Thermal interface material (TIM) is a key component to dissipate the accumulated heat in the majority of power electronic systems. In this work, a facile and solid-state ball-milling method is adopted for the solvent-free reduction of exfoliated graphite nanoplatelets (EGNs) into high-quality ball-milled exfoliated graphite nanoplatelet (BMEGN) fillers. In addition, BMEGN fillers are embedded and uniformly dispersed with polydimethylsiloxane (PDMS) matrix to make a highly stretchable BMEGN-embedded PDMS-TIMs (BMEGN/PDMS) with strongly enhanced thermal conductivity. Furthermore, material characterizations were thoroughly investigated using scanning electron microscopy, transmission electron microscope, Raman spectroscopy, thermogravimetric analysis, and x-ray diffraction. Improvements in the thermal conductivity of TIMs by adding BMEGN were compared, the thermal conductivity was observed for BMEGN fillers with 0- to 48-h ball-milling time, and an enhanced in-plane thermal conductivity of 15.04 to 16.91 W/mK and through-plane thermal conductivity of 1.03 to 1.19 W/mK can be experimentally measured. A strong anisotropy was observed in the range of 14.60 (BMEGN12h/PDMS) to 14.21 (BMEGN48h/PDMS). The results reveal that the ball-milled graphene filler network with branched morphology can effectively provide the synergetic effect of a thermally conductive pathway via diffusion of phonon vibration in flexible composites. The combination of thermal conductivity and thermal stability may facilitate the applications in thermal management.
A symmetrically stacked structure [(a-Si:H(n+)/a-Si:H(i)/CZ wafer (n)/a-Si:H(i)/a-Si:H(n+)] was used to optimize the growth process conditions of the n-type hydrogenated amorphous silicon [a-Si:H(n+)] thin films. Here a-Si:H(n+) film was used as back surface field (BSF) layer for the silicon heterojunction solar cell and all stacked films were prepared by conventional radio-frequency plasma-enhanced chemical vapor deposition. The characterizations of the effective carrier lifetime (τeff), electrical and structural properties, as well as correlation with the hydrogen dilution ratio (R=H2/SiH4) were systematically discussed with the emphasis on the effectiveness of the passivation layer using the lifetime tester, spectroscopic ellipsometry, and hall measurement. High quality of a stacked BSF layer (intrinsic/n-type a-Si:H layer) with effective carrier lifetime of 1.8 ms can be consistently obtained. This improved passivation layer can be primarily attributed to the synergy of chemical and field effect to significantly reduce the surface recombination.
We present an optical-based, rapid method for the in situ porosity measurement of membranes through the electrospinning process. The method was developed by combining an optical method with in situ monitoring of the porosity of the electrospun membranes based on the measured reflected power density. The results showed that the area of bright and dark ratio is consistently proportional to the porosity of the electrospun membranes, which can potentially be used for actual characterization of the membranes. In addition, the effect of different incident angles of a laser beam was performed and compared. The porosity ratio of the electrospun membranes can be empirically evaluated as the determination coefficient R2=0.9945 to 0.9876 can be obtained. The proposed method is successfully demonstrated and validated by the SEM images of the binary method. The potential applications include the in situ monitoring of the electrospinning process for the bioassembly and biomimicking of a human tissue with a great accuracy.
Thermal interface materials (TIMs) are of crucial importance in enhancing heat transfer and minimizing exceedingly high temperatures in high-density electronics. TIMs functionally aim to reduce the microscale crevices by penetrating the gap between the contacting rigid surfaces. We prepared silver nanoparticles (SNPs) and single-wall carbon nanotubes (SWCNTs)-based nanocomposites with graphite nanoplatelets (GNPs) by using a screen printing technique for conformal spreading of SNPs and SWCNTs with various weight-loading ratios on top of a layer containing the GNPs and measured its thermal conductivity and electrical conductivities in both through-plane and in-plane directions. In particular, the 10% SNPs enhanced TIMs showed highly anisotropic behavior in both electrical and thermal conductivities, viz., in-plane electrical conductivity exceeds its through-plane counterpart by three orders of magnitude, the highest in-plane electrical conductivity was 7.85 S/cm, and through-plane electrical conductivity was 0.00287 S/cm. Similarly, anisotropic behavior was found for the in-plane thermal conductivity ∼8.4 W/mK and through-plane thermal conductivity ∼0.35943 W/mK. In addition, scanning electron microscopy (SEM) was performed to reveal the typical morphology and elements’ existence of screen-printed TIMs. The proposed TIMs were put into the actual 15-kW converter to test the thermal management performance.
Ge gate-all-around fin field-effect transistors (Ge FinFETs) with a capacitive effective thickness of a few nanometers have been successfully achieved via atomic-layer-deposited (ALD) high-dielectric Al2O3 on GeO2/Ge and by adopting low-cost thermo ALD equipment. The MOS interface properties of the ZrO2 or Al2O3/GeO2/Ge structures have been studied systematically. It has been found that a GeO2 interfacial layer that is greater than approximately 2.5 nm results in a significant degradation of the MOS interfaces, while an equivalent oxide thickness of <3 nm is still possible while maintaining good GeO2/Ge interface quality. The Ge FinFET’s value has been demonstrated with the Al2O3/GeO2/Ge gate stack prepared using a thermal ALD layer of Al2O3. The experimental results indicate that the MOS interface quality obtained with the technique developed for high-permittivity/Ge gate stacks is also extremely useful for the fabrication of triangle-fin complementary metal oxide semiconductor devices. An Ion/Ioff ratio of 3.2×104 and a subthreshold swing of 103 mV/dec were obtained for the triangular n-type Ge gate-all-around FET with (111) sidewalls. The drain current at VGS−VT=VDS=−1.5 V is 88 mA/mm.
We have demonstrated a fiber-based template process for fabricating site-specific and highly aligned platinum (Pt) microwires (MWs) with a polymeric core utilizing near-filed electrospinning (NFES) and soft transferring technology. Eliminating the indispensably expensive submicron lithographic and metal catalyst/lift-off as well as the extensive etching processes, the proposed method is direct-write, lithography-free, and low cost. As a demonstration, Pt sputtering deposition on top of a 500-μm length of microfibers for 5/10/15 min is sufficient to produce highly conductible Pt MWs with a sheet resistance measured to be 1.46/1.09/0.83 kΩ/sq, with good adhesion between the sputtered MWs and the underlying layers. The measured total resistances were found to increase almost linearly with the length of the MWs and are comparatively more conductive than polypyrrole films. Furthermore, the reliable and repeatable nanofiber arrays can be speedily fabricated, while various patterns and dimensions are predominantly controllable by NFES in a direct-write, addressable manner.
This paper reports a facile and maskless method for fabricating nanofluidic channel arrays using near-field electrospinning (NFES) templates with prescribed patterns and the polydimethylsiloxane (PDMS) molding technique. Nanochannels were fabricated monolithically through three main steps: 1) direct-writing nanofiber arrays onto a silicon substrate using NFES, 2) PDMS molding of the prescribed nanofibers patterns, and 3) plasma treating PDMS substrate to promote the adhesion and bonding process. The nanochannels fabricated in this study had channel widths ranging from 500 to 1300 nm and depths of 70 to 500 nm, and were patterned in a fashion similar to the wire bonding process routinely used in the semiconductor industry. The nanochannel dimensions were predominantly dictated by electrospun nanofibers, showing that NFES is capable of depositing nanofibers with a diameter down to ∼50 nm. Results show that reliable and repeatable nanofluidic channel arrays were speedily fabricated at a very low cost, while nanofluidic patterns and dimensions are predominantly controlled by NFES in a direct-write, addressable manner.
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