Both silicon (Si) and silicon carbide (SiC) are promising materials used in nano-electro-mechanical system (NEMS), however, the understanding on its phonon heat conduction is rare, which restrict the performance improvement of NEMS. Moreover, the effects of the interface between crystals, which could significantly impact the phonon transport, on heat conduction are not sufficient in the existing publication pool. In this paper, two systems, Si/Si and Si/SiC, are simulated at different temperatures and temperature differences using molecular dynamics simulation and the results were analyzed. The temperature of Si inside Si/SiC system was set at 280K, and the temperatures of SiC were set as a certain absolute value based on temperature difference setting. Meanwhile, 6 groups of temperature difference are applied as simulated conditions. In addition, simulated results from Si/Si system are also applied in comparative analysis as a reference group. The results suggested that the existence of the interface of Si/SiC system would reduce the capability of heat conduction compared to the heat conduction of Si/Si and reverse temperature differences are discovered. When the average temperature is higher than 280K, the heat conduction rate of Si/SiC system is higher than that of Si/Si system initially and as the temperature differences between crystals increases to 60.90K, the heat conduction rate of Si/Si system is higher than that of Si/SiC system. Similar conclusion can also be obtained when the average temperature is lower than 280K. This work provides an open opportunity to study the effect of interface on phonon heat conduction between crystals at typical temperature differences and average temperatures.

In plant living tissue, water can flow across cells by different paths, through cell membranes (transcellular path) and plasmodesmata (symplastic path), or through the continuous cell walls matrix (apoplastic path). The relative contribution of these three paths in living tissue is currently unclear and could vary according to species, tissue developmental stage or physiological conditions. Experiments suggested that apoplastic water movement predominates during transpiration. The objective of this study was to investigate the hydraulic process of cellulose cell wall pathway. The effective pore diameter for water flow through the primary wall matrix is between 2 and 20nm. Inside the cell wall polymer porous, there exist hydrophilic/hydrophobic crystal surfaces based on structure anisotropic. Besides, hydrogen bonding and electrostatic interaction and van der Waals (vdW) dispersion force play an important role in water transport inside the Nano cellulose porous. Therefore, the molecular dynamics simulation was applied to reveal the molecular mechanism of surface boundary effect together with various driving force during water passing through cellulose cell wall matrix Nano channel.

With the rapid development of the supersonic aircraft technology, tremendously, the aircraft Mach numbers get higher and higher, but on the other hand, the working condition become worse and worse. The photonic crystal material which is formed by the periodic micro/nanoscale structures can generate the photonic band gaps, and the photonic band gaps could reflect the energy of the electromagnetic wave effectively. Consequently, the photonic crystal material turns into the newly-developing hotspot on the field of thermal protection for the supersonic aircraft. In this paper, the aircraft states of Mach 6 are set as the target operating condition, and 5 optimum proposals are presented for the structures of typical photonic crystal material. The energy which gets into the body material is calculated; Based on the theory of the electromagnetic field, using the method of transmission matrix and Plane Wave Expansion (PWE), the characteristics of the photonic band gaps for one-and-three dimensional photonic crystals are calculated. Finally, the characteristics of the photonic band gaps are discussed, and optimal design for the performance of the photonic crystal material thermal protection are proposed.

Thermophotovoltaic (TPV) energy conversion enables millimeter scale power generation required for portable microelectronics, robotics, etc. In a TPV system, a heat source heats a selective emitter to incandescence, the radiation from which is incident on a low bandgap TPV cell. The selective emitter tailors the photonic density of states to produce spectrally confined selective emission of light matching the bandgap of the photovoltaic cell, enabling high heat-to-electricity conversion efficiency. The selective emitter requires: thermal stability at high-temperatures for long operational lifetimes, simple and relatively low-cost fabrication, as well as spectrally selective emission over a large uniform area. Generally, the selective emission can either originate from the natural material properties, such as in ytterbia or erbia emitters, or can be engineered through microstructuring. Our approach, the 2D photonic crystal fabricated in refractory metals, offers high spectral selectivity and high-temperature stability while being fabricated by standard semiconductor processes. In this work, we present a brief comparison of TPV system efficiencies using these different emitter technologies. We then focus on the design, fabrication, and characterization of our current 2D photonic crystal, which is a square lattice of cylindrical holes fabricated in a refractory metal substrate. The spectral performance and thermal stability of the fabricated photonic crystal thermal emitters are demonstrated and the efficiency gain of our model TPV system is characterized.

The increasing power demands of portable electronics and micro robotics has driven recent interest in millimeter-scale microgenerators. Many technologies (fuel cells, Stirling, thermoelectric, etc.) that potentially enable a portable hydrocarbon microgenerator are under active investigation. Hydrocarbon fuels have specific energies fifty times those of batteries, thus even a relatively inefficient generator can exceed the specific energy of batteries. We proposed, designed, and demonstrated a first-of-a-kind millimeter-scale thermophotovoltaic (TPV) system with a photonic crystal emitter. In a TPV system, combustion heats an emitter to incandescence and the resulting thermal radiation is converted to electricity by photovoltaic cells. Our approach uses a moderate temperature (1000–1200°C) metallic microburner coupled to a high emissivity, high selectivity photonic crystal selective emitter and low bandgap PV cells. This approach is predicted to be capable of up to 30% efficient fuel-to-electricity conversion within a millimeter-scale form factor. We have performed a robust experimental demonstration that validates the theoretical framework and the key system components, and present our results in the context of a TPV microgenerator. Although considerable technological barriers need to be overcome to realize a TPV microgenerator, we predict that 700–900 Wh/kg is possible with the current technology.

Silicon films of thickness near and below one micrometer play a central role in many advanced technologies for computation and energy conversion. Numerous data on the thermal conductivity of silicon thin films are available in the literature, but mainly for the in-plane thermal conductivity of polycrystalline and single-crystal films. Here we use picosecond time-domain thermoreflectance (TDTR), transmission electron microscopy, and phonon transport theory to investigate heat conduction normal to polycrystalline silicon films on diamond substrates. The data agree with predictions that account for the coupled effects of phonon scattering on film boundaries and defects concentrated near grain boundaries. Using the data and the model, we estimate the polysilicon-diamond interface resistance to be 6.5–8 m 2 K GW −1 .

We present in this paper numerical results of the thermal radiative properties of a two-dimensional rectangular SiC grating atop a photonic crystal, which is capable of exciting different resonant modes within a specified spectral band. We demonstrate that diffraction of waves in both extended directions of the two-dimensional grating, even for s -polarized waves, could efficiently give rise to excitation of surface phonon polaritons, which is responsible for large enhancement of thermal emission from the structure. However, the emission curves reveal different angle and geometry dependence for s - and p -polarized waves. Furthermore, due to the interaction of different modes, quasi-diffuse characteristics of thermal radiation from the proposed structure can be found for both polarizations.

An ab initio molecular dynamics study of femtosecond laser processing of germanium is presented in this paper. The method based on the finite temperature density functional theory is adopted to probe the nanostructure change, thermal motion of the atoms, dynamic property of the velocity autocorrelation, and the vibrational density of states. Starting from a cubic system at room temperature (300 K ) containing 64 germanium atoms with an ordered arrangement of 1.132 nm in each dimension, the femtosecond laser processing is simulated by imposing the Nose Hoover thermostat to the electron subsystem lasting for ∼100 fs and continuing with microcanonical ensemble simulation of ∼200 fs . The simulation results show solid, liquid and gas phases of germanium under adjusted intensities of the femtosecond laser irradiation. We find the irradiated germanium distinguishes from the usual germanium crystal by analyzing their melting and dynamic properties.

Solar thermophotovoltaic (STPV) devices provide conversion of solar energy to electrical energy through the use of an intermediate absorber/emitter module, which converts the broad solar spectrum to a tailored spectrum that is emitted towards a photovoltaic cell [1]. While the use of an absorber/emitter device could potentially overcome the Shockley-Queisser limit of photovoltaic conversion [2], it also increases the number of heat loss mechanisms. One of the most prohibitive aspects of STPV conversion is the thermal transfer efficiency, which is a measure of how well solar energy is delivered to the emitter. Although reported thermophotovoltaic efficiencies (thermal to electric) have exceeded 10% [3], [4], previously measured STPV conversion efficiencies are below 1% [5], [6], [7]. In this work, we present the design and characterization of a nanostructured absorber for use in a planar STPV device with a high emitter-to-absorber area ratio. We used a process for spatially-selective growth of vertically aligned multi-walled carbon nanotube (MWCNT) forests on highly reflective, smooth tungsten (W) surfaces. We implemented these MWCNT/W absorbers in a TPV system with a one-dimensional photonic crystal emitter, which was spectrally paired with a low bandgap PV cell. A high fidelity, system-level model of the radiative transfer in the device was experimentally validated and used to optimize the absorber surface geometry. For an operating temperature of approximately 1200 K, we experimentally demonstrated a 100% increase in overall STPV efficiency using a 4 to 1 emitter-to-absorber area ratio (relative to a 1 to 1 area ratio), due to improved thermal transfer efficiency. By further increasing the solar concentration incident on the absorber surface, increased emitter-to-absorber area ratios will improve both thermal transfer and overall efficiencies for these planar devices.

Photonic crystals is a kind of material with band gap, the photons in which frequencies are in the band gap cannot propagate in a structure. Band gap characteristics for an innovative high temperature thermal control structure, integrated with PCs are presented by numerical simulation based on electromagnetic theory. Focused on 3D PCs structure, the plane wave expansion method is applied, and effects of ball radius and dielectric constant on the band gap characteristics are simulated. In addition, the influencing trend of the bandwidth is also discussed.

Recently, several significant progresses have been made on the studies of extracellular and intracellular ice formation based on high-speed camera and cryomicroscope. This experimental methodology could accurately capture rapid formation process of ice crystals at micro-scale. In this paper, we are dedicated to quantify and comparatively investigate the growth rate and morphology of ice crystals growing in DMSO, sucrose and trehalose, respectively via high-speed camera and cryo-microscope. Several impact factors such as the concentration of cryoprotectants and the cooling rate have been investigated. The results indicate that the species and concentration of cryoprotectants and the cooling rate could significantly affect the growth rate and morphology of ice crystals. DMSO is better than trehalose and sucrose as cryoprotectant because of its molecular structure. This work may enhance current understanding of the factors for ice crystals formation and help optimize the cryopreservation process in the near future.

Polyalcohols such as neopentyl glycol (NPG) undergo solid-state crystal transformations that absorb/release sufficient latent heat. These solid-solid phase change materials (PCM) can be used in practical thermal management applications without concerns about liquid leakage and thermal expansion during phase transition. In this paper, microcapsules of NPG encapsulated in silica shell were successfully synthesized with the use of the emulsion technique. The size of the microcapsules was in the range of 0.2–4 μm, and the thickness of the silica shell was about 30 nm. It was found that the endothermic event of the phase change behavior of these NPG-silica microcapsules was initiated at around 39 °C and the latent heat was about 96.0 J/g. A large supercooling of about 43.3 °C was observed in the pure NPG particles without shell. The supercooling of the NPG microcapsules can be reduced to about 14 °C due to the heterogeneous nucleation sites provided by the silica shell. These NPG microcapsules were added into heat transfer fluid PAO to enhance its heat capacity. The effective heat capacity of the fluids can be increased by 56% by adding 20 wt. % NPG-silica microcapsules.

This study investigates the infiltration of water in ZSM-5 zeolite crystals via molecular dynamics simulations and experiments. A zeolite nano-crystal is constructed in the simulations and is surrounded by water molecules which enter and saturate the pores. The average number of water molecules per unit cell of the zeolite is determined along with the radial distribution function of water inside the zeolites. A geometric approximation of the zeolite pores and intersections is proposed and verified. Partial charge on the zeolite atoms is found to be a crucial parameter which governs the water infiltration behavior. ZSM-5 zeolite crystals were also synthesized and water infiltration experiments were conducted using an Instron. The simulation and experimental findings are compared and discussed. The understanding gained from these studies will be important for the development of zeolite based reverse osmosis membranes for water desalination.

Emulsification is an important process in various fields including foods, pharmaceuticals, cosmetics, and chemicals. Emulsification operation is commonly conducted using conventional emulsification devices, such as high-speed blenders, colloid mills, high-pressure homogenizers, and ultrasonic homogenizers. However, these emulsification devices result in the production of polydisperse emulsions with wide droplet size distributions and poor controllability in droplet size and its distribution. In contrast, monodisperse emulsions consisting of monosize droplets have received a great deal of attentions over the past decade due to their high-tech applications, e.g., monosize microparticles as spacers for electronic devices and monosize micro-carriers for drug delivery systems (DDS). Our group proposed microchannel (MC) emulsification as a promising technique to produce monodisperse emulsions in the mid 1990s. Micro/Nanochannel (MNC) emulsification enables generating monosize droplets with the smallest coefficient of variation (CV) of below 5% using MC and nanochannel (NC) arrays of unique geometry. The resultant droplet size, which ranged from 0.5 to 200 μm, can be precisely controlled by channel geometry. Droplet generation for MNC emulsification is very mild and does not require any external shear stress; a dispersed phase that passed through channels is transformed spontaneously into monosize droplets inside a continuous-phase domain. The aim of this paper is to present recent developments in MNC emulsification chips, particularly focusing on asymmetric straight-through MC arrays for large-scale production of monodisperse emulsions. Asymmetric straight-through MC array chips were fabricated using a silicon-on-insulator wafer. Numerous asymmetric straight-through MCs each consisting of a microslot and a narrow MC were positioned in the central region of the chip. Monosize droplets were stably generated via asymmetric straight-through MCs at high production rates. Below a critical droplet production rate, monosize droplets were generated via asymmetric straight-through MCs, with droplet size and size distribution independent of the droplet productivity. The use of a large asymmetric straight-through MC array chip achieved the mass production of monosize tetradecane oil droplets at ∼1 L/h. The simulation results using CFD (computational fluid dynamics) agreed well with the experimental results and provided useful information, such as the movement of the oil-water interface during droplet generation. Monosize submicron droplets were also obtained using NC emulsification chips made of single-crystal silicon.

Spectral and directional control of thermal emission holds substantial importance in different kinds of applications, where heat transfer is predominantly by thermal radiation. Several configurations have previously been proposed, like using gratings, photonic crystals, and resonant cavities. In the present work, we theoretically investigate the influence of periodic microstructures such as micro-scale gratings and photonic crystals on the thermal radiative properties of a structure constituted with these periodic microstructures. The enhanced thermal emission is found to be due to different excitation modes and the coupling between them. In order to offer insight into the mechanisms, we calculate and visualize the electromagnetic field profile at specified emission peaks. Furthermore, the emissivity pattern is calculated as a function of the emission angle and the angular frequency. The results reveal detailed spectral and directional dependence, and omnidirectional feature of thermal emission from the proposed structure. We show that it is possible to flexibly control the emission behavior by adjusting the structure dimensional parameters properly.

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.

The present numerical work describes the simulation and analysis of the absorptance and absorption efficiency of a solar cell, where the effect of utilizing photonic crystals as an active material of the cell was studied. The study was performed by numerical simulations using a computational code based on the Finite Element Method [1]. The results were obtained for photonic crystals with periodicity in both one and two dimensions [2]. In the first one, periodicity, thickness of the active material, and distance with respect to the electrode for hole collection were varied, and two organic materials for the active zone were tested, P3HT:PCBM and TDPT:PCBM. In the case of crystals with periodicity in two dimensions, only the period in one of the two dimensions was varied, based on the cell with the highest efficiency of absorption proposed for cells with periodic photonic crystals in one dimension. All simulations were obtained for waves with TM polarization, zero angle of incidence and wavelengths between 400 and 700 nm.

In this study we introduce our numerical and experimental works for the thermal conductivity reduction by using a porous material. Recently thermal conductivity reduction has been one of the key technologies to enhance the figure of merit (ZT) of a thermoelectric material. We carry out numerical calculations of heat conduction in porous materials, such as, phonon Boltzmann transport (BTE), molecular dynamics simulations (MD), in order to investigate the mechanism of the thermal conductivity reduction of a porous material. In the BTE, we applied the periodic boundary conditions with constant heat flux to calculate the effective thermal conductivity of porous materials. In the MD simulation, we calculated phonon properties of Si by using the Stillinger-Weber potential at constant temperature with periodic boundary conditions in the x , y and z directions. Phonon dispersion curves of single crystal of Si calculated from MD results by time-space 2D FFT are agreed well with reference data. Moreover, the effects of nano-porous structures on both the phonon group velocity and the phonon density of states (DOS) are discussed. At last, we made a porous p-type Bi 2 Te 3 by nano-particles prepared by a beads milling method. The thermal conductivity is one-fifth of that of a bulk material as well as keeping the same Seebeck coefficient as the bulk value. However electrical conductivity was much reduced, and the ZT was only 0.048.

Investigating the factors influencing the characteristics of intracellular ice formation (IIF) is of critical importance for cryopreservation and cryosurgery techniques. However, for the detection of the size of intracellular ice crystals, ∼10nm-0.1μm, there exist serious technical and theoretical difficulties. In this study, a cryomicroscopic method was established to measure the size of intracellular ice crystals in mouse oocytes during their warming processes by investigating the melting point depression of the intracellular ice crystals from extracellular ones. Using the Gibbs-Thomson relation, the size of intracellular ice crystals was calculated and the results range from 4–28 nm, when the molality of the extracellular ethylene glycol and NaCl ranges from 0 to 4 m and 0.15 to 0.6 m , respectively, and the cooling rate is 100K/min.

Atomization has been widely applied in pulmonary drug delivery as a promising technology to transport drug formulations directly to the respiratory tract in the form of inhaled particles or droplets. Because of the targeted treatment, the drug can be delivered directly to the site of inflammation, thus the need for systemic exposure and the possibility of side effects are both reduced. Therefore pulmonary drug delivery has significant advantages over other methods in the treatment of respiratory diseases such as asthma. The most common atomization methods employed in pulmonary drug delivery are jet atomization and ultrasonic atomization. However, the difficulty is in producing monodispersed particles/droplets in a size range of 1–5 micron meter in diameter, necessary for deposition in the targeted lung area or lower respiratory airways, within a controllable fashion. In this paper, we demonstrate surface acoustic wave (SAW) atomization as an efficient technique to generate monodispersed aerosol to produce the required size distribution. The SAW atomizer is made of a 127.86 Y-X rotated single-crystal lithium niobate piezoelectric substrate, which is patterned with chromium-aluminum interdigital transducer (IDT) electrodes via UV lithography. When an alternating electric field is applied onto lithium niobate substrate through the IDT, a SAW, propagating across substrate surface with ten nanometer order amplitudes, is generated. When the SAW meets the liquid which is placed upon substrate, the acoustic energy carried by the wave induces atomization of the working fluid, which contains salbutamol as a model drug. In order to measure the size distribution of the atomized droplets, two methods are used. One is the laser diffraction based Spraytec technique and the other is an in-vitro lung modelthe one stage glass twin impinger. The former revealed that the mean diameter of the aerosol atomized was around 3 um which were confirmed by the lung model that demonstrated that nearly 80% of atomized drug aerosol was deposited in the simulated lung area. Moreover, the SAW atomizer only requires 1–3 W driving power, suggesting that it can be miniaturized for portable consumer use.