Thin-film Thermoelectric Materials Research Articles

Thermoelectric microstructures of Bi 2Te 3/Sb 2Te 3 for a self-calibrated micro-pyrometer

The fabrication of thermopiles suitable for thermoelectric cooling and energy generation using Bi 2Te 3 and Sb 2Te 3 as n- and p-type layers, respectively, is reported. The thin-film thermoelectric material deposition process, thin-film electronic characterization and device simulation is addressed. The thermoelectric thin-films were deposited by co-evaporation of Bi and Te, for the n-type element and Sb and Te, for the p-type element. Seebeck coefficients of −190 and +150 μV K −1 and electrical resistivities of 8 and 15 μΩ m were measured at room temperature on Bi 2Te 3 and Sb 2Te 3 films, respectively. These values are better than those reported in the literature for films deposited by co-sputtering or electrochemical deposition and are close to those reported for films deposited by metal-organic chemical vapour deposition and flash evaporation. A small device with a cold area of 4 mm × 4 mm 2 and four pairs of p–n junctions was fabricated on a Kapton ® substrate, showing the possibility of application in Peltier cooling, infrared detection and energy generation. Small devices fabricated on a polyimide (Kapton ®) substrate and micro-devices fabricated on a silicon nitride substrate were simulated using finite element analysis. The simulations show the possibility of achieving near 20 K cooling over 1 mm 2 areas.

Cross-plane Thermoreflectance Imaging of Thermoelectric Elements

ABSTRACTThis paper presents the first cross-plane thermoreflectance image of the temperature distribution in a thermoelectric (TE) element under bias. Using the technique of lock-in CCD thermoreflectance imaging, we can map the temperature distribution of an operational device with submicron spatial resolution and a temperature resolution of 10 mK. As such it offers a complete picture of the quasi-equilibrium transport within the device. The submicron resolution of the thermoreflectance image enables clear determination of localized heating due at interfaces - for example to due contact resistance - and thermal impedance mismatch within samples. The high spatial resolution is ideal for the characterization of thin-film thermoelectric materials where data from conventional techniques (such as the transient Harman method) are difficult to interpret. This paper also presents the first thermoreflectance data we are aware of for BiTe-based material systems. Identification and separation of the Peltier and Joule components of the heating are possible, and finite difference simulations of the devices are presented for comparison with experiment. In this way it is possible to simultaneously acquire information about the Seebeck coefficient, electrical conductivity, and thermal conductivity of the thermoelectric material. The measurements demonstrate the feasibility of non-contact thermal measurements at the sub-micron scale.

Approaches to thermal isolation with thin-film superlattice thermoelectric materials

ABSTRACTWe propose the use of hetero-structured high ZT materials in thermally isolated thermoelectric couples for the purpose of improving overall device efficiency. Use of high ZT material incorporated in convective flow devices will yield a two-fold improvement in COP over isothermal TE devices. The analytical approach presented in this paper requires optimization of TE element aspect ratio according to available ΔT at location in the device. We discuss the limitations on thermoelectric element characteristics and geometry to achieve high COP. We also utilize moderate temperature differentials to meet the target of high COP. If space requirements are restrictive, or materials processing is limited, the materials described herein provide the unique ability to alter Seebeck coefficient, thermal conductivity, and electrical such that the required aspect ratio is suitable. Improvement in Seebeck coefficient and electrical resistivity independently increase COP by as much as 25% or more when applied to thermally isolated device considered.

Performance of Integrated Thin-film Thermoelectrics in cooling “hot-spots” on Microprocessors – Experimental setup and Results

ABSTRACTThe improving performance of microprocessors is increasing the Thermal Design Power [TDP] and cause localized power densities commonly referred to as “Hot-spots”, which reach in excess of 200 W/cm2. The focus of this work is to present Thin-film Superlattice Thermoelectric materials [TFST] as a solution for microprocessor die hot spot cooling. TFST have measured ZT values of ∼2 at 300K, response times of the order of 15μs and potential to pump heat flux of several hundred Watts/cm2. TFST devices appear suitable for managing the junction temperatures at such hot spots such that the recommended maximum operational temperature, Tjmax, is not exceeded. This promotes stable processor performance and increased reliability. To test this theory, hot spots were identified on the microprocessor die by infrared (IR) imaging during operation and a suitable TFST module was mounted onto the processor. Thermal transients at the hot spot show that active heat pumping and spreading by TFST significantly reduces the constraints on the thermal design. This investigation suggests that in the future, with the transition of Superlattice material performance into higher module ZT values; these hot spots may be effectively removed.

Thin-film thermoelectric devices with high room-temperature figures of merit.

Thermoelectric materials are of interest for applications as heat pumps and power generators. The performance of thermoelectric devices is quantified by a figure of merit, ZT, where Z is a measure of a material's thermoelectric properties and T is the absolute temperature. A material with a figure of merit of around unity was first reported over four decades ago, but since then-despite investigation of various approaches-there has been only modest progress in finding materials with enhanced ZT values at room temperature. Here we report thin-film thermoelectric materials that demonstrate a significant enhancement in ZT at 300 K, compared to state-of-the-art bulk Bi2Te3 alloys. This amounts to a maximum observed factor of approximately 2.4 for our p-type Bi2Te3/Sb2Te3 superlattice devices. The enhancement is achieved by controlling the transport of phonons and electrons in the superlattices. Preliminary devices exhibit significant cooling (32 K at around room temperature) and the potential to pump a heat flux of up to 700 W cm-2; the localized cooling and heating occurs some 23,000 times faster than in bulk devices. We anticipate that the combination of performance, power density and speed achieved in these materials will lead to diverse technological applications: for example, in thermochemistry-on-a-chip, DNA microarrays, fibre-optic switches and microelectrothermal systems.

Properties of thin film thermoelectric materials: application to sensors using the Seebeck effect

The main thermoelectric properties, i.e. Seebeck coefficient α, electrical resistivity ϱ, thermal conductivity K and the coefficient Z of thermoelectric merit, are determined for narrow-gap V 2VI 3 semiconductors and semimetals. Variations in α, K and ϱ depending on the thickness e of the thin film are also measured. The essential technical characteristics such as the sensitivity S f to flux, the time constant τ and the noise equivalent power of a wide-band radiation detector are modelled according to the adapted thermal conductance eK. The most significant results concerning specific applications are described. Knowledge of these data is useful for the production of sensors based on the Seebeck effect, such as thermocouples, thermopiles, radiation detectors, hyperfrequency power sensors and electrical converters.

Thermoelectric thin-film high-temperature measurements by computer control

This paper describes the design and performance of a novel automated apparatus for measuring the Seebeck coefficient (S) and the electrical conductivity (σ) of thin-film thermoelectric materials continuously over the temperature range of 20 to 1000 °C. An inert substrate coated with a thin film thermoelectric material is increased and decreased in temperature at a predetermined rate set by program control while maintaining a constant temperature gradient across the sample. A fully automated computer data acquisition system is used to control instrumentation, measurements, and data storage. This approach is particularly useful for studying the transition from amorphous to crystalline structure in the semiconducting materials. Thin-film thermoelectric materials were prepared by magnetron sputtering and rf glow discharge. Parameters S and σ were measured as a function of temperature, histology, type, and amount of doping. Materials studied had the compositions Bi2Te3, GeTe, Ge80Te20, PbTe, and S.