Improved Heat Removal Research Articles

Current Rerouting Improves Heat Removal in Few-Layer WSe2 Devices.

Few-layer (FL) transition-metal dichalcogenides have drawn attention for nanoelectronics applications due to their improved mobility, owing to the partial screening of charged impurities at the oxide interface. However, under realistic operating conditions, dissipation leads to self-heating, which is detrimental to electronic and thermal properties. We fabricated a series of FL-WSe2 devices and measured their I-V characteristics, while their temperatures were quantified by Raman thermometry and simulated from first principles. Our tightly integrated electrothermal study shows that Joule heating leads to a significant layer-dependent temperature rise, which affects mobility and alters the flow of current through the stack. This causes the temperatures in the top layers to increase dramatically, degrading their mobility and causing the current to reroute to the bottom of the FL stack where thermal conductance is higher. We discover that this current rerouting phenomenon improves heat removal because the current flows through layers closer to the substrate, limiting the severity of self-heating and its impact on carrier mobility. We also observe significant lateral heat removal via the contacts because of longer thermal healing length in the top layers and explore the optimum number of layers to maximize mobility in FL devices. Our study will impact future device designs and lead to further improvements in thermal management in van der Waals (vdW)-based devices.

Evaluation of graphene based nano fluids with minimum quantity lubrication in turning of AISI D3 steel

Minimum quantity lubrication is an extensively accepted lubrication system owing to its improved heat removal in the machining areas. The purpose of the current study is to examine the effectiveness of graphene based nanofluids in turning on D3 tool steel with minimum quantity lubrication. Experiments were executed with Taguchi’s L27 orthogonal matrix array. The effectiveness were examined with the machining parameters under environment (dry condition, flood lubrication and minimum quantity lubrication), cutting speed, feed rate and depth of cut in terms of surface roughness, cutting temperature, tool wear and chip morphology. A technique known as Preference Ranking Organization Method for Enrichment Evaluations were used to determine the imperative parameters governing the responses. The results reveal that graphene nanofluids with minimum quantity lubrication found to be a right substitute in reducing the surface roughness, tool wear and cutting temperature.

Optimal mechanical and thermal architecture of high-conductivity inserts for cooling an electronic piece

Highly conductive inserts improve heat removal from electronic components remarkably; however, using these inserts is associated with a side effect: the mechanical strength deterioration of the piece. In this research, based on the constructal theory, the architecture of these inserts is optimized in such a way that the lowest maximum temperature and highest mechanical strength of the disc-shaped piece are achieved. Here, three patterns for the configuration of inserts embedded in the disc are studied, namely radial, dendritic with one level of pairing and dendritic with two levels of pairing. The total amount of high-conductivity inserts material is fixed, while the geometrical parameters of the inserts like length, the bifurcating locations and the angles of bifurcations are considered as degrees of freedom. It was observed that when 16 inserts were invoked, maximum temperature and stress were minimized. Results showed that changing the structure from radial to dendritic with one bifurcation causes a decrease in the maximum temperature and stress of about 17% and 3.2%, respectively. Also, it was seen that increasing the complexity of the structure by increasing the number of bifurcations from one to two leads to 5.6% reduction and 1% increment in minimal maximum temperature and optimal maximum stress, respectively.

Comparing thermal comfort and air quality performance of two active chilled beam systems in an open-plan office

The traditional air distribution and supply devices in ventilated rooms are not always able to effectively remove excess heat from the space. Therefore, chilled beams, especially the active systems, are used to achieve the desired cooling demand. The focus of this paper was the potential benefit of a newly designed active chilled beam (ACB) system, to improve heat removal effectiveness local thermal condition and indoor air quality in the occupants’ breathing zone. The system based on 1-way flow design (1W-ACB) was installed in an open-plan office and its performance was studied by analysing the temperatures, velocities and tracer gas concentrations in predetermined risky zones. The system was compared against a traditional 4-way flow design (4W-ACB).The obtained results showed that heat removal effectiveness was slightly higher for the 1W-ACB system compared to the 4W-ACB system. The local thermal condition was very good close to the workstations when using 1W-ACB. The benefits of the new system were also shown in the occupied zone by analysing the mean age of air and air-change effectiveness (ACE) in the breathing level at the workstation locations. The 1W-ACB system provided air with lower mean age (fresher air), and therefore higher ACE, near the breathing zone at the workstations compared to the 4W-ACB. On the other hand, the 4W-ACB system had the advantage of providing high thermal and mean age of air uniformity throughout the room.

Waterproof Perovskite-Hexagonal Boron Nitride Hybrid Nanolasers with Low Lasing Thresholds and High Operating Temperature

Solid-state perovskites have recently emerged as promising coherent light sources, due to their efficient gain. While the continuous-wave (CW) optically pumped perovskite laser has been achieved at low temperatures, the final frontier of an electrical perovskite-based laser diode remains challenging due to the heat management and intrinsic instability of perovskite materials. Here, we demonstrate waterproof perovskite-hexagonal boron nitride (hBN) hybrid nanolasers with low lasing thresholds and high operating temperature. After capping with the hBN flake, which possesses superb and anisotropic thermal conductivity, heat dissipation of the perovskite nanolaser is accelerated and an overheated hot spot is avoided. This results in the significant reduction of lasing thresholds (17.05% to 60.15%) in 16 measured samples and clear lasing behavior under a temperature as high as 75.6 °C. Moreover, hBN with high environmental stability can effectively protect the perovskite from the polar solvents. The hBN encapsulated CsPbI3 nanolaser can incessantly lase in water for an hour, and the lasing behavior can be retained even after 24 h of immersion in water. The reduction of lasing threshold, improved heat removal, and higher temperature tolerance of the hybrid structure nanolaser marks a major step toward a CW-pumped perovskite laser at room temperature, while also allowing perovskites to be integrated into high power density optoelectronic devices and future electrically driven lasers. In addition, as the sandwiching of perovskites with hBN can withstand polar solvents, this method may serve as a reliable method for both the future fabrication of perovskite-based devices and sensor-based applications in solvent systems.

Segmented Fuel Cell for Verification of Variable Temperature Flow Field Concept

Variable temperature flow field concept allows maintaining close to 100% relative humidity along the entire flow field of the anode and the cathode side without external humidification. In previously reported research the concept of variable temperature was successfully applied to the cathode channel only. However, with appropriate temperature and flow distribution it may be possible to achieve water transfer to the anode side and keep hydrogen close to 100% relative humidity too. In order to prove this an interactive combination of CFD modeling and experimental verification is applied. For experimental verification a 5-segment fuel cell is designed, constructed and tested. It consists of 5 linearly placed molded graphite segments 40×40×5 mm, insulated electrically and thermally from the neighboring segments by a polycarbonate matrix. The five parallel, straight channels with rectangular 1×1 mm cross-section are machined in the polycarbonate matrix and the molded graphite plates with 1 mm spacing, thus forming a 250 mm long and 10 mm wide flow field (25 cm2). The molded graphite segments are electrically connected in parallel by copper current collectors. The connecting rods of the current collectors are of equal length in order to achieve similar ohmic losses of each segment. Current shunts are soldered on each connecting rod and calibrated to enable spatially resolved in-situ current density monitoring on each segment. Peltier thermoelements are individually attached to each segment of copper current collector. Each Peltier thermoelement has an attached aluminium heat sink and a fan for improved heat removal rates. The temperature of each Peltier thermoelement can be prescribed independently via in-house built PID control in LabVIEW®software. In order to ensure equal pressure force on each segment, thus equal contact resistance, specially designed pressure rods are installed which also serve to fix each Peltier thermoelement, heat sink and the fan to the construction. The structural components are fixed between two stainless steel end plates, actually frames, with 12 nuts and bolts. Figure 1 shows the exploded view of the segmented fuel cell. Relative humidity and temperature sensors are placed between the neighboring segments in order to enable insertion of specially designed fittings with RH and temperature sensors directly inside the reactant channels. The overall experimental setup assembly enables: i) measurement of the relative humidity and temperature profiles directly inside the channels before and after each segment in total of 12 points, 6 along each side, ii) measurements of temperature on each segmented current collector terminal, total of 10 measurements, 5 on each side of the cell, iii) measurements of the current density distribution along the cell via 5 current shunts, iv) regulation of the temperature of each segment separately along the anode and cathode side via total of 10 Peltier thermoelements driven by the in-house built relay PID control, 5 on each side of the cell, v) establishment of desired temperature distribution along the cell to the actual water vapor saturation temperature profile, vi) calculation of net local water transport through the membrane for each segment. The segmented fuel cell is tested at several operating conditions, namely at isothermal conditions 60°C with fully and partially humidified reactants, and with a prescribed variable temperature flow field with dry reactants. Air and hydrogen are in counterflow. Very good agreement between modelled and experimentally obtained data is achieved at all operating conditions. The results indicate that it is possible to achieve 100% RH along the entire flow field with dry reactants on both sides at the cell inlet. Such operation along the cathode side can be achieved by prescribing the variable temperature flow field profile extracted from Mollier's h-x chart along the cathode side. By selecting an appropriate membrane thickness, thereby manipulating the back-diffusion water flux, it is also possible to humidify hydrogen on the anode side. The first segment on each side, where the dry reactants enter, is humidified from the opposite side of the membrane, keeping the membrane hydrated in those regions as well. Figure 1

Optimized SESAMs for kilowatt-level ultrafast lasers.

We present a thorough investigation of surface deformation and thermal properties of high-damage threshold large-area semiconductor saturable absorber mirrors (SESAMs) designed for kilowatt average power laser oscillators. We compare temperature rise, thermal lensing, and surface deformation of standard SESAM samples and substrate-removed SESAMs contacted using different techniques. We demonstrate that for all cases the thermal effects scale linearly with the absorbed power, but the contacting technique critically affects the strength of the temperature rise and the thermal lens of the SESAMs (i.e. the slope of the linear change). Our best SESAMs are fabricated using a novel substrate-transfer direct bonding technique and show excellent surface flatness (with non-measureable radii of curvature (ROC), compared to astigmatic ROCs of up to 10 m for standard SESAMs), order-of-magnitude improved heat removal, and negligible deformation with absorbed power. This is achieved without altering the saturation behavior or the recovery parameters of the samples. These SESAMs will be a key enabling component for the next generation of kilowatt-level ultrafast oscillators.

Optimisation of cavity parameters for lasers based on AlGaInAsP/InP solid solutions

We have studied the effect of laser cavity parameters on the light – current characteristics of lasers based on the AlGaInAs/GaInAsP/InP solid solution system that emit in the spectral range . It has been shown that optimisation of cavity parameters (chip length and front facet reflectivity) allows one to improve heat removal from the laser, without changing other laser characteristics. An increase in the maximum output optical power of the laser by has been demonstrated due to cavity design optimisation.

Manufacturing technology of intra- and interchip interconnects for modern ULSIs: Review and concepts of development

Technological operations involved in manufacturing multilevel ULSI interconnections by the dual damascene (DD) method have been analyzed. Problem issues are considered, including (i) etching of porous intralevel insulation with ultralow values of dielectric constant, (ii) application of metal barrier and seed layers onto the porous surface of etched dielectric spacers, in which copper conductors have to be formed, (iii) low mechanical strength of porous dielectric spacers between copper conductors, (iv) insufficient stability of copper conductors with respect to failures as a result of electromigration, and (v) rapid growth in the resistivity of copper conductors with decreasing width, and limited possibility of compensating this factor by increasing conductor height (thickness). Possible approaches to solving these problems are proposed. According to one of these, horizontal copper conductors are formed first and then a porous dielectric is incorporated into the gaps between conductors by the method of chemical sol-gel deposition from solution by spin-on-dielectric (SOD) method on a centrifuge. The horizontal conductors can then be formed by the standard single-damascene process of copper deposition into temporal mask that is subsequently removed and replaced by the porous dielectric. According to another method, copper conductors are formed by local electrochemical deposition on the wafer surface that is preliminary covered by the barrier layer (BL), seed layer (SL), and a temporal mask, in which a pattern of conductors is prepared by etching down to the SL surface. In this case, the deposited copper conductors initially possess a textured crystalline structure, which facilitates the formation of elongated crystallites during subsequent thermal recrystallization. The surface of conductors can be also covered by local electrochemical process with an electroless barrier layer (e.g., of CoWP). Then, the BL and SL are removed and a porous ultralow-dielectric-constant insulator is incorporated into the gaps between copper conductors. The next stage of forming interconnections in multicrystal ULSIs consists of a three-dimensional (3D) vertical assembly of thinned chips by connecting them with copper conductors formed using through substrate via (TSV) technology. In order to improve heat removal from the assembly and increase its mechanical strength, the filling of vertical through channels in chips by copper is supplemented by copper plating of side edges and the formation of microbumps on crystal surfaces.

Metal Cover에 의한 EMC 분석 및 Noise 저감 방법

The ECU which is operating inductive actuator like motor or solenoid should be designed with considering the heat removal performance and the EMC performance. In most cases, these two performances are trade-off. Especially, the metal cover with plastic housing for improving heat removal performance can affect EMC performance by coupling the noise source to harness cable and connector. Therefore, after analyzing the EMC effectiveness of the metal cover, countermeasures for EMC noise reduction should be established. In this paper, by simulating and testing of AHB Gen3 ECU, the influence of the metal cover to EMC performance is analyzed. And based on this result, we propose solutions for EMC noise reduction.

Dimerization of ethene in a fluidized bed reactor using Ni-based Supported Ionic Liquid Phase (SILP) catalysts

Ethene dimerization to 2-butenes was catalyzed by Ni-SILP complexes for the first time in a fluidized bed to improve heat removal.

Finite Element Modeling in 3D of the Impact of Superfluid Helium Filled Micro-Channels on the Heat Transfer through LHC Type Cable Insulation

For a future luminosity upgrade of CERN's Large Hadron Collider, a drastically improved heat removal in the inner triplet quadrupole magnets is required. One of the necessary improvements involves the cable insulation. A porous all-polyimide insulation scheme has been proposed recently. Essentially the insulation features a network of micro channels filled with superfluid helium that significantly increases the heat transfer through the insulation layer. A three dimensional Finite Element model required to simulate and study the enhanced heat transfer through the micro channels is presented here. The thermal coupling between heated cable and helium as well as the heat flux through the micro-channels are investigated. The model is validated by comparison of results with published measured data. Finally a sensitivity analysis is performed concerning the stability of the cables in magnet windings.

Influence of airflow intensity on phytase production by solid-state fermentation

Phytase production by Aspergillus niger F3 by solid state fermentation (SSF) on citrus peel was evaluated at pilot scale under different aeration conditions. The best airflow intensity was 1VkgM (Lair kgmedium−1min−1), which allowed to produce 65units of phytases per gram in dry basis (65Ug−1d.b.) as it removed the metabolic heat generated by the microorganism, Agitation did not improve heat removal. Airflow intensity was considered as scale-up criterion. When the airflow intensity was maintained at 1VkgM for SSF with 2 and 20kg of medium, the kinetics parameters for biomass and enzyme concentration at the end of fermentation differed by less than 2. The air flow intensity was required to maintain the temperature and cool the SSF and to provide oxygen for microbial growth. Air flow intensity is a key a factor that must be considered when scale-up of SSF is attempted.

Study of a novel cathode tool structure for improving heat removal in electrochemical micro-machining

In this paper, the effects of a hollow structure of cathode tool and radial flow of electrolyte on heat removal are studied using COMSOL software. A multi-physics (electrical field, flow field and heat transfer) model, which implement the bilateral interactions with customized PDE, is proposed to simulate the EMM process with a moving cathode tool. The ALE method is used to track the moving interface. The simulations show that the electrolyte flow rate and flushing time are the most important factors for the heat removal. A hollow structure of the cathode tool is proposed to improve the stability of flow rate, which will remove the heat generated during the EMM process effectively.

Effects of Spray Angle on Spray Cooling of Extruded Aluminum Alloy Plate

Spray cooling technology relying on phase-change mechanism has a notable ability to remove high heat fluxes. Spray cooling can be used to transfer large amounts of energy through the latent heat of evaporation. Heat transfer rates much higher than can be attained in pool boiling are possible with spray cooling since the vapor can be removed from the surface more easily. Therefore, spray cooling has a number of applications in the various fields such as continuous casting process, electronics thermal control and metal quenching process which require a high flux cooling technique [1–3]. One important area which would benefit from spray cooling to improve heat removal is press quenching of extruded aluminum alloy plate.

Novel Ways of Heat Removal from Highly Irradiated Superconducting Windings in Accelerator Magnets

Novel ideas of heat removal from superconducting windings in accelerator type magnets are investigated with the help of a recently developed and validated thermal model of a magnet cold mass implemented in COMSOL Multiphysics. Here the focus is on how to improve heat removal from the midplane of a superconducting quadrupole magnet, the area exposed to the highest radiation heat load. In addition, this part of the coil windings has the longest thermal path towards the heat sink and several thermal design improvements proposed in the past are not very effective here. It is shown that with minor changes in the geometrical design, the cooling of the midplane conductors can be strongly increased

Effect of natural convection on the current-voltage characteristic of a DC discharge in neon at intermediate pressures

The parameters of the positive column of a glow discharge in neon are calculated with allowance for the induced hydrodynamic motion. It is shown that natural convection in the pressure range of ∼0.1 atm significantly affects the profiles of the parameters of the positive column and its current-voltage characteristic. The convection arising at large deposited energies improves heat removal, due to which the temperature in the central region of the discharge becomes lower than that calculated from the heat conduction equation. As a result, the current-voltage characteristic is shifted. With allowance for convection, the current-voltage characteristic changes at currents much lower than the critical current at which a transition into the constricted state is observed. This change is uniquely related to the Rayleigh number in the discharge. Thus, a simplified analysis of thermal conduction and diffusion, even with detailed account of kinetic processes occurring in the positive column, does not allow one to accurately calculate the current-voltage characteristic and other discharge parameters at intermediate gas pressures.

Optimizing CMOS technology for maximum performance

Since power dissipation is becoming a dominant limitation on the continued improvement of CMOS technology, technologists must understand the best way to design transistors in the presence of power constraints. The primary objective is to obtain as much performance as possible for a fixed amount of power, and it is chip performance, not device performance, that matters. In order to investigate this regime, we have captured in simplified models the basic elements for determining chip performance, including intrinsic transistor characteristics, circuit delay, tolerance issues, basic microprocessor composition, and power dissipation and heat removal considerations. These models have been assembled in a processor-level technology-optimization program to study the characteristics of optimal technology across many generations of CMOS. The results that are presented elucidate the limits of future CMOS technology improvements, the optimal energy consumption conditions, and the relative benefits of various proposed technology enhancements, including high-k gate insulators, metal gates, high-mobility semiconductors, improved heat removal, and the use of multiple layers of circuitry.

High-efficiency AlGaInP thin-film LEDs using surface-texturing and waferbonding with conductive epoxy

In thin-film light-emitting diodes (LEDs) absorption of the generated light in the substrate is avoided by the transfer of the epitaxial LED layers on a reflecting carrier. The extraction efficiency can be further enhanced by texturing the top surface. We developed a structure for high-efficiency surface-textured thin-film AlGaInP LEDs with improved heat dissipation by waferbonding onto a carrier substrate using conductive epoxy. The improved heat removal toward the substrate through the conductive epoxy increases the maximum allowable current density by more than a factor of two, compared to LEDs bonded on the carrier substrate with benzocyclobutene. As a result, an optical power of 65 mW is achieved for unencapsulated devices emitting at 650 nm. Using this technology, we also demonstrate an external quantum efficiency of 43% for unencapsulated LEDs at a peak wavelength of 650 nm, which is the highest reported efficiency for AlGaInP thin-film LEDs. The external quantum efficiency increases to 52% at 100 K.

Fischer–Tropsch synthesis on monolithic catalysts with oil circulation

Monolithic catalysts made of cordierite and γ-Al 2O 3 have been prepared and tested for the Fischer–Tropsch (FT) synthesis. When operated without oil circulation, washcoated cordierite monoliths have previously been shown to be as active and selective as the corresponding powder catalyst provided that the monoliths have low washcoat loadings. Two-phase operation, i.e. with oil/product circulation during reaction, resulted in improved heat removal and temperature control, in lower apparent activity and faster deactivation, but the C 5+ selectivity was equal to or even better than without oil circulation. The lower apparent activities obtained with oil circulation seem to be a combination of catalyst deactivation and flow-related problems in the present experimental set-up.