thermal conductivity in an AlN∕GaN∕AlN heterostructure than that in a single GaN thin film with the same total structure thickness. Such thermal conductivity can also be tuned higher or lower by adjusting the core or cladding layer thickness without changing the total thickness of the structure. Obtained results have quantitatively shown that it is possible to improve heat conduction in semiconductor nanostructures through phonon spectrum engineering, in agreement with other recent studies.

An ultra-thin-walled solenoid for the CELSIUS/WASA experiments

Abstract An ultra-thin-walled superconducting warm bore solenoid has been developed for the WASA detector set-up at the CELSIUS accelerator of the The Svedberg Laboratory (TSL), Sweden. It gives a central magnetic field up to 1.3 T , and can be operated in persistent mode. The total thickness of the superconducting coil is 9 mm with an inner diameter of 554 mm and an energy-to-cold-mass ratio of 6 kJ / kg . The total wall thickness is a record low 0.18 in units of the radiation length ( X 0 ) including the material of the cryostat. The solenoid has been successfully operated in combination with the WASA detector set-up since 1998. The solenoid has a coil split in two parts with a central gap of 40 mm for a tube guiding small frozen hydrogen target pellets into the accelerator beam. The solenoid is fixed by glass-fibre supports. It has a vacuum vessel with 1 mm thick corrugated walls of aluminium and it has high-purity aluminium strips in axial direction to improve heat conduction. The solenoid is enclosed by an iron yoke which shields the detector readout electronics from the magnetic field. A diagnostics, control and safety system has been developed for the solenoid itself and for the associated cryogenics.

A numerical study of nonlinear instability phenomena in solid rocketmotors

An existing nonlinear instability model has been extended and improved to allow it to more realistically treat the longitudinal shock-wave type of combustion instability frequently encountered in tactical solid rocket motors. Results obtained utilizing this model to investigate limiting amplitude and velocity-coupling phenomena in solid rocket motors are presented. An advanced finite difference integration technique capable of accurately describing shocks and contact discontinuities has been incorporated into the computer program, as well as an improved heat conduction solution, an heuristic velocity-coupling model, and a spectral analysis capability. Solutions demonstrating that limiting amplitude is independent of the characteristics of the initial disturbance are presented, as are results of a preliminary study of nonlinear velocity coupling demonstrating such phenomena as triggering, mean pressure shift, modulated limit cycles, and threshold velocity effects.

An Improved Method of Producing Fluorine 18 for Use in Medium Flux Reactors

Fluorine 18 has been prepared for patient use in the University of Wisconsin 250 kw medium flux reactor (1.5 × 1012 n/cm2sec.). using a modification of the method of Thomas et al. (1). In their technic 6Li2CO3 in powder form is irradiated in a reactor and 18F is produced by the reaction 16O + t → 18F + n where the tritons are the result of thermal neutron capture by 6Li. After irradiation, the Li2CO3 is dissolved in sulfuric acid and HF is distilled into a receiving flask containing dilute NaOH. Thomas et al. report that an irradiation of 300 mg of powder for 1.25 hours in a flux of 2.5 × 1013 n/cm2sec. produces 5.3 mCi with a yield of about 70 per cent efficiency. Because the flux in the University of Wisconsin reactor is less by a factor of 17, modifications had to be introduced to produce a sufficient quantity of 18F for clinical scintiscanning. A 60 per cent increase in the irradiation time (two hours) will produce only 43 per cent more 18F; a further increase in time becomes even less advantageous. The obvious step of increasing the amount of powdered 6Li2CO3 in the irradiation sample produced difficulties with sample size and led to overheating of the sample during irradiation. Problems were also associated with dissolving the larger amount of powder in preparation for distillation. The technic evolved to overcome these difficulties consists in dissolving the 6Li2CO3 in acid prior to irradiation. One necessary precaution is that all dissolved CO2 must be driven off before irradiation to avoid pressure build-up which might exceed the structural limits of the container. It is possible to concentrate more than 2 g of 6Li2CO3 in 10 cc of 1:1 H2SO4. The liquid form provides improved heat conduction. Processing after removal from the reactor is expedited because the time-consuming step of carefully dissolving the 6Li2CO3 in acid before distillation is eliminated. The time between removal from the reactor and patient injection is generally less than forty-five minutes. This includes transportation from the reactor facility to the radioisotope laboratory plus sterilization and radioassay procedures. An additional advantage of the modified technic is that the residual solution containing 6Li can be re-used at a later date. One solution has been recycled five times with no appreciable loss in yield. The quantity of material available for injection is between 3 and 4 mCi. The chief disadvantage of the modified technic is the handling of a strongly acid liquid which is also highly radioactive.