Jikun Chen

Glass formation mechanism of minor yttrium addition in CuZrAl alloys

Effects of yttrium additions on glass-forming ability of the model system (Cu0.48Zr0.48Al0.04)100−xYx (x=0–7at.%) were studied in detail by using a “perturbation theory” approach. Introduction of the atomic-level strain energy into the driving force for the crystal nucleation well explained the beneficial effects of yttrium additions. It was found that the atomic-level strain energy is related closely to the atomic packing efficiency of the precipitated crystals and less yttrium amount is needed to suppress the precipitation of crystal phase with a higher atomic packing efficiency, which is consistent with the experimental observations.

Laser deposition and direct-writing of thermoelectric misfit cobaltite thin films

A two-step process combining pulsed laser deposition of calcium cobaltite thin films and a subsequent laser induced forward transfer as micro-pixel is demonstrated as a direct writing approach of micro-scale thin film structures for potential applications in thermoelectric micro-devices. To achieve the desired thermo-electric properties of the cobaltite thin film, the laser induced plasma properties have been characterized utilizing plasma mass spectrometry establishing a direct correlation to the corresponding film composition and structure. The introduction of a platinum sacrificial layer when growing the oxide thin film enables a damage-free laser transfer of calcium cobaltite thereby preserving the film composition and crystallinity as well as the shape integrity of the as-transferred pixels. The demonstrated direct writing approach simplifies the fabrication of micro-devices and provides a large degree of flexibility in designing and fabricating fully functional thermoelectric micro-devices.

Photo-induced enhancement of the power factor of Cu2S thermoelectric films

Element doping is commonly used to adjust the carrier concentrations in semiconductors such as thermoelectric materials. However, the doping process unavoidably brings in defects or distortions in crystal lattices, which further strongly affects the physical properties of the materials. In this work, high energy photons have been used to activate the carriers in Cu2S thermoelectric films. As a result, the carrier concentrations, and the respective electrical conductivity as well as Seebeck coefficient are further changed. The photon-induced electrical transport properties are further analyzed utilizing a Parallel circuit model. Due to the realization of optimized carrier concentrations by photon activation, the power factor of Cu2S film is improved more than 900 times as compared with the dark data. As compared to the traditional doping process, the approach using photon activation can realize the tuning of carrier concentrations without affecting crystal lattice. This method provides an opportunity to investigate the intrinsic physical properties of semiconductor materials without involving traditional element doping process that usually brings in additional lattice defects or distortions.

Direct tuning of electrical properties in nano-structured Bi2Se0.3Te2.7 by reversible electrochemical lithium reactions

Lithium intercalation and de-intercalation processes have been used to fabricate bulk Bi(2)Se(0.3)Te(2.7) with internal nanostructures. The doped Li content can be precisely controlled through this method. It provides a chance to directly optimize electrical properties when preparing nano-structured materials, leading to the optimum carrier concentration for improved thermoelectric figure of merit.

Enhanced thermoelectric performance in rare-earth filled-skutterudites

A series of rare-earth filled skutterudites are successfully fabricated by using a non-equilibrium synthesis approach involving super-cooling and high pressure sintering. The filler filling fractions are significantly improved to realize optimum carrier concentrations and hugely suppressed lattice thermal conductivity, thereby leading to significantly enhanced zT.

Electrical and thermal transport properties of Y bxCo4Sb12 filled skutterudites with ultrahigh carrier concentrations

For filled skutterudites, element Yb is one of the most common and important fillers. However, the optimal carrier concentration range in Y bxCo4Sb12 filled skutterudites has not been determined as a result of the low Yb filling fraction limit. In this study, a non-equilibrium fabrication process (MS-SPS process), consisting of a melt-spinning method and a spark plasma sintering technique, has been applied to prepare Y bxCo4Sb12 samples. The Yb filling fraction is successfully extended to 0.35, which provides the possibility to clarify the optimal carrier concentration range for Yb-filled skutterudites. High carrier concentrations, with a maximum of around 1 × 1021 cm−3, were achieved in the MS-SPS Y bxCo4Sb12 samples due to the significantly enhanced Yb filling fractions. The phase compositions, lattice parameters, electrical and thermal transport properties of the MS-SPS Y bxCo4Sb12 samples with high carrier concentrations were systematically investigated. An optimal carrier concentration range of around 5 ∼ 6 × 1020 cm−3, corresponding to the actual Yb filling fraction of around 0.21∼0.26, has been determined, which displays the highest thermoelectric performance in Y bxCo4Sb12 thermoelectric materials.

Analytical investigations on the thermal properties of microscale inorganic light-emitting diodes on an orthotropic substrate

The microscale inorganic light-emitting diodes (μ-ILEDs) create novel opportunities in biointegrated applications such as wound healing acceleration and optogenetics. Analytical expressions, validated by finite element analysis, are obtained for the temperature increase of a rectangular μ-ILED device on an orthotropic substrate, which could offer an appealing advantage in controlling the heat flow direction to achieve the goal in thermal management. The influences of various parameters (e.g., thermal conductivities of orthotropic substrate, loading parameters) on the temperature increase of the μ-ILED are investigated based on the obtained closed-form solutions. These results provide a novel route to control the temperature distribution in the μ-ILED system and provide easily interpretable guidelines to minimize the adverse thermal effects.

Composition control of pulsed laser deposited copper (I) chalcogenide thin films via plasma/Ar interactions

We present how the applied Ar background pressure correlates to the cation and anion composition of Cu(I) chalcogenide thin films grown by pulsed laser deposition (PLD). The as-grown films produced at ∼10−2 mbar pAr show a more pronounced deficiency in lighter composition, as compared with using quasi-vacuum or ∼10−1 mbar. The thermoelectric performance of the as-grown Cu2Se varies consistently with the respective changes in Cu/Se ratio vs.pAr. The optimum thermoelectric performance of Cu2Se is achieved by growing in vacuum or quasi-vacuum, where dense and even thin film morphology is obtained while the cation/anion composition is more congruent than at higher pAr.摘要本文阐述了在利用脉冲激光沉积法生长亚铜基硫族化合物薄膜过程中, 所使用的氩气背景气体压力与薄膜化学组分的关系. 当氩气压力在∼10−2 mbar时, 薄膜材料中原子量较轻的成分缺失更为严重. 与之相比, 当利用准真空或∼10−1 mbar时薄膜材料化学组分与所用靶材组分更为接近. 以硒化亚铜为例, 研究结果显示所生长薄膜材料的电输运性能与铜硒组分比例随压力的变化相一致. 当生长条件为真空或准真空时, 所生长的硒化亚铜薄膜具有致密的结构和最佳的化学组分, 因而具有最优的热电性能.