Numerical investigation on the temperature uniformity of micro-pin-fin heat sinks with variable density arrangement

Temperature non-uniformity on chips has drawn the attention of researchers due to unwanted thermal stress development on chips resulting in a reduction in their life cycle and performance. In the present investigation, the heat transfer and flow characteristics of the coolant in the micro-pin-fin heat sink with variable density arrangement have been conducted numerically. The dimensions of the micro-pin-fin heat sink are 18.0 mm × 19.0 mm × 4.0 mm, and the height of the micro-pin-fin is 2 mm. Circular micro-pin-fin with diameters of 400, 500, and 600 μm, respectively have been considered. The power supply is 50 W with a heat source area of 10.0 mm × 10.0 mm. Water was the working fluid used while aluminum was used for the solid part of the heat sink. The operating pressure drop between the inlet and the outlet of the heat sink is fixed at 1500 Pa, 3000 Pa, and 5000 Pa. Ansys-Fluent was employed for the analysis. The results indicate that the temperature uniformity on the heat source for heat sink with variable density arrangement is better than that with staggered arrangement for about the same number of micro-pin-fins. The best effective thermal resistance is noted as 0.258 K/W among the heat sinks with all the configurations. In addition, the temperature difference per unit length on the heat source for heat sink with convergent arrangement at the pressure difference of 5000Pa and micro-pin-fin diameter of 600 μm was 1.34 K/mm, which is lower than the previously reported literature

Improvement of Lighting Uniformity and Phosphor Life in Field Emission Lamps Using Carbon Nanocoils

The lighting performances and phosphor degradation in field emission lamps (FELs) with two different kinds of cathode materials—multiwalled carbon nanotubes (MWCNTs) and carbon nanocoils (CNCs)—were compared. The MWCNTs and CNCs were selectively synthesized on 304 stainless steel wire substrates dip-coated with nanosized Pd catalysts by controlling the growth temperature in thermal chemical vapor deposition, and the film uniformity can be optimized by adjusting the growth time. FELs were successfully fabricated by assembling these cathode filaments with a glass bulb-type anode. The FEL with the CNC cathode showed much higher lighting uniformity and light-spot density and a lower current at the same voltage than that with the MWCNT cathode filament, and its best luminous efficiency was as high as 75 lm/W at 8 kV. We also found that, for P22, the phosphor degradation can be effectively suppressed by replacing MWCNTs with CNCs in the cathode, due to the much larger total bright spot area and hence much lower current density loading on the anode

Fabrication and Simulation of Self-Focusing Field Emission X-ray Tubes

A self-focusing field emission (FE) X-ray tube with a large-area cathode design was simulated and fabricated. The designed X-ray tube had a cylindrically symmetric geometry; the diameter of the cathode and the anode was 15 mm, and the cathode-anode distance was 20 mm. Owing to the unique cup-shaped design of the cathode, the electron beam emitted from the large-area cathode was focused onto the anode without using magnetic lenses or extra biased electrodes. Carbon nanocoils, which were grown on the bottom of the circular cup-shaped cathode, were used as electron emitters because of their excellent FE properties. A simulation of the electron trajectories for various cup heights revealed that the optimal focal spot size (0.1 mm) was obtained at a cup height of 5 mm when a voltage of 50 kV was applied. To verify this result, an X-ray tube was fabricated and tested. An X-ray photograph of the tested tooth and electric circuits showed good resolution and X-ray intensity. The large cathode area effectively reduces the current density and thereby improves the lifetime of the cathode

An Automatic Foreign Matter Detection and Sorting System for PVC Powder

In the present study, an automatic defect detection system has been assembled and introduced for Polyvinyl chloride (PVC) powder. The average diameter for PVC powder is approximately 100 μm. The system hardware includes a powder delivery device, a sieving device, a circular platform, an image capture device, and a recycling device. A defect detection algorithm based on YOLOv4 was developed using CSPDarkNet53 as the backbone for feature extraction, spatial pyramid pooling (SPP) and path aggregation network (PAN) as the neck, and Yoloblock as the head. An auto-annotation algorithm was developed based on a digital image processing algorithm to save time in feature engineering. Several hyper-parameters have been employed to improve the efficiency of detection in the process of training YOLOv4. The Taguchi method was utilized to optimize the performance of detection, in which the mean average precision (mAP) is the response. Results show that our optimized YOLOv4 has a test mAP of 0.9385, compared to 0.8653 and 0.7999 for naïve YOLOv4 and Faster RCNN, respectively. Additionally, with the optimized YOLOv4, there is no false alarm for images without any foreign matter

A Holey Graphene Additive for Boosting Performance of Electric Double-Layer Supercapacitors

We demonstrate a facile and effective method, which is low-cost and easy to scale up, to fabricate holey graphene nanosheets (HGNSs) via ultrafast heating during synthesis. Various heating temperatures are used to modify the material properties of HGNSs. First, we use HGNSs as the electrode active materials for electric double-layer capacitors (EDLCs). A synthesis temperature of 900 °C seems to be optimal, i.e., the conductivity and adhesion of HGNSs reach a compromise. The gravimetric capacitance of this HGNS sample (namely HGNS-900) is 56 F·g−1. However, the volumetric capacitance is low, which hinders its practical application. Secondly, we incorporate activated carbon (AC) into HGNS-900 to make a composite EDLC material. The effect of the AC:HGNS-900 ratio on the capacitance, high-rate performance, and cycling stability are systematically investigated. With a proper amount of HGNS-900, both the electrode gravimetric and volumetric capacitances at high rate charging/discharging are clearly higher than those of plain AC electrodes. The AC/HGNS-900 composite is a promising electrode material for nonaqueous EDLC applications

Using Graphene-Based Composite Materials to Boost Anti-Corrosion and Infrared-Stealth Performance of Epoxy Coatings

Corrosion prevention and infrared (IR) stealth are conflicting goals. While graphene nanosheets (GN) provide an excellent physical barrier against corrosive agent diffusion, thus lowering the permeability of anti-corrosion coatings, they have the side-effect of decreasing IR stealth. In this work, the anti-corrosion properties of 100-μm-thick composite epoxy coatings with various concentrations (0.01–1 wt.%) of GN fillers thermally reduced at different temperatures (300 °C, 700 °C, 1100 °C) are first compared. The performance was characterized by potentiodynamic polarization scanning, electrochemical impedance spectroscopy, water contact angle and salt spray tests. The corrosion resistance for coatings was found to be optimum at a very low filler concentration (0.05 wt.%). The corrosion current density was 4.57 × 10−11 A/cm2 for GN reduced at 1100 °C, showing no degradation after 500 h of salt-spray testing: a significant improvement over the anti-corrosion behavior of epoxy coatings. Further, to suppress the high IR thermal signature of GN and epoxy, Al was added to the optimized composite at different concentrations. The increased IR emissivity due to GN was not only eliminated but was in fact reduced relative to the pure epoxy. These optimized coatings of Al-GN-epoxy not only exhibited greatly reduced IR emissivity but also showed no sign of corrosion after 500 h of salt spray test