Small Thermal Resistance Research Articles

High-Current Back-Illuminated Partially Depleted-Absorber p-i-n Photodiode With Depleted Nonabsorbing Region

We demonstrate a high-current back-illuminated InGaAs/InP p-i-n photodiode (PD), whose depleted region comprises partially depleted-absorbing, depleted-absorbing, and depleted-nonabsorbing layers to increase RF power output. The back-illuminated PD has an advantage of small thermal resistance between a photoabsorber and a heat sink for avoiding catastrophic thermal failure. The thermal resistance decreases with an increase in the detecting area; however, it simultaneously increases the capacitance, imposing a limitation on the RF response. To reduce the capacitance, we have incorporated a depleted-nonabsorbing layer into a partially depleted absorber PD structure that photogenerated electrons can drift trough. We have fabricated two samples, one with a detecting area of 50 μm, and the other with a detecting area of 70 μm in diameter. Both PDs show high RF power outputs of 28.7 and 29.0 dBm at a frequency of 5 GHz, and 25.7 and 26.7 dBm at each -3-dB frequency of 10.5 and 7 GHz, respectively.

Thermal properties of carbon nanotube-copper composites for thermal management applications.

Carbon nanotube–copper (CNT/Cu) composites have been successfully synthesized by means of a novel particles-compositing process followed by spark plasma sintering (SPS) technique. The thermal conductivity of the composites was measured by a laser flash technique and theoretical analyzed using an effective medium approach. The experimental results showed that the thermal conductivity unusually decreased after the incorporation of CNTs. Theoretical analyses revealed that the interfacial thermal resistance between the CNTs and the Cu matrix plays a crucial role in determining the thermal conductivity of bulk composites, and only small interfacial thermal resistance can induce a significant degradation in thermal conductivity for CNT/Cu composites. The influence of sintering condition on the thermal conductivity depended on the combined effects of multiple factors, i.e. porosity, CNTs distribution and CNT kinks or twists. The composites sintered at 600°C for 5 min under 50 MPa showed the maximum thermal conductivity. CNT/Cu composites are considered to be a promising material for thermal management applications.

Effect of the angle of inclination of a plate shield on the thermal and hydraulic performance of a plate-fin heat sink

Using CFD software FLUENT, we investigated the effect of the angle of inclination of a plate heat shield on the thermal and hydraulic performance of a plate-fin heat sink. The variation of this angle causes a substantial and complicated variation of the flow field in space both upstream and downstream near such a heat sink. This distinctive behavior modifies the pressure drop between the inlet and outlet of the investigated duct, but that variation influences only slightly the flow field in the space from fin to fin, and thus the thermal resistance of the heat sink. This trend is further smoothed with increasing Reynolds number and height of the heat sink. As a compromise between the demands of small thermal resistance and a small pressure drop, the angle of inclination of a plate heat shield must be chosen carefully.

Reduction of Thermal Resistance for Chip Test Technology by Using Super Thermal Conductivity Material and Mirror Finished Silicon

In recent years, an innovative 3D-integrated circuit has been developed. Furthermore, innovative chip test technology has also been developed whereby the test is conducted without the need to dice the wafer. Chip tests must be performed at various temperatures. The 3D-integrated circuit has a higher heat generation density than a conventional circuit, and so highly efficient cooling technology is required. We selected an indirect spray cooling technology. With indirect spray cooling, there is a material between the heat generating chips and the sprayed coolant. Therefore, to achieve more efficient spray cooling, the thermal resistance of the material and the contact thermal resistance between the material and the chip must be reduced. In this research, we focused on super thermal conductive composites (STC) to reduce the thermal resistance of the material. Measurement results showed that the thermal resistance of STC is very small and the key issue with the spray cooling chip test is the reduction of the contact thermal resistance between the material and the chip surface. We then evaluated the contact thermal resistance of silicon with a mirror-like finish experimentally. We applied a small amount of liquid between silicon wafers with a mirror-like finish and obtained a very small contact thermal resistance of about 0.2 × 10 –4 m 2 K/W.

Comparison of thermal performances of plate-fin and pin-fin heat sinks subject to an impinging flow

In this paper, we compare thermal performances of two types of heat sinks commonly used in the electronic equipment industry: plate-fin and pin-fin heat sinks. In particular, heat sinks subject to an impinging flow are considered. For comparison of the heat sinks, experimental investigations are performed for various flow rates and channel widths. From experimental data, we suggest a model based on the volume averaging approach for predicting the pressure drop and the thermal resistance. By using the model, thermal resistances of the optimized plate-fin and pin-fin heat sinks are compared. Finally, a contour map, which depicts the ratio of the thermal resistances of the optimized plate-fin and pin-fin heat sinks as a function of dimensionless pumping power and dimensionless length, is presented. The contour map indicates that optimized pin-fin heat sinks possess lower thermal resistances than optimized plate-fin heat sinks when dimensionless pumping power is small and the dimensionless length of heat sinks is large. On the contrary, the optimized plate-fin heat sinks have smaller thermal resistances when dimensionless pumping power is large and the dimensionless length of heat sinks is small.

Chaotic behavior of pulsating heat pipes

Chaotic behavior of closed loop pulsating heat pipes (PHPs) was studied. The PHPs were fabricated by capillary tubes with outer and inner diameters of 2.0 and 1.20 mm. FC-72 and deionized water were used as the working fluids. Experiments cover the following data ranges: number of turns of 4, 6, and 9, inclination angles from 5° (near horizontal) to 90° (vertical), charge ratios from 50% to 80%, heating powers from 7.5 to 60.0 W. The nonlinear analysis is based on the recorded time series of temperatures on the evaporation, adiabatic, and condensation sections. The present study confirms that PHPs are deterministic chaotic systems. Autocorrelation functions (ACF) are decreased versus time, indicating prediction ability of the system is finite. Three typical attractor patterns are identified. Hurst exponents are very high, i.e., from 0.85 to 0.95, indicating very strong persistent properties of PHPs. Curves of correlation integral versus radius of hypersphere indicate two linear sections for water PHPs, corresponding to both high frequency, low amplitude, and low frequency, large amplitude oscillations. At small inclination angles near horizontal, correlation dimensions are not uniform at different turns of PHPs. The non-uniformity of correlation dimensions is significantly improved with increases in inclination angles. Effect of inclination angles on the chaotic parameters is complex for FC-72 PHPs, but it is certain that correlation dimensions and Kolmogorov entropies are increased with increases in inclination angles. The optimal charge ratios are about 60–70%, at which correlation dimensions and Kolmogorov entropies are high. The higher the heating power, the larger the correlation dimensions and Kolmogorov entropies are. For most runs, large correlation dimensions and Kolmogorov entropies correspond to small thermal resistances, i.e., better thermal performance, except for FC-72 PHPs at small inclination angles of θ < 15°.

A molecular dynamics study on heat transfer characteristics at the interfaces of alkanethiolate self-assembled monolayer and organic solvent

In this paper, we present molecular dynamics (MD) simulations of interfaces composed of self-assembled monolayers (SAMs) and solvents in order to investigate the heat transfer characteristics at the interface. Two typical normal alkylthiolate SAMs with different chain lengths, i.e., 1-propanethiol C(3)H(7)SH and 1-dodecanethiol (C(12)H(25)SH) chemically adsorbed on Au(111) substrate surfaces, were used, and toluene was adopted as the organic solvent. In addition to the SAM systems, an interface composed of the bare solid substrate and solvent (without SAMs) was analyzed for comparison. Nonequilibrium MD simulations, in which a temperature gradient perpendicular to the interface was imposed, were performed and the difference in thermal boundary resistance in the interface systems was discussed. We observed that the SAM interfaces have smaller thermal resistance when compared with that of the bare solid interface. In order to understand the mechanisms of the small resistance at the SAM-solvent interfaces, the vibrational character of molecules in each phase, which contacted each other at the interface was analyzed and a detailed adsorbed structure of solvent molecule in the interface region was extracted. As a result, a clear difference in these characters was found between the SAM interfaces and bare solid interface.

Thermal Shunts in Thermoelectric Energy Scavengers

The necessity for an additional component in thermoelectric generators and low-dimensional thermopiles for improving the efficiency of energy scavengers is discussed. Energy scavengers usually work at a small temperature difference and high thermal resistance of the heat source and sink, i.e., of the environment. The name “thermal shunt” is given to this component because, firstly, it is connected thermally in parallel to a certain zone of the environment in the vicinity of a thermopile and, secondly, it shunts this zone thermally. As a result, the heat flow through the thermopile increases, improving its performance characteristics.

Thermal Conductivity and Interfacial Thermal Resistance: Measurements of Thermally Oxidized SiO2 Films on a Silicon Wafer Using a Thermo-Reflectance Technique

This article describes the development of a method to measure the normal-to-plane thermal conductivity of a very thin electrically insulating film on a substrate. In this method, a metal film, which is deposited on the thin insulating films, is Joule heated periodically, and the ac-temperature response at the center of the metal film surface is measured by a thermo-reflectance technique. The one-dimensional thermal conduction equation of the metal/film/substrate system was solved analytically, and a simple approximate equation was derived. The thermal conductivities of the thermally oxidized SiO2 films obtained in this study agreed with those of VAMAS TWA23 within ± 4%. In this study, an attempt was made to estimate the interfacial thermal resistance between the thermally oxidized SiO2 film and the silicon wafer. The difference between the apparent thermal resistances of the thermally oxidized SiO2 film with the gold film deposited by two different methods was examined. It was concluded that rf-sputtering produces a significant thermal resistance ((20 ± 4.5) × 10−9 m2·K·W−1) between the gold film and the thermally oxidized SiO2 film, but evaporation provides no significant interfacial thermal resistance (less than ± 4.5 × 10−9 m2·K·W−1). The apparent interfacial thermal resistances between the thermally oxidized SiO2 film and the silicon wafer were found to scatter significantly (± 9 × 10−9 m2·K·W−1) around a very small thermal resistance (less than ± 4.5 × 10−9 m2·K·W−1).

Development of a Heat-Pipe-Based Hot Plate for Surface-Temperature Measurements

The development of a flat heat-pipe-based hot plate operating in the temperature range from ambient to 200 °C is presented here. The aim of the design was to improve the heat-transfer performance in order to provide almost ideal thermal uniformity of surface and volume temperature profiles, thereby ensuring a minimal temperature gradient and a small effective thermal resistance. The device was investigated to evaluate its performance in view of its potential use as a calibration apparatus for surface-temperature sensors. An accurate characterization was carried out to estimate the surface-temperature stability and uniformity under different thermal conditions. Measurement results showed temperature stability within 0.03 °C and uniformity of the heat-pipe (HP) hot-plate surface better than 0.08 °C. As a result of its high thermal conductivity, a small perturbation (<0.2 °C) of the temperature field both on the surface and inside the device chamber, when a contact probe is applied on its surface, was obtained. The study of the HP hot plate and the performed tests suggest that such a device has potential as a calibration apparatus for surface-temperature sensors.

Comparison of Fluid Flow and Thermal Characteristics of Plate-Fin and Pin-Fin Heat Sinks Subject to a Parallel Flow

In this study, we compare the thermal performances of the two types of heat sinks most commonly used in the electronic equipment cooling: plate-fin and pin-fin heat sinks. In order to obtain the fluid flow and thermal characteristics of heat sinks, an experimental investigation is conducted. Based on the experimental results of the present study and the available data from the existing literature, the correlations of the friction factor and the Nusselt number are suggested for each type of heat sink. Correlations for the pin-fin heat sinks are newly developed, while correlations for the plate-fin heat sinks are selected from previous models. By using the appropriate correlations, thermal resistances of the optimized plate-fin and pin-fin heat sinks are compared under fixed pumping power conditions. Finally, a contour map, which depicts the ratio of the thermal resistances of the optimized plate-fin and pin-fin heat sinks as a function of dimensionless pumping power and dimensionless length, is presented. The contour map indicates that optimized plate-fin heat sinks possess lower thermal resistances than optimized pin-fin heat sinks when dimensionless pumping power is small and the dimensionless length of heat sinks is large. On the contrary, the optimized pin-fin heat sinks have smaller thermal resistances when dimensionless pumping power is large and the dimensionless length of heat sinks is small.

Thermal transient characteristics of die attach in high power LED PKG

The reliability of packaged electronics strongly depends on the die attach quality because any void or a small delamination may cause instant temperature increase in the die, leading sooner or later to failure in the operation. Die attach materials have a key role in the thermal management of high power LED packages by providing the low thermal resistance between the heat generating LED chips and the heat dissipating heat slug. In this paper, thermal transient characteristics of die attach in high power LED PKG have been studied based on the thermal transient analysis using the evaluation of the structure function of the heat flow path. With high power LED packages fabricated by die attach materials such as Ag paste, solder paste and Au/Sn eutectic bonding, we have demonstrated for characteristics such as cross-section analysis, shear test and visual inspection after shear test of die attach and how to detect die attach failures and to measure thermal resistance values of die attach in high power LED PKG. From the differential structure function of the thermal transient characteristics, we could know the result that die attach quality of Au/Sn eutectic bonding with the thermal resistance of about 3.5 K/W was much better than this of Ag paste and solder paste with the thermal resistance of about 11.5–14.2 K/W and 4.4–4.6 K/W, respectively. From this results, it is possible to fabricate high power LED with a small thermal resistance and a good die attach quality by applying Au/Sn eutectic bonding die attach with a high reliability and a good repeatability.

An experimental study on the heat transfer characteristics of a heat pipe heat exchanger with latent heat storage. Part II: Simultaneous charging/discharging modes

In this part of the paper, the performance of the simultaneous charging/discharging operation modes of the heat pipe heat exchanger with latent heat storage is experimentally studied. The experimental results show that the device may operate under either the fluid to fluid heat transfer with charging heat to the phase change material (PCM) or the fluid to fluid heat transfer with discharging heat from the PCM modes according to the initial temperature of the PCM. The melting/solidification curves, the performances of the heat pipes and the device, the influences of the inlet temperature and the mass flow rate of the cold water on the operation performance are investigated by extensive experiments. The experimental results also disclose that under the simultaneous charging/discharging operation mode, although the heat transfer from the hot water directly to the cold water may vary, it always takes up a major part of the total heat recovered by the cold water due to the very small thermal resistance compared with the thermal resistance of the PCM side. The melting/solidification processes taking place in the simultaneous charging/discharging operation are compared with those in the charging only and discharging only processes. By applying a simplified thermal resistance analysis, a criterion for predicting the exact operation modes was derived and used to explain the observed experimental phenomena.

COMPUTATION OF THREE-DIMENSIONAL DEVELOPING PRESSURE-DRIVEN LIQUID FLOW IN A MICROCHANNEL WITH EDL EFFECT

Advances in miniature fabrication technology have allowed the possibilities of using microchannels in ultracompact, very efficient heat exchangers, which capitalize on the large surface area-to-volume ratio of the channels, to transport high heat fluxes with small thermal resistance. This article describes a developing flow model by using 3-D Navier-stoke equations with flow pressure as a driving force to simulate the developing liquid flow phenomenon in microchannels. The concept of electric double layer (EDL) is introduced to explain the microscale deviation between flow in microscale channels and large-scale channels. Governing equations are stated for developing rectangular microchannel flows. An additional source term, related to the electric potential and resulting from the EDL effect, is introduced in the conventional momentum equation as a body force, thereby modifying the flow and heat transfer characteristics. The electric potential distribution is simulated using a Poisson-Boltzmann equation. A finite-volume scheme is used to solve the differential equations. Results comparing the performance of the microchannel under different factors such as Reynolds number, nondimensionalized electric potential, etc., are shown and discussed. Nusselt numbers for each case, with or without EDL effect, are also included, so that the actual effectiveness of the microchannel can be understood.

NUMERICAL STUDIES OF DEVELOPING FLOW IN MICROCHANNEL

International Journal of Computational Engineering ScienceVol. 04, No. 02, pp. 389-392 (2003) Design, Modelling and SimulationNo AccessNUMERICAL STUDIES OF DEVELOPING FLOW IN MICROCHANNELS. T. TAN and E. Y. K. NGS. T. TANFerrotec Corporation, Kallang Basin Industrial estate, Singapore 339412, SingaporeSchool of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue Singapore 639798, Singapore Search for more papers by this author and E. Y. K. NGSchool of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue Singapore 639798, Singapore Search for more papers by this author https://doi.org/10.1142/S1465876303001344Cited by:6 PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail AbstractMicrochannels have the potential to be ultra compact, very efficient heat exchangers, which capitalize on the large surface area to volume ratio of the channels, to transport high heat fluxes with small thermal resistance. This paper describes a 3-D developing flow model by using Navier stoke equations with electric double layer (EDL) effect to model the flow field in microchannel. The electric potential distribution is simulated using a Poisson-Boltzmann equation. Results comparing the performances of the microchannel under different nondimensionalized electric potential are shown and discussed. Nusselt numbers for each case such as with or without EDL effect is also calculated; so that the actual effectiveness of the micro-channel can be understand.Keywords:numericaldeveloping flowmicrochannelEDLNavier-Stoke References G. M. Mala, D. Li and J. D. Dale, Int. J. Heat Mass Transfer 40(13), 3079 (1997). Crossref, Google ScholarE. Y. K. Ng and S. T. Poh, Journal of Computer Modeling in Engineering & Sciences 3(3), 351 (2002). Google ScholarC. Yang, D. Li and J. H. Masliyah, International Journal of Heat and Mass Transfer 41, 4229 (1998). Crossref, Google Scholar S. V. Patankar , Numerical heat transfer and fluid flow ( McGraw-Hill , New York , 1980 ) . Crossref, Google Scholar FiguresReferencesRelatedDetailsCited By 6A NOVEL INVESTIGATION OF HEAT TRANSFER CHARACTERISTICS IN HYBRID MICRO-CHANNEL HEAT SINK STRUCTURE: OPPOSITION-BASED ANTLION OPTIMIZATIONR. DEENA and G. SAI SUNDARA KRISHNAN22 August 2019 | Surface Review and Letters, Vol. 27, No. 05Study of EDL effect on 3-D developing flow in microchannel with Poisson–Boltzmann and Nernst–Planck modelsE. Y. K. Ng and S. T. Tan1 January 2007 | International Journal for Numerical Methods in Engineering, Vol. 71, No. 7LATTICE BOLTZMANN SIMULATION OF VISCOUS DISSIPATION IN ELECTRO-OSMOTIC FLOW IN MICROCHANNELSZHEN-HUA CHAI, BAO-CHANG SHI, and LIN ZHENG21 November 2011 | International Journal of Modern Physics C, Vol. 18, No. 07Simulation of electro-osmotic flow in microchannel with lattice Boltzmann methodZhenhua Chai and Baochang Shi1 Apr 2007 | Physics Letters A, Vol. 364, No. 3-4Numerical Analysis of EDL Effect on Heat Transfer Characteristic of 3-D Developing Flow in a MicrochannelS. T. Tan and E. Y. K. Ng22 September 2006 | Numerical Heat Transfer, Part A: Applications, Vol. 49, No. 10Computational studies of the EDL effect in 3-D developing flow in microchannelS T Tan and E Y K Ng25 April 2006 | Journal of Physics: Conference Series, Vol. 34 Recommended Vol. 04, No. 02 Metrics History Keywordsnumericaldeveloping flowmicrochannelEDLNavier-StokePDF download

Investigation of thermal contact effect on thermoelectric coolers

The effect of the thermal and contact resistances of ceramic plates plays a vital role in thermoelectric (TE) coolers with relatively short thermoelements, especially in TE microcoolers. Through first introducing equivalent impedances, like equivalent thermal conductance, electrical resistance etc., general performance formulae with familiar forms are developed to take into account the thermal contact effect on TE coolers. Then, two new approaches are presented to obtain simplified formulae at maxima for temperature difference, cooling rate and coefficient of performance. Compared numerically with real maximum cooling performances of TE coolers, simplified formulae are proved to be good approximations at small thermal contact resistance and not-too-short thermoelements. In addition, some errors in previous investigators’ models are redressed.

High performance single mode GaAs quantum cascade lasers

We report on the realization of distributed feedback quantum cascade lasers in the AlGaAs/GaAs material system. Strong coupling is achieved by the use of a metalized surface relief grating, allowing the realization of short lasers. Continuous wave operation was achieved for an AlAs/GaAs chirped superlattice laser with an emission wavelength of 11.8 μm . The used double plasmon enhanced optical waveguide consists only of the binary material GaAs and not of a ternary compound which results in a small thermal resistance. Further, the emission wavelength of the distributed feedback laser is continuously tunable according to the temperature dependence of the refractive index. Additionally, we want to report about pulsed single mode operation up to 335 K for a laser material, where Al 0.45Ga 0.55As is used for the barriers and GaAs for the wells. The emission wavelength is 9.5 μm and the optical peak power in pulsed mode operation from one facet of the distributed feedback laser is 0.3 W at a temperature of 0°C.

Surface Chemistry and Characteristics Based Model for the Thermal Contact Resistance of Fluidic Interstitial Thermal Interface Materials

Microprocessor powers are increasing at a phenomenal rate, which requires very small thermal resistance between the die (chip) and the ambient, if the current economical methods of conduction and convection cooling are to be utilized. A typical thermal solution in flip chip technology utilizes two levels of thermal interface materials: between the die and the heat spreader, and between the heat spreader and the heat sink. Phase change materials and thermal greases are among the most prominent interstitial thermal interface materials (TIM) used in electronic packaging. These TIMs are typically polymeric matrix loaded with highly conducting filler particles. The dwindling thermal budget has necessitated a better understanding of the thermal resistance of each component of the thermal solution. Thermal conductivity of these particle-laden materials is better understood than their contact resistance. A careful review of the literature reveals the lack of analytical models for the prediction of contact resistance of these types of interstitial materials, which possess fluidic properties. This paper introduces an analytical model for the thermal contact resistance of these types of interstitial materials. This model is compared with the experimental data obtained on the contact resistance of these TIMs. The model, which depends on parameters such as, surface tension, contact angle, thermal conductivity, roughness and pressure matches very well with the experimental data at low pressures and is still within the error bars at higher pressures.

Development of Micro Channel Heat Exchanging

In order to investigate the performance of the micro channel heat exchanger, three-dimensional numerical simulations and experiments on heat transfer behavior and pressure loss were carried out. So far as the heat transfer phenomena is concerned, results obtained using a silicon chip micro channel model showed a very small thermal resistance, about 0.1 (Kcm2/W). And, measured pressure loss showed good agreement with that of analytical result obtained on the basis of fully developed laminar pipe flow assumption. Furthermore, a practical setup was made with a micro channel heat exchanger to clarify the possibility of using the micro channel heat exchanger in electrical equipment. As a result, it was confirmed that the performance of the micro channel heat exchanger system is sufficient to cool a silicon chip which generates a large amount of heat, and the scale of the system is compact compared to that of the whole setup of electrical equipment.

A.C. scanning thermal microscopy: Tip–sample interaction and buried defects modellings

Modellings describing the different thermal interactions between a sample surface and a probe in case of modulated heated probe are given and analysed. To obtain more reliable thermal data for a full calibration of thermal microscope or quantitative interpretations, these models are developed in three dimensions. The scattering of the thermal waves on buried small thermal resistances is theoretically studied. Lateral resolutions for the amplitude and phase of the probe's a.c. thermal response are obtained.