This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band (±10% or less) about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are (small) systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. [J. Appl. Phys. 81, 6692 (1997)], was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

A Benchmark Study on the Thermal Conductivity of Nanofluids

Integrated microfluidic devices

Power electronics has progressively gained an important status in power generation, distribution, and consumption. With more than 70% of electricity processed through power electronics, recent research endeavors to improve the reliability of power electronic systems to comply with more stringent constraints on cost, safety, and availability in various applications. This paper serves to give an overview of the major aspects of reliability in power electronics and to address the future trends in this multidisciplinary research direction. The ongoing paradigm shift in reliability research is presented first. Then, the three major aspects of power electronics reliability are discussed, respectively, which cover physics-of-failure analysis of critical power electronic components, state-of-the-art design for reliability process and robustness validation, and intelligent control and condition monitoring to achieve improved reliability under operation. Finally, the challenges and opportunities for achieving more reliable power electronic systems in the future are discussed.

http://www.tf.uni-kiel.de/etit/LEA-download/publications/2014/80629_jestpe-2013-04-0032_postprint.pdf

Transitioning to Physics-of-Failure as a Reliability Driver in Power Electronics

This paper proposes a synthesis of different electrical methods used to estimate the temperature of power semiconductor devices. The following measurement methods are introduced: the voltage under low current levels, the threshold voltage, the voltage under high current levels, the gate-emitter voltage, the saturation current, and the switching times. All these methods are then compared in terms of sensitivity, linearity, accuracy, genericity, calibration needs, and possibility of characterizing the thermal impedance or the temperature during the operation of the converter. The measurement of thermo-sensitive parameters of wide bandgap semiconductors is also discussed.

/pdf/temperature-measurement-of-power-semiconductor-devices-by-1nz3bww95c.pdf

Temperature Measurement of Power Semiconductor Devices by Thermo-Sensitive Electrical Parameters—A Review

Heat and fluid flow in high-power LED packaging and applications

Power electronic systems play an increasingly important role in providing high-efficiency power conversion for adjustable-speed drives, power-quality correction, renewable-energy systems, energy-storage systems, and electric vehicles. However, they are often presented with demanding operating environments that challenge the reliability aspects of power electronic techniques. For example, increasingly thermally stressful environments are seen in applications such as electric vehicles, where ambient temperatures under the hood exceed 150 °C, while some wind turbine applications can place large temperature cycling conditions on the system. On the other hand, safety requirements in the aerospace and automotive industries place rigorous demands on reliability.

/pdf/improved-reliability-of-power-modules-a-review-of-online-3yephrhs9n.pdf

Improved Reliability of Power Modules: A Review of Online Junction Temperature Measurement Methods

The measurement of the junction temperature with thermosensitive electrical parameters (TSEPs) is largely used by electrical engineers or researchers, but the obtained temperature value is generally not verified by any referential information of the actual chip temperature distribution. In this paper, we propose to use infrared (IR) measurements in order to evaluate the relevance of three commonly used TSEPs with insulated gate bipolar transistor chips: the saturation voltage under a low current, the gate-emitter voltage, and the saturation current. TheIR measurements are presented in detail with an estimation of the emissivity of the black paint deposited on the power module. The temperatures obtained with IR measurements and with the different TSEPs are then compared in two cases: the use of only one chip and the use of two paralleled chips.

/pdf/comparison-of-junction-temperature-evaluations-in-a-power-4p3nwn5uk1.pdf

Comparison of Junction Temperature Evaluations in a Power IGBT Module Using an IR Camera and Three Thermosensitive Electrical Parameters

Power conversion systems are dependent on the performance and reliability of static converters. However, they are subject to frequent functional and environmental strains, which can induce failures. The anticipation of these failures is difficult but important so the operation of a system can be halted before a breakdown occurs. In the case of photovoltaic (PV) power plants, the system can be simplified into two distinct blocks: the solar panels/modules and the power inverter. A breakdown in either of these blocks can cause significant downtime in the system. Nevertheless, multiple solar module failures can often be tolerated and not lead to a total breakdown of the entire array, whereas a single component failure in the inverter can lead to a collapse of the entire system. Furthermore, solar panel/module manufacturers often offer warranties of up to 20 years, while warranties for inverters rarely reach the ten-year mark [1]. As such, the overall cost of a PV inverter can increase by a factor of two or three if it undergoes one or two failures during the life of the system. Although the market competition between PV inverter manufacturers has traditionally focused on the efficiency of their product, a failure that induces a downtime of just a few days can easily negate the yield attained through a 1% efficiency improvement. Oversizing the inverter or introducing redundancy into the inverter system is one option to improve reliability; however, often, this is not economical.

Condition Monitoring: A Decade of Proposed Techniques

An increase in power densities in electronic devices is a direct consequence of their miniaturization and performance improvements. We propose the use of flat miniature heat pipes with micro capillary grooves to spread heat flux across a heat sink. Models of the structure were developed to calculate heat transfer limitations and temperature drops. A brass/water prototype was fabricated to demonstrate the feasibility of heat spreading using this type of heat pipe. Simulation and experimental results obtained with the prototype are described. The dissipated power reached 110 W/cm/sup 2/ without heat transfer limitations. The results are then extended to the design of this type of heat pipe in silicon. Thermal performance was calculated. Simulation, experimental results and the fabrication process are presented.

Yvan Avenas

Papers

Temperature Measurement of Power Semiconductor Devices by Thermo-Sensitive Electrical Parameters—A Review

Improved Reliability of Power Modules: A Review of Online Junction Temperature Measurement Methods

Comparison of Junction Temperature Evaluations in a Power IGBT Module Using an IR Camera and Three Thermosensitive Electrical Parameters

Condition Monitoring: A Decade of Proposed Techniques

Silicon heat pipes used as thermal spreaders