Development of a Measurement Method for the Thermal Conductivity of a Thick Film Prepared by a Screen-Printing Technique

We propose a method that allows us to evaluate the thermal conductivity of a conductive material that has thickness on the order of microns. The key feature of the proposed method is use of a complete thermoelectric device with electrodes and a substrate, while conventional methods measure the temperature gradient of thermoelectric materials directly without electrodes. The measured thermal conductivity of a ZnSb film annealed at 380°C in N₂ ambient for 16?min to 26?min is 1.2?W/m?K to 1.4?W/m?K. The measurement shows that thermoelectric film prepared by a screen-printing technique has lower thermal conductivity than bulk material (2.2?W/m?K to 2.4?W/m?K) because the screen-printing technique generates high porosity in the film. The lower measured thermal conductivity of the porous films compared with bulk material supports the reliability of the proposed measurement method.     

Measurement of Thermal Conductivity Using Steady-State Isothermal Conditions and Validation by Comparison with Thermoelectric Device Performance

A new technique for measuring thermal conductivity with significantly improved accuracy is presented. By using the Peltier effect to counterbalance an imposed temperature difference, a completely isothermal, steady-state condition can be obtained across a sample. In this condition, extraneous parasitic heat flows that would otherwise cause error can be eliminated entirely. The technique is used to determine the thermal conductivity of p-type and n-type samples of (Bi,Sb)₂(Te,Se)₃ materials, and thermal conductivity values of 1.47?W/m?K and 1.48?W/m?K are obtained respectively. To validate this technique, those samples were assembled into a Peltier cooling device. The agreement between the Seebeck coefficient measured individually and from the assembled device were within 0.5%, and the corresponding thermal conductivity was consistent with the individual measurements with less than 2% error.     

Thermal characterization of embedded electronic features by an integrated system of CCD thermography and self-adaptive numerical modeling

This work provides a practical application of a coupled experimental-computational system devised for the full characterization of the thermal behavior of complex three-dimensional active submicron electronic devices. A thermoreflectance thermography (TRTG) technique is used to non-invasively measure the 2D surface temperature field of an activated device, with submicron spatial resolution. The measured planar temperature distribution field is then used as input for an ultra-fast inverse computational solution to derive the three-dimensional temperature distribution throughout the device. For the purposes of this investigation, test micro-heater devices were constructed on epitaxial layers of natural (Si) and isotopically pure (Si²⁸) silicon. Then, all devices were activated and measured with the TRTG technique. In order to demonstrate the coupled experimental-computational system, the measured temperature fields of the samples whose thermal properties are known (Si) were used to extract critical physical parameters (the oxide layer thickness and the effective heater length). Then, since the devices with unknown thermal properties (Si²⁸) share the same construction with the Si devices, the extracted parameters were used together with the measured planar temperature fields to derive the thermal conductivity of Si²⁸. The extracted oxide layer thickness and thermal conductivity of Si²⁸ compared very closely to values obtained by other independent direct methods.     

Measurement of directional thermal properties of biomaterials

This paper presents an experimental technique to measure the directional thermal conductivity and thermal diffusivity of materials. A heated thermistor heats the sample and a sensing thermistor placed about 2.5 mm away measures the temperature rise due the heating pulse at the heated thermistor. An empirical relation between the power delivered by the first thermistor and the temperature rise recorded by the sensing thermistor is used to measure the thermal conductivity of the material along the line joining the thermistors. Diffusivity of the material is determined from the delay between the power pulse in the heated thermistor and the temperature pulse at the sensing thermistor. Signal processing was done to eliminate errors in the measurement due to change of base line temperature. Uncertainty of the measurement technique was found to be 5% when tested in media of known thermal properties. The thermal conductivity and thermal diffusivity of swine left ventricle in normal and ablated conditions were measured using this technique. The thermal conductivity of the tissue dropped significantly from 0.61 to 0.50 W.m(-1).K(-1) after ablation while the diffusivity dropped from 2.1 x 10(-7) to 1.7 x 10(-7)m2.s(-1).     

高温功率半导体器件连接的低温烧结技术

综述了功率电子器件和模块的连接和封装工艺,介绍了粉末致密烧结技术和用于电子封装的现状,对纳米银金属焊膏烧结技术进行了讨论。研究表明,纳米银可有效降低烧结温度,提高设备的高温稳定性、导热性、导电性、机械强度、抗疲劳性等。由于银的熔点较高,这种新技术可应用于高温功率器件的封装。     

Thermal Cycling, Mechanical Degradation, and the Effective Figure of Merit of a Thermoelectric Module

Thermoelectric modules experience performance reduction and mechanical failure due to thermomechanical stresses induced by thermal cycling. The present study subjects a thermoelectric module to thermal cycling and evaluates the evolution of its thermoelectric performance through measurements of the thermoelectric figure of merit, ZT, and its individual components. The Seebeck coefficient and thermal conductivity are measured using steady-state infrared microscopy, and the electrical conductivity and ZT are evaluated using the Harman technique. These properties are tracked over many cycles until device failure after 45,000 thermal cycles. The mechanical failure of the TE module is analyzed using high-resolution infrared microscopy and scanning electron microscopy. A reduction in electrical conductivity is the primary mechanism of performance reduction and is likely associated with defects observed during cycling. The effective figure of merit is reduced by 20% through 40,000 cycles and drops by 97% at 45,000 cycles. These results quantify the effect of thermal cycling on a commercial TE module and provide insight into the packaging of a complete TE module for reliable operation.     

衬底热导对Mn-Co-Ni-O薄膜热敏 红外探测器性能的影响

研究了用Mn-Co-Ni-O薄膜材料制备的热敏红外探测器件,并主要通过调控衬底热导改进了器件性能。对于较薄的探测元,研究了具有微桥结构的红外器件。结果表明,该器件的响应率比非微桥器件高80%左右,探测率高44%左右。这主要是因为其独特的结构形式降低了器件热导,且对光的全反射效应提高了探测元对光的吸收率。对于较厚的探测元,在衬底上增加了抛光的散热铜片,使器件衬底的导热系数提高了47%左右,但响应电压降低了50%左右。因此需要合理选择散热铜片,以获得合适的时间常数和响应率。     

Use of Photothermally Generated Seebeck Voltage for Thermal Characterization of Thermoelectric Materials

A simple and accurate experimental procedure to measure simultaneously the thermal properties (conductivity, diffusivity, and effusivity) of thermoelectric (TE) materials using their Seebeck voltage is proposed. The technique is based on analysis of a periodically oscillating thermoelectric signal generated from a TE material when it is thermally excited using an intensity-modulated laser source. A self-normalization procedure is implemented in the presented method using TE signals generated by changing the laser heating from one side to another of the TE material. Experiments are done on a polyaniline carbon nanohybrid (6.6 wt.% carbon nanotubes), yielding a thermal conductivity of 1.106 ± 0.001 W/m-K. The results are compared with the results from photothermal infrared radiometry experiments.     

Thermal Transport in Conductive Polymer–Based Materials

The demand for flexible conductive materials has motivated many recent studies on conductive polymer–based materials. However, the thermal conductivity of conductive polymers is relatively low, which may lead to serious heat dissipation problems for device applications. This review provides a summary of the fundamental principles for thermal transport in conductive polymers and their composites, and recent advancements in regulating their thermal conductivity. The thermal transport mechanisms in conductive polymer–based materials and up‐to‐date experimental approaches for measuring thermal conductivity are first summarized. Effective approaches for the regulation of thermal conductivity are then discussed. Finally, thermal‐related applications and future perspectives are given for conductive polymers and their composites.     

A noninvasive electromagnetic conductivity sensor for biomedicalapplications

The authors present the theory and practice of using a simple coil sensor operated at high frequency (1-10 MHz) to measure changes in conductivity in a biological sample. Explicit results are obtained for symmetric configurations that are useful for calibrating the device and making order-of-magnitude estimates of the results of applying the technique to humans to monitor the onset and progress of brain edema. A simple model suggests that the technique may be able to successfully measure the onset of some medical complications using the technology described     

Innovative wick design for multi-source, flat plate heat pipes

We present a novel micro-heat pipe wick design and fabrication technique to significantly boost the effective thermal conductivity of the heat pipe relative to the monolithic substrate material. Extensive porous flow modeling of the process has provided critical information on the key parameters and the resulting anisotropic wick designs have shown robust performance improvements. A methanol charged copper device reported in this paper showed a maximum thermal conductivity of 760 W/m K prior to dry out. This represents a 1.9× increase over the conductivity of solid copper.     

Temperature determination in optoelectronic waveguide modulators

Optoelectronic devices are particularly sensitive to temperature changes induced by the absorption of light and the passage of current. In order to study the thermal issues arising in a InGaAsP-based Mach-Zehnder (MZ) optical modulator, a nonlinear finite-element thermal model of the device was constructed. The model considers the variation with temperature of both the thermal conductivity of the semiconductors composing the device and the optical absorption. To that effect, the optical absorption was measured inside the waveguide as a function of temperature, An experimental method using liquid crystals to measure the surface temperature was also developed. Both were used to evaluate the temperature inside a variable optical attenuator present on the modulator. Good agreement with the model and the experiment is found over a wide range of operating conditions. These tools are expected to play a key role in understanding thermal issues in future photonic devices, in view of the desire to integrate multiple devices on a common substrate and the continuous increase of the optical powers in fiber systems     

Characterization of solder interfaces using laser flash metrology

Thermal conductivity and normalized thermal resistance measurements on eight bulk solders and three tri-layer samples were performed using the laser flash technique. In this study, the laser flash technique is demonstrated to be capable of measuring thermal conductivity of 0.7-mm-thick solders ranged from 20 to 90 W/m K with an average uncertainty of 11%. Laser flash was then used to measure the intrinsic thermal resistance of Cu/Solder/Cu, tri-layer sandwich structures. Analysis shows that the total intrinsic thermal resistance measurement is confounded by sample voiding. C-SAM scanning ultrasonic microscope metrology was shown to be useful in providing information regarding solder voiding and the quality of the solder/copper interfaces. The laser flash method also proved to be useful in assessing voiding effects resulting from different assembly processes and different contacting surfaces on package thermal performance.     

Submicron Mapping of Thermal Conductivity of Thermoelectric Thin Films

A tool for evaluating thin-film thermal conductivity to submicron spatial resolution has been developed. The micro-instrumentation utilizes the thermoreflectance (TR) technique to characterize thermal conductivity and material uniformity. The instrument consists of a heating element for creating temperature gradients and an Invar bar with in?situ temperature monitoring for heat flux measurements. The thin-film sample is sandwiched between the heater and Invar bar while a microscope is used to direct light onto a cross-section of the sample and reflected light is collected with a camera. By using this technique, we can achieve submicron spatial resolution for thermal conductivity and eliminate contributions from thermal contact resistance, thereby also eliminating the need for sample preparation other than cleaving. The method offers temperature resolution of 10?mK, spatial resolution of 200?nm, and thermal conductivity measurement with 0.01?±?0.001?W/mK resolution. The thermal conductivity of a 0.6% ErAs:InGaAlAs thermoelectric (TE) element, prepared by molecular beam epitaxy (MBE) growth, obtained with the new instrument is 2.3?W/mK, while the average thermal conductivity obtained with the 3-omega method is 2.5?W/mK. Energy-dispersive x-ray (EDX) spectroscopy is also used to prove that the elemental composition has uniformity consistent with the material variation observed by the TR technique. Moreover, a temperature profile across a 0.6% ErAs:InGaAlAs TE element on InP substrate is imaged. Two different slopes, corresponding to different thermal conductivities, have been observed, showing that the thermal conductivity of the TE element is lower than that of the InP substrate as expected.     

多晶硅发射极微波功率管的三维热电耦合模型

文章结合双层多晶硅发射极双极型微波功率管器件结构,采用步内建模法,首次建立了三维热电耦合模型,并进行了直流稳态模拟.模拟结果表明,新的三维热电耦合模型可准确预测功率管结温的均匀状况.与单子胞器件CI相比,多子胞器件C2的中心区域结温变化平缓,结温温度约为390K,比单子胞器件C1的结温温度下降约10K.此外,低热传导率...     

Thermal modeling and measurement of AlGaN-GaN HFETs built on sapphire and SiC substrates

We present thermal modeling and measurement results of AlGaN-GaN heterojunction field effect transistors fabricated on sapphire and SiC substrates, respectively. The device structures are identical except for the substrate material used to grow the AlGaN-GaN heterostructure. One objective is to study the effect of substrate material on the thermal and electrical performance of the resulting devices. To compute the temperature profiles, in-house PAMICE code developed for a three-dimensional structure was used. To measure the temperatures on the chip surface, nematic liquid crystal thermography was used. This technique is nondestructive and can be performed in realtime during device operation. It has submicrometer spatial resolution and /spl plusmn/1/spl deg/C temperature accuracy. The measured temperatures agree well with the calculated ones. The relationship between the measured temperature and power is almost linear for both types of devices. The junction-to-case thermal resistance of the device fabricated on sapphire substrate is 4.4 times that of the device built on SiC substrate.     

Anisotropic Thermal Conductive Composite by the Guided Assembly of Boron Nitride Nanosheets for Flexible and Stretchable Electronics

Owing to the growing demand for highly integrated electronics, anisotropic heat dissipation of thermal management material is a challenging and promising technique. Moreover, to satisfy the needs for advancing flexible and stretchable electronic devices, maintaining high thermal conductivity during the deformation of electronic materials is at issue. Presented here is an effective assembly technique to realize a continuous array of boron nitride (BN) nanosheets on tetrahedral structures, creating 3D thermal paths for anisotropic dissipation integrated with deformable electronics. The tetrahedral structures, with a fancy wavy shaped cross‐section, guarantee flexibility and stretchability, without the degradation of thermal conductivity during the deformation of the composite film. The structured BN layer in the composites induces a high thermal conductivity of 1.15 W m^?1 K^?1 in the through‐plane and 11.05 W m^?1 K^?1 in the in‐plane direction at the low BN fraction of 16 wt%, which represent 145% and 83% increases over the randomly mixing method, respectively. Furthermore, this structured BN composite maintains thermal dissipation property with 50% strain of the original length of composite. Various electronic device demonstrations provide exceptional heat dissipation capabilities, including thin film silicon transistor and light‐emitting diode on flexible and stretchable composite, respectively.     

Performance Improvement of Flexible Thermoelectric Device: FEM-Based Simulation

Possibilities for improving the performance of the flexible thermoelectric (TE) device were discussed on the basis of heat conduction analysis by the finite element method. The flexible TE device consists of two flexible substrates and thin films of n- and p-type TE materials placed between the substrates. To enhance the device performance, the use of higher-performance TE materials and improvement of the flexible substrate will be effective. In the present study, the effect of the thermal conductivity of the materials used in the device on the output voltage was examined. The calculations indicated that there is a certain combination of thermal conductivities of the components which gives the maximum output voltage. Although a lower thermal conductivity of the TE material leads to higher output voltage, influence of the thermal conductivity on the maximum voltage was not significant under the condition of the present study. As a result, it is effective to improve device performance by choosing an appropriate combination of TE material and substrate material. According to the calculations, approximately 60% increase in output voltage is expected compared with that of the present combination of materials used in the prototype device.     

Size-Dependent Lattice Thermal Conductivity of Nanostructured Bulk Semiconductors

Recently, nanostructuring of bulk semiconductors has emerged as an effective approach to develop high-efficiency thermoelectric materials for large-scale device applications, where the thermal conductivity reduction predominates in the enhanced figure of merit of these materials. In this work, a quantitative nanothermodynamic model was established to calculate the lattice thermal conductivity of semiconductor nanocomposites considering the interface scattering effects. It is found that the lattice thermal conductivity can be significantly reduced in nanostructured bulk semiconductors compared with their bulk counterparts. The findings in this work may provide new insights into the fundamental understanding of phonon transport in nanocomposites and also the development of high-performance thermoelectric materials.     

SIMOX硅片埋氧层垂直方向热导率的测量

提出了一种简单、有效而且准确的测量SOI硅片埋氧层垂直方向热导率的方法,并采用这种方法测量了用SIMOX工艺制作的SOI硅片的埋氧层垂直方向的热导率.测量结果显示至少在5 5nm以上的尺度上对于SIMOX硅片的埋氧层垂直方向经典的热导率定义仍然成立,且为一明显小于普通二氧化硅的热导率(1 4W/mK)的常数1 0 6W/mK .测量中发现硅/二氧化硅边界存在边界热阻,并测量了该数值.结果表明,边界热阻在SOI器件尤其是薄二氧化硅背栅的双栅器件热阻的计算中不可忽略.