G. De Mey

Thermal impedance plots of micro-scaled devices

The complex thermal impedance Zth of a microelectronic heat source on the surface of a silicon wafer has been calculated semi-analytically as a function of the frequency. By representing the results in a Nyquist plot, almost perfect circular curves are obtained. This result is analogous to complex loci of the dielectric constant obtained for some materials.

On the optimum efficiency of endoreversible thermodynamic processes

The classical theory of endoreversible thermodynamics gives rise to an optimum efficiency for the power output given by 1-T2/T11/2 in contrast to the Carnot efficiency 1-T/T1 used in reversible thermodynamics. It will be shown that many systems show the same efficiency, although a system giving an optimum efficiency involving T2/T11/3 has been found.

A Fixed-Angle Heat Spreading Model for Dynamic Thermal Characterization of Rear-Cooled Substrates

During a period of almost 40 years already, various fixed-angle heat spreading models have been developed in the literature. These models can be used by thermal engineers as approximations for the thermal steady state resistance of a heat source on a rear-cooled substrate. In this paper an extension of these models to dynamic (time-dependent) phenomena is proposed. The heat dissipated by a square source (side a) is assumed to spread out into the substrate (thickness ts) under a fixed angle Phi. An analytical solution for the complex thermal impedance Zth(jomega) in phasor notation is derived. The obtained expression, in which Phi is used as a fitting parameter, is compared with exact results. A very good agreement is observed (average relative error less than 6%) for a wide range of the normalized thickness lambda = ts/a. A compact expression for the optimal heat spreading angle as a function of lambda is given. Finally the temperature response to a heat power step is investigated. A simple formula for the thermal rise time is provided, allowing a thermal designer to make quick yet accurate estimations about the dynamic behavior of the device.

Determination of the electric field in a Hall generator under influence of an alternating magnetic field

Abstract A method is described to calculate the electric field in a Hall generator under influence of an alternating magnetic field. The problem is divided into two separate problems, the first of which describes the Hall effect while the second gives the behaviour of the eddy currents. Each of these problems can be reduced to an integral equation, which is easily solved on a digital computer. The method is illustrated for a rectangular Hall generator.

Performance study of screen-printed textile antennas after repeated washing

Abstract The stability of wearable textile antennas after 20 reference washing cycles was evaluated by measuring the reflection coefficient of different antenna prototypes. The prototypes’ conductive parts were screen-printed on several textile substrates using two different silver-based conductive inks. The necessity of coating the antennas with a thermoplastic polyurethane (TPU) coating was investigated by comparing coated with uncoated antennas. It is shown that covering the antennas with the TPU layer not only protects the screen-printed conductive area but also prevents delamination of the multilayered textile fabric substrates, making the antennas washable for up to 20 cycles. Furthermore, it is proven that coating is not necessary for maintaining antenna operation and this up to 20 washing cycles. However, connector detachment caused by friction during the washing process was the main problem of antenna performance degradation. Hence, other flexible, durable methods should be developed for establishing a stable electrical connection.

An integral equation approach to ac diffusion

In Fig. 2, the experimental data by Krall rt trl. and Filetti et ul. are compared with authors’ results. The slopes of Filetti’s results show a good agreement with authors’ in the limited range of xR/L. It is interesting to note that Nu~,,_~ is approximated by (4) without any distinction of short or long stall. In conclusion, NUT,, maX is solely dependent on a stall length, xR/L, and decreases with increasing stall length. The accuracy of NUL, m.,X estimated by the empirical relation (4) is within k 10”); for 4 x IO3 < RP[. i 8 x 10“ and 0.2 < .YJL i 16.0. REFERENCES

Vacuum deposition rate measurements on thin polymer films

Abstract The evaporation of polymer films is described by a simple theory, which indicates clearly that electronic rate measurement gives no correct result due to the evaporation mechanism. The theory is illustrated with experimenal results made on PTFE.

Chimney Effect on Natural Convection Cooling of a Transistor Mounted on a Cooling Fin

The thermal impedance Z(th)(j omega) has been measured for the junction of a transistor mounted on a cooling fin. The setup was put inside a chimney in order to enhance the natural convection cooling. From the thermal impedance Z(th)(j omega) the influence of the chimney on the reduction in thermal resistance R-th=Z(th)(j omega=0) is clearly visible. A maximum improvement of almost 20% has been observed experimentally. Measuring the thermal impedance as a function of the angular frequency omega provides specific information about the thermal path being influenced by the chimney.

Electrical circuit model of elastic and conductive yarns produced by hollow spindle spinning

Abstract The implementation of electronic textiles which are capable of being applied as electrodes, sensors or heating elements is crucial in several fields of application, ranging from automotive to sports, from rehabilitation to art and design. However, current electronic textiles often suffer from poor mechanical properties, low drape and unstable electroconductive characteristics. One possibility to overcome these shortcomings is to introduce the electroconductive properties at a fibre or yarn level by combining traditional fibres and yarns with metal wires or metalised yarns. In this paper, we describe the production of multicomponent yarns which combine electroconductive properties with elasticity, drape and mechanical strength. We discuss the electroconductive properties of the yarns produced with hollow spinning technology. An electrical model based on Kirchhoff’s law was designed and validated with the abovementioned measurements.

Current Distribution Modelling in Electroconductive Textiles

In this paper the current distribution within woven electroconductive textile sheets was investigated using a computer program written for the purpose of the simulations. An iterative method of solving very large resistor networks used to model textile sheets is discussed. The factors taken into account are the conductivity of the fibres, the contact resistance between the fibres, the contact angle a between the fibres and the electrodes, the size and the aspect ratio of the textile sheets. Maps of the current distribution generated by the program are included.