Thermoelectric Microdevices Research Articles

Integrated micro thermoelectric devices with self-power supply and temperature monitoring: Design and application in power grid early warning

The Internet of Things (IoT) and wearable technology have led to the development of wireless sensors at an unprecedented pace. Within the IoT framework, smart grid systems require real-time temperature monitoring of various switch contacts. Especially in the process of power transformation (China: ultra-high voltage → 10 kV → 380 / 220 V), poor switch contact will lead to overheating, resulting in equipment damage and major safety accidents. Micro-thermoelectric devices (micro-TEDs) exhibit a significantly greater thermoelectromotive force than metal-based thermocouples. By establishing two bivariate first-order equations for hot-end and cold-end temperatures, accurate detection of both temperatures and simultaneous self-power supply can be achieved. In this work, we observe that the Seebeck coefficient of hot extruded n-type Bi2Te2.7Se0.3 and p-type Bi0.5Sb1.5Te3 thermoelectric couple exhibits slight fluctuations around 435 μV/K, while its resistivity follows an almost linear trend with temperature from room temperature to 120 °C. By utilizing these two bivariate first-order equations, the temperatures at both the hot and cold ends can be determined. Based on this, we have fabricated a micro-TED (size: 14 × 6 × 2.5 mm3, TE leg: 0.8 × 0.8 × 1.6 mm3) with a temperature measurement accuracy of up to ± 0.1 °C and a power density of 10.12 mW/cm2 under a temperature difference of 30 °C. Furthermore, we used thirteen micro-TEDs and one circuit board to build a self-power supply temperature monitor. When this TE sensor device was deployed on the plum blossom connector surface and conducted online assessments for the 10 kV substation over a span of approximately 200 h, it continuously outputs a temperature signal and the power supply maintained normalcy, exhibiting no aberrations. This temperature monitoring system can be extended to other fields that require all-weather wireless temperature monitoring, which provides a new direction for the development of micro-TEDs.

Additive Manufacturing of Thermoelectric Microdevices for 4D Thermometry.

Thermometry, the process of measuring temperature, is one of the most fundamental tasks not only for understanding the thermodynamics of basic physical, chemical, and biological processes but also for thermal management of microelectronics. However, it is a challenge to acquire microscale temperature fields in both space and time. Here, a 3D printed micro-thermoelectric device that enables direct 4D (3D Space + Time) thermometry at the microscale is reported. The device is composed of freestanding thermocouple probe networks, fabricated by bi-metal 3D printing with an outstanding spatial resolution of a few µm. It shows that the developed 4D thermometry can explore dynamics of Joule heating or evaporative cooling on microscale subjects of interest such as a microelectrode or a water meniscus. The utilization of 3D printing further opens up the possibility to freely realize a wide range of on-chip, freestanding microsensors or microelectronic devices without the design restrictions by manufacturing processes.

A fully automated measurement system for the characterization of micro thermoelectric devices near room temperature

In this work, we report the customized design and development of a fully automated measurement system to characterize the temperature-dependent properties of microscale thermoelectric devices. The featured system can characterize the devices up to a dimension of 8.79 × 8.79 mm2 in an atmospheric environment. Commercially available bismuth telluride-based thermoelectric modules (both thermoelectric generator and thermoelectric cooler) are used to validate the feasibility and accuracy of the developed system. Their performance data is analysed and compared with the available manufacturer datasheet. The measured data from these devices are found to be in good agreement with the anticipated values and showed an acceptable deviation of less than 4% in the output performances. The developed setup is simple to operate and suitable for performance evaluation of both macro and micro-thermoelectric devices. The system accurately reproduces application conditions the module may be subjected to in a real-world environment.

Fluctuation-dissipation in thermoelectric sensors

Thermoelectric materials exhibit correlated transport of charge and heat. The Johnson-Nyquist noise formula 4k B T R for the spectral density of voltage fluctuations accounts for fluctuations associated solely with Ohmic dissipation. Applying the fluctuation-dissipation theorem, we generalize the Johnson-Nyquist formula for thermoelectrics, finding an enhanced voltage fluctuation spectral density 4k B T R(1 + Z D T) at frequencies below a thermal cut-off frequency f T , where Z D T is the dimensionless thermoelectric device figure of merit. The origin of the enhancement in voltage noise is thermoelectric coupling of temperature fluctuations. We use a wideband , integrated thermoelectric micro-device to experimentally confirm our findings. Measuring the Z D T enhanced voltage noise, we experimentally resolve temperature fluctuations with a root mean square amplitude of at a mean temperature of 295 K. We find that thermoelectric devices can be used for thermometry with sufficient resolution to measure the fundamental temperature fluctuations described by the fluctuation-dissipation theorem.

High-Precision and Low-Damage Microchannel Construction via Magnetically Assisted Laser-Induced Plasma Ablation for Micro-Thermoelectric Devices.

Thermoelectric devices are developing toward high density and miniaturization with a large filling factor for new applications in chip thermal management and microenergy harvesting. Pulsed laser etching has become one of the most effective tools for the patterning construction of highly integrated micro-thermoelectric devices. However, the laser spot size and Gaussian laser energy distribution restrict the processing size and accuracy of microchannels. Moreover, the rapid temperature rise caused by laser energy injection would also raise serious problems such as element volatilization, cracks, and recast layers. Herein, a liquid-assisted nanosecond laser ablation technology with magnetically controlled plasma is proposed to etch microchannels on thermoelectric thick films. By evaluating the size and shape of microchannels, we theoretically investigated the influence of cavitation bubbles on the laser optical path and surface roughness in laser-induced plasma ablation. In addition, the energy criterion for high-precision ablation is revealed, and the effect of magnetic field on ablation threshold is explained by magnetic constraint on energy and kinetic properties of the laser-induced charged plasma plume. Finally, the high-precision and low-damage microchannels are achieved on Bi2Te3 thermoelectric thick films with a minimum line width of 19.12 μm and a small sidewall inclination degree of tan θ = 0.085. This work provides a promising alternative for the fabrication of high-density three-dimensional (3D) patterning in semiconductor microdevices.

Micro-thermoelectric devices

Micro-thermoelectric devices

Rapid Selective Ablation and High-Precision Patterning for Micro-Thermoelectric Devices Using Femtosecond Laser Directing Writing.

Highly integrated miniature thermoelectric (TE) devices are desirable for applications of chip thermal management and self-powered energy harvesting. Currently, further performance improvement of micro-TE devices is largely limited by micro-nano-patterned processing, which shows the incompatibility with high-performance TE material fabrication or contradiction between machining accuracy and efficiency. This work presents a useful method to flexibly achieve high-precision array patterning for the micro-TE device through the femtosecond laser direct writing technique. By experimentally examining the material ablation process and numerically analyzing the electron-lattice temperature, the laser energy threshold for different materials is determined to obtain the selective removal between TE materials and metallic electrodes. Furthermore, the evaluation criteria are established between the formation quality of microgroove in the array structure and the laser pulse energy distribution, and the shape-control and property-control pattern processing can be realized through the reasonable control of the laser energy. Consequently, the Bi2Te3-based TE pattern with a competitive leg density (496 pairs/cm2) and a high filling factor (55%) is successfully constructed.

Interface performance of Bi<sub>2</sub>Te<sub>3</sub>-based micro thermoelectric devices optimized synergistically by surface modification engineering

The miniaturization of thermoelectric devices raises a strong requirement for the excellent interfacial properties of thermoelectric elements. Thus, achieving a heterogeneous interface with low interfacial contact resistivity and high interfacial bonding strength is a prerequisite for the successful fabrication of high-performance and high-reliability Bi<sub>2</sub>Te<sub>3</sub>-based micro thermoelectric devices. In this work, we adopt the acid pickling method to modify the surface structure of Bi<sub>0.4</sub>Sb<sub>1.6</sub>Te<sub>3</sub> material to synergistically optimize the interfacial properties of Bi<sub>0.4</sub>Sb<sub>1.6</sub>Te<sub>3</sub>/Ni thermoelectric elements. The acid pickling process effectively modulates the work function of Bi<sub>0.4</sub>Sb<sub>1.6</sub>Te<sub>3</sub> material, which dramatically reduces the contact barrier height of Ni/Bi<sub>0.4</sub>Sb<sub>1.6</sub>Te<sub>3</sub> heterojunction from 0.22 to 0.02 eV. As a consequence, the corresponding interfacial contact resistivity of the element is greatly reduced from 14.2 to 0.22 μΩ·cm<sup>2</sup>. Moreover, the acid pickling process effectively adjusts the surface roughness of the matrix, forming a V-shaped pit of 2–5 μm in depth on the substrate surface and leading to a pinning effect. This significantly enhances the physical bonding between the material surface and the Ni layer, which, together with the metallurgical bond formed by the interfacial diffusion reaction zone of about 50-nm-thick Ni/Bi<sub>0.4</sub>Sb<sub>1.6</sub>Te<sub>3</sub>, greatly enhances the interfacial bond strength from 7.14 to 22.34 MPa. The excellent interfacial properties are further validated by the micro-thermoelectric devices. The maximum cooling temperature difference of 4.7 mm× 4.9 mm micro thermoelectric device fabricated by this process achieves 56.5 K, with hot side temperature setting at 300 K, and the maximum output power reaches 882 μW under the temperature gradient of 10 K. This work provides a new strategy for realizing the synergetic optimization of interfacial properties and opens up a new avenue for improving the performance of micro thermoelectric devices.

Morphology Control of Cu Added Electrodeposited Bi-Sb-Te Thick Films for Micro-Thermoelectric Devices

In this research, bismuth and tellurium based thermoelectric materials are studied and electrodeposition method was adapted to fabricate the devices. To improve the thermoelectric performance, the thickness of the patterned films was attempted to be increased up to 30 µm. Film properties, such as morphology, composition, resistivity and thermoelectric output, of p-type Cu-added Bi-Sb-Te films were investigated. It was found that the thermoelectric properties were improved with an increase in the film thickness. An annealed buffer layer was created as substrate to control the morphology of Bi-Sb-Te films to be more compact and denser. Finally, 30 µm thick p-type Bi-Sb-Te patterned-films were successfully fabricated, indicating the applicability to micro-thermoelectric devices.

A synergy of strain loading and laser radiation in determining the high-performing electrical transports in the single Cu-doped SnSe microbelt

Semiconducting microbelts are key components of the thermoelectric micro-devices, and their electrical transport properties play significant roles in determining the thermoelectric performance. Here, we report heavily Cu-doped single-crystal SnSe microbelts as potential candidates used in thermoelectric microdevices, fabricated by a facile solvothermal route. The considerable Cu-doping concentration of ~11.8% up to the solubility contributes to a high electrical conductivity of ~416.6 S m−1 at room temperature, improved by one order of magnitude compared with pure SnSe (38.0 S m−1). Meanwhile, after loading ~1% compressive strain and laser radiation, the electrical conductivity can be further improved to ~601.9 S m−1 and ~589.2 S m−1, respectively, indicating great potentials for applying to thermoelectric microdevices. Comprehensive structural and compositional characterizations indicate that the Cu+ doping state provides more hole carriers into the system, contributing to the outstanding electrical conductivity. Calculations based on first-principle density functional theory reveal that the heavily doped Cu lowers the Fermi level down into the valence bands, generating holes, and the 1% strain can further reduce the bandgap, strengthening the ability to release holes, and, in turn, leading to such an excellent electrical transport performance. This study fills the gaps of finding novel materials as potential candidates used in the thermoelectric microdevices and provides new ideas for micro/nanoscale thermoelectric material design.

Fabrication of micro-thermoelectric devices for power generation and the thermal management of photonic devices

This work demonstrates and discusses the fabrication of cross-plane configured micro thermoelectric devices for the power generation and thermal management of the photonic devices. The device is fabricated using a cost-effective electrodeposition technique on the silicon wafer with 210 pairs of the electrodeposited p-type BiTe and n-type CuTe pillars. The complete device is fabricated using the flip-chip bonding technique. Our focus in this work is on the challenges in the device fabrication and the solutions employed to overcome the obstacles thereby successfully fabricating the micro thermoelectric device.

Micro‐Thermoelectric Devices: Three‐Mode Modulation Electrochromic Device with High Energy Efficiency for Windows of Buildings Located in Continental Climatic Regions (Adv. Sustainable Syst. 3/2019)

Micro‐Thermoelectric Devices: Three‐Mode Modulation Electrochromic Device with High Energy Efficiency for Windows of Buildings Located in Continental Climatic Regions (Adv. Sustainable Syst. 3/2019)

Design Guidelines for Micro‐Thermoelectric Devices by Finite Element Analysis

AbstractEnergy harvesting and thermal management are required for applications in the internet‐of‐things, autarkic sensors, or highly integrated electronic devices. Thermoelectric devices, both generators and coolers, are promising specialized technologies for localized energy harvesting and thermal management. These devices are well optimized for near‐room temperature operation at the macroscopic scale. However, the high integration density of today's most significant applications requires an increasing degree of miniaturization. Understanding the design guidelines for micro‐thermoelectric devices with realistic materials properties, and with concurring size and geometry constraints, is a challenge that has not been fully addressed up to now. Here, finite element analysis is used to understand the interplay between thermal and electrical heat fluxes in micro‐thermoelectric devices. The relevant design guidelines for metallic top and bottom contacts and thermoelectric elements, as well as an optimal packing density of the thermoelectric elements are identified. The results demonstrate that on the micrometer scale, the effects of net electrical and thermal resistances of the individual components of the devices (i.e., thermoelectric leg, top, and bottom contacts) are of comparable magnitude. This makes it necessary to apply design strategies specific to the micrometer scale, in order to geometrically optimize the device.

Micro-thermoelectric devices with large output power fabricated on a multi-channel glass template

Micro-TE (thermoelectric) devices have received increasing attention because of their promising applications as energy resources for micro-systems. However, poor performance and complicated fabrication process has limited their application. This work demonstrates that micro-TEGs (thermoelectric generators) with a large output power of 218 μW and output voltage of 34.7 mV under a larger temperature difference of 98 K can be fabricated using electro-deposition based on the multi-channel glass template, which is much higher than that of other reported work. The micro-TEGs consisted of four pairs of thermocouples. Each thermocouple was composited by about 200 p-type legs of Sb2Te3 and 200 n-type legs of Bi2Te3 60 μm in diameter and 200 μm in height, and were well filled in the glass template by impulse voltage deposition. The length of the TE legs can be adjusted by increasing the length of the channel, which led the large temperature difference and high output power and output voltage. Additionally, the glass template which shows low thermal conductivity and is electrically insulating, can be retained after being filled with TE materials, which simplifies the device fabrication process and also helps to enhance the rigidity of the TE modules. Moreover, this work also reveals that the output voltage can be further improved by increasing the numbers of thermocouples via decreasing the numbers of legs in each thermocouple.

Microthermoelectric devices keep their cool

Microthermoelectric devices keep their cool

Design and Simulation of a Novel Thermoelectric Micro-Device with Electrodeposited Bi-Te Alloys

A novel thermoelectric micro-device was designed with n-type and p-type Bi-Te materials alloys via a template electrodeposition process. The glass template including 250 holes in 10×10 mm2with a thickness of 200~ 400 µm. The diameter of the holes is 50~ 80 µm and the distance of adjacent centers of the holes is 200 µm. According to the design, the performance of heat transference and thermoelectric energy generation are simulated by COMSOL Multiphysics. In order to simplify model, there are 16 units in total, and each unit is made up of 16 (4 × 4) pillars. In the simulation, the largest temperature difference is 7.8K on the conditions of 500 W/m2K in convection heat transfer coefficients and the maximum output potential of the module is 21.7 mV. The maximum output power achieved 96.9 µW under 500 W/m2K of heat transfer coefficient and 10 mA of current. Under ideal conditions, the value of open circuit voltage and maximum output power increases to nine times as the model, but short circuit current remains the same. When the heat transfer coefficient is 500 W/m2K and the current density is 10 mA, the maximum output power of the actual product achieved 871.7 µW.

Fabrication of ZnO-Based Thermoelectric Micro-Devices by Electrodeposition

In order to fabricate micro-thermoelectric devices, ZnO films were prepared via electrodeposition, and the effect of deposition condition on the films was investigated. It was found that morphology and crystallinity of ZnO films were strongly affected by applied potential, bath composition and bath temperature, and using the optimized condition, ZnO-based micro-thermoelectric devices were fabricated and device performance was evaluated. For electrodeposition, acetate ion was added into the bath to prevent several drawbacks such as formation of pits and cracks. From the solution containing zinc nitrate and sodium acetate as a source of acetate ion, ZnO films were electrodeposited. The films with smooth morphology and sufficient adhesion strength were obtained at −0.9 V vs. Ag/AgCl with the addition of 10 mmolL−1 sodium acetate. The films with higher crystallinity and thermoelectric properties were obtained from the bath with higher temperature of 80°C. The micro-thermoelectric device, which consisted of four arrays and each array had 55 pairs of thermoelectric elements, was fabricated and it generated 0.73 nW as maximum power. The power density was approximately 40 times larger than that of the ZnO thin films, which demonstrated that electrodeposited ZnO could be applied to thermoelectric micro-devices.

Electrodeposition of ZnO from Acetate Bath for Thermoelectric Devices

We attempted to form ZnO films with smooth surface for the application of thermoelectric micro-devices, using electrodeposition process. In order to reduce the defects of the films such as cracks and pits mainly caused by the gas evolution, the bath containing acetate in addition to nitrate ions was developed to reduce the gas evolution during the deposition. Using this bath, formation of ZnO, rather than Zn metallic films, was confirmed by Raman spectroscopy. The films had no cracks and the surface became smoother than those electrodeposited from the conventional nitrate-based bath. In order to fabricate the micro-devices, The ZnO was electrodeposited into the patterns with diameter of 200 μm. As a result, ZnO successfully filled the pattern keeping the smooth surface conditions, which demonstrated the applicability of the ZnO films to micro-device fabrication.

(Invited) Electrodeposition of PbTe Thick Films for Micro Thermoelectric Devices

Heat is the natural by-product of energy conversion processes. Of the 4.25 × 1020 J of energy the United States consumes every year, more than 60% is wasted in the form of heat. Therefore, waste heat recovery is a crucial step to improve the energy generation and utilization efficiency. Thermoelectric (TE) materials, which can directly convert rejected or waste heat into usable electric power, has been extensively developed for this issue. Thick-film-based devices have advantages over conventional TE module because of its compact size. By shrinking the size of thermoelectric devices, it not only allows the device to operate under smaller temperature gradients, but by so doing it expands its capability to handle a wider range of thermal and power management microelectronic systems. The combination of electrochemical deposition of compound semiconductors (metal tellurides) with standard integrated circuit technique makes fabrication of thermoelectric microdevices possible. In our work, we demonstrated a rapid deposition process (180 µm/h) in alkaline solution to synthesize dense thick PbTe film (>50 µm) for thermoelectric applications. Compared to the commonly used acidic baths with deposition rate about 4 µm/h, our electrodeposition process enhanced the film growth rate 48 folds higher. The electrodeposition mechanism of PbTe was examined. The thermoelectric property of thick PbTe films were optimized by introducing energy barriers from crystal grain boundary and tellurium nanoinclusion, which was achieved by tailoring composition and crystal structures of the thick PbTe films. Additionally, control over the composition and crystal structures was realized by tuning the electrodeposition conditions, such as the ratio of [TeO3 2-], [Pb2+], [EDTA4-]/[Pb2+], solution pH, and applied potential, as well as post-annealing process.

Non-epitaxial pulsed laser deposition of Ag2Se thermoelectric thin films for near-room temperature applications

Non-epitaxial growth of Ag2Se thermoelectric thin films on (La, Sr) (Al, Ta)O3 or glass substrates at a relatively low temperature of 100°C has been achieved by using pulsed laser deposition (PLD). The deposition kinetics has been optimized by varying the background interactions with laser induced plasma plume. The shockwave plasma expansion is to be critical for the structure, composition and morphologies of thin films. As-grown Ag2Se thin films under the optimized deposition condition exhibit comparable transport performances as compared to the reported Ag2Se bulk materials at similar carrier concentrations. The non-epitaxial correlation between the Ag2Se thin films and their substrates is expected to benefit for the further fabrication of in plan thermoelectric micro-devices for room temperature applications.