Electrothermal Characterization Research Articles

Thermal and Optical Characterization of Silicon-Based Tunable Optical Thin-Film Filters

In this paper, we report on the successful fabrication and comprehensive optical and electrothermal characterization of a silicon-based tunable optical filter. Of particular interest here are the static and transient electrothermal characteristics of the filter membrane. The filter is configured as a Fabry-Perot resonator with dielectric Bragg reflectors and is fabricated as a stress-compensated multilayer of dielectric thin-films. Tunability is achieved by changing the refractive index of the solid-state cavity material through thermal modulation, accomplished by Joule heating in metal thin-film resistors. The filter layers are configured as a membrane to provide good thermal isolation and its electrothermal behavior was characterized in steady-state and dynamically. A maximum stable temperature difference of 450 K and a heating efficiency of 13.4 K mW-1 were measured with a thermal time constant for the filter hotplate of 3 ms. The filter's spectral width is 0.28 nm at a wavelength of 1550 nm and through thermal modulation it was possible to shift the filter peak over more than 29 nm.

Electrothermal characterization of tungsten-coated carbon microcoils for micropropulsion systems

Laser-induced chemical vapor deposition is used to deposit tungsten-coated carbon microcoils from the gas phase. Because the microcoils are used as heating elements in cold/hot gas microthrusters in nanosatellites, it is important to characterize their electrothermal behavior so that the performance of the thruster may be predicted. The coils are deposited using an argon-ion laser (wavelength 514.5nm) at 360mW and 930mbar of C2H4. Bent arms are then deposited at the ends of the 1mm long coils using 600mW of laser power and 700mbar of C2H4. The arms serve as electrical contacts and as mechanical supports to hold the coils in the thruster by a locking mechanism. A layer of tungsten is then applied to the carbon coils by the hydrogen-reduction of WF6 using a 20:1 (H2:WF6) pressure ratio at a total pressure of 105mbar and 400mW of laser power. High-resolution scanning electron microscopy analysis showed the tungsten coating to be 1.5–3.5μm thick on the body of the coil and less than 2μm on the contact arms. The tungsten coating reduced the resistance of the carbon coils by an order of magnitude and reached higher temperatures at lower voltages. In vacuum, the tungsten coating is the dominant current carrier below 1300–1700°C; above this range carbon dominates. Peak temperatures for the tungsten-coated coils are 2050°C in vacuum and 1940°C in N2 – several hundred degrees higher than the non-coated coils.

The electrothermal feasibility of carbon microcoil heaters for cold/hot gas microthrusters

With the miniaturization of spacecraft the need for efficient, accurate and low-weight attitude control systems is becoming evident. To this end, the cold/hot gas microthruster system of this paper incorporates carbon microcoils—deposited via laser-induced chemical vapor deposition—for heating the propellant gas (nitrogen) before the nozzle inlet. By increasing the temperature of the propellant gas for such a system, the specific impulse (Isp) of the microthruster will increase. The benefits of a higher Isp are lower propellant mass, higher thrust and shorter burning times. Therefore, the feasibility of achieving this increase with the carbon microcoils is investigated. The carbon microcoils have been characterized experimentally with respect to their electrothermal performance, i.e. resistance, temperature, parasitic heat losses and degradation in ambient. The resulting heat losses from the heater and the heated gas have been estimated through a combination of experiments, numerical simulation and approximate analytical expressions. At high powers, degradation of the carbon material leads to coil failure in ambient where trace oxygen was present. Thus, the next generation of carbon microcoils to be tested will have a protective coating to extend their lifetime. Theoretical modeling showed that an increase in the propellant gas temperature from 300 to 1200 K and a corresponding two-fold increase in the Isp can be achieved if 1.0 W of power is supplied to each coil in a three-coil thruster. These simulation results show that if the coils are capable of dissipating 1 W of heat at 1700 K coil temperature, the doubling of the Isp may be achieved. Comparing to the electrothermal characterization results we find that the carbon coils can survive at 1700 K if protected, and that they can be expected to reach 1700 K at power below 1 W.

First step in the reliability assessment of ultracapacitors used as power source in hybrid electric vehicles

Regarding the new power requirements in hybrid electric vehicles, ultracapacitors seem to be very attractive. The examination of their existing characteristics, combined with the pulsed charge-discharge current mode, leads us to propose an original approach for electro-thermal characterization of ultracapacitors. Therefore, a dedicated test bench has been designed allowing both electric and thermal behaviour investigations. The principle of this study is to investigate the ultracapacitors thermal behaviour thanks to several specific cyclic current profiles. The use of these profiles will constitute the very first step in an assessment of the ultracapacitors reliability devoted to power source operation in automotive applications.

Test procedures and lifetime data analysis for electro-thermal endurance characterisation of EPR-insulated cables

The endurance of EPR-insulated cables is investigated by means of laboratory tests on cable models aged by thermal and electrical stresses, single or combined. The electrical, thermal and multi-stress life lines obtained by tests carried out according to a factorial procedure are reported. A significant contribution to plot the life lines is provided by the combined analysis method, which allows the electro-thermal life surface to be drawn. Indices for electrical, thermal and combined-stress endurance characterisation can thereby be supplied. The test results give high values of electrical, thermal and multi-stress endurance coefficients for the tested cables. This confirms the good electrical and thermal behaviour of EPR-insulated cables. Finally, a comparison of the results with a life model, able to provide life estimations, is made, obtaining good data fitting.

Electrothermal Vaporization and Characterization of the Graphite Surface at Elevated Temperatures

Electrothermal Vaporization and Characterization of the Graphite Surface at Elevated Temperatures

Electrothermal Characterization for a Fusible Link

A low energy fusible link was characterized by its electrothermal analog model. Thermal transient testing apparatus, procedures, and methodology were used to determine the required parameters. The thermal time constant was determined by a novel comparison technique. Typical data and its relevance are presented.

Electrothermal considerations in display applications of light-emitting diodes

Some of the possibilities and problems of high-density integration and packaging of light-emitting diode displays are explored, using a new electrothermal characterization of light-emitting diodes. After a brief discussion of thermal effects in spontaneous light-emitting diodes, the characterization is derived. It reveals a peak in luminance with increasing electric power due to self heating. Experimental results confirm the accuracy of the characterization for gallium arsenide phosphide monolithic displays. Finally, the characterization is used to evaluate some projected high power-density applications. These particularly involve dense diode arrays and the use of higher levels of integration, together with magnification by projection of an image onto a screen, to reduce the cost per element associated with processed chip area. Self heating is found to cause a strong dependence of the luminance on the number of elements powered in a dense array, and to limit severely the achievable magnification even with improved packaging.