Journal of Electronic Materials

Recent theoretical studies indicate that the existence of resonant states near the Fermi level can lead to a significant improvement in the thermoelectric figure of merit. Experimental investigation of this concept requires determination of the existence of these resonant states. In this paper, we report a theoretical calculation on the I–V characteristic of a thermoelectric/insulator/superconductor-based tunneling junction. The results show that the resonant states can be detected by measuring the I–V characteristic of such a superconducting tunneling junction, which provides a theoretical basis for development of a measurement technique to detect the resonant states for thermoelectric materials.

To improve the efficiency of power modules in environmentally friendly vehicles, silicon-carbide (SiC) chips and silicon-nitride (Si3N4) active metal-brazed (AMB) substrates were bonded by low-pressure silver (Ag) sintering at 220°C and 1 MPa using Ag paste. The initial bond strength of the sintered joint was 35.7 MPa, and the void content and bonding-layer thickness of the sintered joint were 4–7% and 94–99 μm, respectively. To verify the reliability of the silver-sintered joints, we conducted thermal cycling tests (TCTs) and high-temperature storage tests (HTSTs). The bond strength of the SiC chip/Si3N4 AMB substrate after the TCT decreased to 16.9 MPa, but increased to 43.6 MPa after the HTST. Thermomechanical fatigue cracks from the difference in the thermal-expansion coefficient occurred at the sintered joint interface during the TCT, which decreased the sintered joint strength. As the sintering process progressed continuously during the long test time at a high temperature, the densification of the sintered joint increased to 98.4%, which lead to an increase in bonding strength. In this study, a SiC device was sinterbonded to a Si3N4 AMB substrate using silver paste at a low pressure, and optimization at a commercialized level was achieved.

X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy were used to conduct studies of both the crystalline and molecular structure of copper hexadecafluoro-phthalocyanine (F16CuPc). Bulk AC electrical conductivity and dielectric characteristics of F16CuPc were displayed in the pellet form over a temperature range of 293 K to 393 K and a frequency range of 50 Hz to 20 MHz. In this study, the universal power law $$\sigma_{{{\text{ac}}}} \left( {\omega ,T} \right) = A_{ \circ } \omega^{S}$$ has been utilized to explain the frequency exponent response of AC conductivity. The correlated barrier hopping model (CBH) governs the AC conductivity of F16CuPc. The determination of activation energy for the alternating current process was indexed. The barrier height was discovered to be 0.25 eV according to the Giuntini model. The complex impedance $$\left( {Z^{\prime } {\text{vs}} Z^{\prime \prime } } \right)$$ was found to be semicircular. The variation of $$\varepsilon^{\prime } ,\varepsilon^{\prime \prime }$$ as the frequency changes is also investigated. At various temperatures, the bulk of F16CuPc obtained complex permittivity and complex electric modulus.

Realistic microstructures of compacted powders formed by spark plasma sintering or field-activated sintering technology were modeled using the discrete finite-element method. Two key thermoelectric characteristics were studied: (1) the effect of the electric current pattern, i.e., direct current (DC) and pulsed current, on temperature distributions in the compacted powders, and (2) the effect of compaction modes, i.e., isostatic compaction and uniaxial compaction, on conductivity. Simulations showed that, for the same electric power input, pulsed current offered faster heating and more uniform temperature distribution in the compact than did DC. Additionally, using uniaxial compaction, the effective conductivity of the compact in the compaction direction was higher than in the transverse direction, by as much as 20%. Experimental measurements confirmed the existence of anisotropy of conductivity in the compact.