Thermal Conductivity of Single-Crystal Silicon Microbridges Measured by Electrical Resistance Thermometry and Time-Domain Thermoreflectance

Accurate thermal conductivity values are essential to the modeling, design, and thermal management of microelectromechanical systems (MEMS) and devices. However, the experimental technique best suited to measure thermal conductivity, as well as thermal conductivity itself, varies with the device materials, fabrication conditions, geometry, and operating conditions. In this study, the thermal conductivity of boron doped single-crystal silicon-on-insulator (SOI) microbridges is measured over the temperature range from 77 to 350 K. The microbridges are 4.6 mm long, 125 μm tall, and two widths, 50 or 85 μm. Measurements on the 85 μm wide microbridges are made using both steady-state electrical resistance thermometry and optical time-domain thermoreflectance. A thermal conductivity of ∼ 77 W/mK is measured for both microbridge widths at room temperature, where both experimental techniques agree. However, a discrepancy at lower temperatures is attributed to differences in the interaction volumes and in turn, material properties, probed by each technique. This finding is qualitatively explained through Boltzmann transport equation modeling under the relaxation time approximation.

A First-Principles Method of Determining Van Der Waals Forces in a Dissipative Media

Lifshitz theory of van der Waals (vdW) force and energy is strictly valid when the location at which the stress tensor is calculated is in vacuum. Generalization of Lifshitz theory to the case when the stress tensor is to be calculated in a dissipative material, as opposed to vacuum, is a surprisingly difficult undertaking because there is no expression for the electromagnetic stress tensor in dissipative materials. Here, we derive the expression for vdW force in planar dissipative media by calculating the Maxwell stress tensor in a fictious layer of vacuum, that is eventually made to vanish, introduced in the structure, without employing the complicated quantum field theoretic method proposed by Dzyaloshinskii, Lifshitz, and Pitaevskii. Even though this work has proven to be a corroboration of Dzyaloshinskii et al., it has thrown new light on our understanding of vdW forces and suggests that it should be possible to achieve the similar result for objects with arbitrary shapes.

Thermal Radiative Properties of Vertical Graphitic Petal Arrays

In this work, thermal radiative properties of vertical graphene petal arrays are theoretically and experimentally investigated to show that they are superior absorbers of radiation. Finite difference time domain (FDTD) simulations are first performed to calculate optical properties of vertical graphitic arrays of different configurations, namely, graphitic gratings, periodic graphitic cavities, and random graphitic cavities. The effect of polarization of incident radiation on optical properties of such structures is systematically evaluated. When the incident electric field is parallel to the graphitic plane (S polarization) in graphitic gratings, the absorptance is very high, but the reflectance low but still significant when compared to reflectance from a MWCNT array. On the other hand, when the electric field is polarized perpendicular to the graphitic plane (P polarization), the absorptance is significantly lower, as well as the reflectance. This contrast is due to the stronger optical response for the S polarization. Ordered graphitic petal cavity arrays show optical properties falling between the above two cases because of the presence of both polarizations. The random graphitic petal cavity arrays with various angles of orientation show similar properties with ordered petal arrays, and the simulated reflectance agrees very well with experimental data measured on a fabricated thin graphite petal sample.

Thermal Conductivity Enhancement of Room Temperature Ionic Liquids (RTILs) With Various Magnetic Nanoparticles

This paper reports an experimental study of thermal conductivity of room temperature ionic liquids (RTILs) based magnetic nanofluids. Various magnetic nanoparticles of metal oxides with high thermal conductivity, such as CuO, Al2O3, Fe3O4 and Carbon Nano Tubes (CNTs), were used to prepare magnetic nanofluids, while RTIL, trihexyl (tetradecyl) posphonium dicyanamide was used as the base fluid. Two major parameters that affect to the thermal conductivity enhancement of fluids were investigated. The effect of particle concentration and external magnetic fields were tested. It was observed that the magnetic nanofluids thermal conductivities increase with increment of particle concentration and external magnetic field parallel to the temperature gradient. Besides, it was observed that under higher magnetic fields, thermal conductivity enhancement tends to approach a saturation state. Surfactant was used to disperse magnetic nanoparticles within the RTILs. The transient hot wire method was used for this investigation.

Electrokinetic Power Generation by Forward Osmosis

In this paper, an innovative direct power generation technique from salinity gradient is proposed and demonstrated. The basis of this novel method encompasses forward osmosis (FO) and electrokinetic (EK) principles. Tapping the concentration difference between seawater and river fresh water, forward osmosis (FO) is utilized to allow for spontaneously transporting water across a semi-permeable membrane. The flow of water is then directed towards array of microchannels in the form of porous medium where power is produced from the electrokinetical streaming potential. Experimentally, NaCl solution and DI water were used to model as seawater and fresh river water, respectively. Both glass and polymer based porous media and commercial flat sheet FO membranes were employed herein. Results show power density could reach the order of 101W/m2. Having features of ease of fabrication, simple configuration and no mechanical moving parts, this method provides a feasible mean to harvest enormous energy from salinity gradient. Thus the proposed technique could contribute greatly to renewable energy and towards sustainable future.

Study on Condensation of Biomass Pyrolysis Gas by Spray Bio-Droplets

Bio-oil, produced from biomass feedstock like rice hull, straw, wood flour and other biomass wastes by fast pyrolysis where the biomass feedstock is heated and pyrolized with a rapid rate and the pyrolysis gases produced are condensed rapidly, is an interesting potential alternative fuel oil. Cooling rate of the biomass pyrolysis gas is an important factor effecting the production of bio-oil. In order to speed up the cooling rate, the high temperature biomass pyrolysis gas is cooled and condensed by spray droplets of produced bio-oil with low temperature in this paper. It was assumed that the chemical reactions among the components of pyrolysis gas can be ignored, a theoretical model based on the classic film model and the Maxwell-Stefan equation was presented to simulate the heat and mass transfer characteristics of the spray condensation of biomass pyrolysis gas. The effects of the initial pyrolysis gas temperatures, the initial bio-oil droplets temperatures and diameters, and the flow ration of the gas and the liquid droplets on the heat and mass transfer between the gas and the liquid droplets were analyzed by the model.

Numerical Study on Near-Wall Natural Convection Over a Microscale Heating Wire

The study of the natural convection over a very small heat sources is important in the analysis of heat transfer problems in the electronics industry. However, the characteristics of the spatial distribution of the velocity in the near wall region, which is crucial to the mechanisms of heat transfer process in natural convection around a microscale object, is not well understood. In this investigation, the microscale natural convection in the near wall region of a platinum micro heat source was investigated numerically, using FLUENT, a commercially available computational fluid dynamics (CFD) software, and compared with corresponding experimental results. The influence of the nanoparticles on the natural convection was observed using the single-phase or two-phase models available in FLUENT. The temperature and velocity fields were obtained, with which the Brownian diffusion coefficient was deduced. The results indicate that the temperature gradient induced Brownian diffusion and thermophoresis in the near wall region plays an important role in the microscale natural convection in the water/nanoparticle mixture investigated and are in good agreement with the results from a corresponding experimental investigation.

Analysis of USP Laser Induced Ablation Threshold in Transparent Aqueous Tissue

The ultrashort pulsed (USP) laser induced plasma-mediated ablation in transparent media is modeled and studied in this work. We propose that a certain number of free electrons are required to trigger the avalanche ionization for the first time. Based on this assumption, the ablation process is postulated as two separate processes — the multiphoton and avalanche ionizations. For USP laser induced ablation in the transparent corneal epithelium at 800 nm, the critical seed free-electron density and the time to initialize the avalanche ionization for pulse widths from picoseconds down to the femtoseconds range are calculated. It is found that the critical seed free-electron density decreases as the pulse width increases, obeying a tp−5.65 rule. Moreover, this model is also extended to the estimation of crater sizes in USP laser ablation of polydimethylsiloxane (PDMS). The crater sizes ablated in a PDMS by a 900 fs pulsed laser at wavelength 1552 nm are modeled using the present model, and the results match with the existing experimental measurements.

A Study on the Effects of Ceramic Content of CNT / Metal Composite Surface Coatings Fabricated by Cold Spray for Boiling Heat Transfer Enhancement

In this study, the enhancement of boiling heat transfer through surface modification of heat transfer materials by surface coating was investigated. Specifically, the study evaluated how the boiling heat transfer performance of CNT/metal composite coatings, fabricated by the cold gas dynamic spray process, was affected by the addition of different ceramic particles on the raw materials of these surface coatings. These raw materials were composite powders of CNT, Cu powder, and ceramic powders (SiC, AlN, and BN) that were mechanically alloyed using an attrition ball mill. The boiling heat transfer tests were performed in a pool of R134a at a gauge pressure of 2.5 kg/cm2 and liquid temperature of 4.8°C. Test results showed that the addition of ceramic particles in the CNT/Cu composite powders can further augment boiling heat transfer. The augmented boiling heat transfer performance varied with the kind of ceramic powder added and with their quantity. The highest enhancement ratio was 2.57. This was exhibited by a Cu plate specimen that was cold spray coated with (5 vol.% CNT + 95 vol.% Cu) + 20 vol.% AlN composite powder.

Temperature Sensitivity of Nanohole Array Sensors

Small, sensitive temperature sensors are required to develop chip-scale calorimeters for pharmaceutical and related industries. Laser illuminated nanohole array apertures (NHA) that produce extraordinary optical transmission (EOT) perform as temperature sensors and may be suitable for micro-calorimetry. We investigated NHA sensors by an experimental parametric study to determine the most sensitive configuration. Temperature sensitivity of EOT is discussed, and the results suggest that nanohole arrays enable thermal measurements with microscale spatial resolution. The sensing chip is a glass substrate with 105nm thick gold surface, illuminated with a helium–neon laser. 15 different designs were milled in a formation of 3×5 matrix. Each row has a different array size (3×3, 5×5 and 10×10) and each column has a different pitch size varying from 250nm to 450nm in 50nm increments. The aperture size was fixed at 150 nm, thus the overall size of the array varies from 0.65μm×0.65μm to 4.20μm×4.20μm. The highest sensitivity was achieved with 350nm and 400nm pitch sensors and a 10×10 array (up to8% intensity gain per 0.10°C). These conditions correspond to a predicted peak wavelength region with high transmission gradients, due to the transmission maxima, causing higher sensitivity. This behavior was consistent in all array sizes. Results also showed that even the smallest sensors are sensitive to temperature changes, and they suggest a means for designing future NHA sensors to accommodate different light sources and fluids.