Chun-Kai Liu

Thermal conductivity of Si/SiGe superlattice films

We have evaluated the thermal conductivity of Si/SiGe superlattice films by theoretical analysis and experiment. In experiments, the ultrahigh vacuum chemical vapor deposition is employed to form the Si/Si0.71Ge0.29 and Si/Si0.8Ge0.2 superlattice films. The cross-plane thermal conductivities of these superlattice films are measured based on the 3ω method. In the theoretical analysis, the phonon transport in Si/Si1−xGex superlattice film is explored by solving the phonon Boltzmann transport equation. The dependence of the thermal conductivity of the Si/Si1−xGex superlattice films on the superlattice period, the ratio of layer thicknesses, and the interface roughness is of interest. The calculations show that when the layer thickness is on the order of one percentage of the mean free path or even thinner, the phonons encounter few intrinsic scatterings and consequently concentrate in the directions having high transmissivities. Nonlinear temperature distributions are observed near the interfaces, arising fr...

Characteristics of Thermal Resistance for High Power LEDs

In this paper the accurate and fast measurement equipment was developed and applied to study the thermal characteristics of high power LEDs. The forward-voltage based method was conducted to measure the junction temperature of high power LEDs. Conduction type method is adopted to measure the temperature sensitivity parameter (TSP) with small magnitude of error compared with the traditional method. The experiment time was reduced from 3~4 hours to 10 minutes for one sample. It was demonstrated that the repeatability of the measurement system was well after the repeatability test. LEDs used here were 5 W single chip LED and 50 W multi-chip LED with 36 chips inside the LED. Thermal resistance of junction-to-case as function of input power and case temperature was discussed. It was shown that the 5 W LED revealed an increasing trend of thermal resistance with the input power at each case temperature but the contrary trend of 5OW LED. The results also exhibited the dependency of thermal resistance and case temperature. With the increasing case temperature, the value of thermal resistance became higher under each input power. Three factors affected the thermal performance including: the first, the relation between light output efficiency and junction temperature; the second, the effect of internal series electrical resistance Rin and external electrical resistance Rex; and the third, the materials degeneration of each part inside the LEDs package as the junction temperature increased. To combine the three factors could explain the thermal characteristics of high power LEDs.

Interfacial reactions between PbTe-based thermoelectric materials and Cu and Ag bonding materials

The development of reliable bonding materials for PbTe-based thermoelectric modules that can undergo long-term operations at high temperature is carried out. Two cost-effective materials, Cu and Ag, are isothermally hot-pressed to PbTe-based thermoelectric materials at 550 °C for 3 h under a pressure of 40 MPa by the rapid hot-pressing method. Scanning electron microscopy, electron probe micro-analysis, and X-ray diffraction analysis are employed to identify intermetallic compounds, chemical reactions, and microstructure evolution after the initial assembly and subsequent isothermal aging at 400 °C and 550 °C. We find that Cu diffuses faster than Ag in PbTe. Neither Cu nor Ag is a good bonding material because they both react vigorously with Pb0.6Sn0.4Te. In order to be able to use Cu electrodes, it would be necessary to insert a diffusion barrier to prevent Cu diffusion into PbTe.

IGBT power module packaging for EV applications

Insulated gate bipolar transistor (IGBT modules are widely used in power electronics applications such as vehicle and household applications as well as industry to realize energy savings with higher efficiency, smaller size, low cost, higher reliability and more environmental safety. However, due to the large power generation of IGBT and diode chips and harsh application environment of vehicle applications, reliability is an important issue. In this paper, the 600V, 450A IGBT high power module packaging technologies for electrical vehicle are developed and verified. The integrated optimum design of thermal, stress and electrical designs is established. Power module assembly technologies with high reliability include chip and DBC (direct bond copper) bonding with low void rate, heavy Al wire bonding and module encapsulation are developed. Finally, the performance of power module is verified by module testing and EV platform testing. The results show that the optimum design and assembly process can reduce junction temperature, thermal stress and parasitic effects of IGBT power modules. The power modules have good performance that can successfully pass through the module and EV platform tests.

Assembly of N type Bi2(Te, Se)3 thermoelectric modules by low temperature bonding

Abstract Solid liquid interdiffusion technique is employed to attach N type Bi2(Te0·92Se0·08)3 thermoelectric material to alumina substrate. The surface finishes of the bonding surfaces for Bi2(Te0·92Se0·08)3 and alumina are Ni and Ag respectively. The bonding layer is pure Sn. The growth kinetic of intermetallic compounds and their microstructure evolution during aging at 200°C was established. The success of this bonding technique provides a cost effective way to assemble thermoelectric modules at a low temperature for higher temperature applications.

Evaluation of Temperature-Dependent Effective Material Properties and Performance of a Thermoelectric Module

We devised a novel method to evaluate the temperature-dependent effective properties of a thermoelectric module (TEM): Seebeck coefficient (Sm), internal electrical resistance (Rm), and thermal conductance (Km). After calculation, the effective properties of the module are converted to the average material properties of a p–n thermoelectric pillar pair inside the module: Seebeck coefficient (STE), electrical resistivity (ρTE), and thermal conductivity (kTE). For a commercial thermoelectric module (Altec 1091) chosen to verify the novel method, the measured STE has a maximum value at bath temperature of 110°C; ρTE shows a positive linear trend dependent on the bath temperature, and kTE increases slightly with increasing bath temperature. The results show the method to have satisfactory measurement performance in terms of practicability and reliability; the data for tests near 23°C agree with published values.

Thermo-mechanical analysis of thermoelectric modules

The present paper studies the thermo-mechanical performance of thermoelectric modules by utilizing the Finite Element Analysis FEA simulation software ANSYS. A typical type TEG device with 32 pairs of legs was constructed. Three different thickness for the pads with 100um, 500um, and 1000um were given for investigating the geometry effect. The thermo-electric results got well confirmed compared with analytical solution. The maximum Von Mises stress occurs on the contact surface between top pad and top substrate due to the large CTE mismatch between the copper pad and the A1N substrate, especially in the higher temperature case. This stress might lead to module failure and reduce the reliability.

600V, 450A IGBT power module for 50kw electrical vehicle

Insulated gate bipolar transistor (IGBT) power modules are widely used in household, industry and vehicle applications due to the features of higher efficiency, smaller size, low cost and higher reliability. However, the large power dissipation of IGBT and diode chips and harsh application environment of vehicle, reliability is an important issue. In this paper, the 600V, 450A IGBT power module packaging technologies for 50kW electrical vehicle (EV) are developed and verified. The integrated electrical, thermal, and stress design is established. Module assembly technologies include chip on direct bond copper (DBC) substrate bonding, DBC substrate on base plate bonding, heavy Al wire bonding are developed. Finally, the performance of power module is verified by module testing, system platform testing and EV vehicle testing. The results show that the IGBT power modules have good performance and successfully pass through the system platform and EV vehicle tests.

Non-refrigerant thermoelectric air conditioning technique on vehicles

An electrical compressor technique of thermoelectric (TE) based climate control system in passenger vehicles provides solid state design and construction, non-refrigerants and the ability of providing faster time to comfort, beltless solution that enables engine-off air conditioner operation and modified the adversely impact by belt-driven vapor compression air conditioners, improved efficiency of positive temperature coefficient (PTC) heater systems. A complete scale TE air conditioning module was designed and adjusted to appropriate performance with CAE analyzing approaches, the well designed module proceeded practically constructed and tested on vehicle system to validate the design concept. In the design, system integration and energy management algorithms have been improved to further increase system level efficiency. To analyze the COP and cooling ability of the TE vehicle cooling system, a mathematical model is proposed, and a robust control algorithm is applied. The test results are presented and discussed.

Direct liquid cooling For IGBT power module

Power electronics systems trend to decrease size, increase switching frequency and voltage ratings. This has resulted in higher power dissipation dnesities of IGBT modules and higher junction temperature of IGBT devices. Thermal management becomes an important package design criteria. From the thermal resistance network of conventional indirect IGBT power module, the thermal interface material between IGBT power module and system cooler occupies a significant part. The present paper studies the thermal effects of IGBT power module with direct liquid cooling design. The microchannel cold plate is bonded with DBC substrate directly and elimitation thermal interface material. Thermal simulations of IGBT power with indirect liquid cooling and direct liquid cooling are complished and compared. The direct liquid cooling using microchannel cold plate can reduce thermal resistance and cooler size of IGBT power module effectively. Additionally, the microchannel cold plate can decrease the warpage and increase reliability of IGBT power module.