Reducing Popularity Bias in Recommender Systems through AUC-Optimal Negative Sampling

Popularity bias is a persistent issue associated with recommendation systems, posing challenges to both fairness and efficiency. Existing literature widely acknowledges that reducing popularity bias often requires sacrificing recommendation accuracy. In this paper, we challenge this commonly held belief. Our analysis under general bias-variance decomposition framework shows that reducing bias can actually lead to improved model performance under certain conditions. To achieve this win-win situation, we propose to intervene in model training through negative sampling thereby modifying model predictions. Specifically, we provide an optimal negative sampling rule that maximizes partial AUC to preserve the accuracy of any given model, while correcting sample information and prior information to reduce popularity bias in a flexible and principled way. Our experimental results on real-world datasets demonstrate the superiority of our approach in improving recommendation performance and reducing popularity bias.Comment: 20 page

Graphene ultrathin film electrode for detection of lead ions in acetate buffer solution

Few-layer graphene ultrathin films were synthesized via solid-state carbon diffusion from amorphous carbon (a-C) thin layers sputtering coated on Si substrates with or without a SiO2 layer, which an a-C layer was covered by a nickel (Ni) layer as a catalyst. When the Ni/a-C bilayer coated samples were heated at 1000 °C the carbon (C) atoms from the a-C layers diffused into the top Ni layers to form a C rich surface. Upon rapid cooling, the C atoms accumulated on the surface of the Ni layers and formed graphene ultrathin films through nucleation and growth processes. The formation of graphene ultrathin films was confirmed by Raman spectroscopy, high resolution transmission electron microscopy (HR-TEM), electron diffraction, field-emission scanning electron microscopy (FE-SEM) and 4-point probe. The synthesized graphene ultrathin films were used as working electrodes for detection of trace heavy metal ions (Pb2+, as low as 7 nM) in acetate buffer solutions (pH 5.3) using square wave anodic stripping voltammetry (SWASV). The effects of substrate surface condition and Ni layer thickness on the structure and electrochemical properties of graphene ultrathin film electrodes were investigated in detail. Compared to conventional diamond-like carbon (DLC) electrodes, the graphene electrodes developed in this study had better repeatability, higher sensitivity and higher resistance to passivation caused by surface active species

Wear behaviour of martensitic NiTi shape memory alloy under ball-on-disk sliding tests

Ball-on-disk sliding wear tests were conducted on martensitic NiTi shape memory alloy against an alumina counter ball. The wear features and coefficients of friction of the samples subjected to different loads and sliding cycles were investigated to reveal the degradation process of the NiTi surface. Three stages of degradation were identified, namely, a near-zero wear stage, a transition wear stage and an abrasive wear stage. A special wear feature, herein referred to as the “crown-like structure”, was produced under near-zero and transition wear conditions and was observed for the first time in this study. The mechanisms of the surface degradation process are further discussed with respect to the deformation distribution in the vicinity of the contact area under different loads

Tribological behavior of polyurethane immersed in acidic solution

Polyurethane (PU) samples were immersed in a 1 M HCl solution for different durations from 0 to 90 days to investigate the influence of immersion time on the tribological performance of the samples. The effects of thermal, mechanical, and surface properties of the PU on its tribological performance were also investigated with respect to immersion time. It was found that the friction coefficient of the PU measured against a steel ball significantly decreased with increased immersion time due to the lubricating and cooling effects of absorbed water in the PU during the immersion. The decreased friction of the PU led to the improved wear resistance of the polymer, though the mechanical strength of the PU was reduced by the prolonged immersion due to the increased plasticization of the polymer. The experimental results clearly indicated that the immersion of the PU in the acidic solution for a certain period significantly influenced the tribological performance of the polymer

Thermal, mechanical and tribological properties of polycarbonate/acrylonitrile-butadiene-styrene blends

Polycarbonate (PC) and acrylonitrile-butadiene-styrene (ABS) were blended by varying ABS content. The thermal, mechanical and tribological properties of the PC/ABS blends were systematically characterized. Increasing ABS content in the PC/ABS blends decreased thermal stability of the blends, as a result of the lower thermal stability of the ABS than that of the PC. Although the tensile strength of the PC/ABS blends apparently decreased with increased ABS content, the PC/ABS blend with 10 wt% of ABS had the highest tensile strength, because of improved processability of the blend. The friction and wear of the ABS measured against a steel ball of 6 mm in diameter were higher than those of the PC. As a result, a higher ABS content in the PC/ABS blends resulted in higher friction and wear of the blends. The scratch results showed that scratching with a 5 mm steel ball generated a scratch with a shorter length and lower depth on the PC than on the ABS, which indicated better scratch resistance of the PC. Therefore, the PC/ABS blend with 50 wt% of PC had better scratch resistance than the ABS, due to the influence of the PC embedded in the blend

Processing and characterization of shape memory alloy thin films for micro-grippers with multi-degree-of freedom and other applications

In the present project, the magnetron sputtering technique was used to fabricate NiTi-base SMA thin films. The project began with fabrication of the SMA thin films through exploring various deposition parameters. In order to obtain thin films with shape memory effect and good mechanical properties, composition control and microstructure control are the two most important issues. After successfully fabricated the thin films with shape memory effect, various factors that may affect the performance of the thin films were systematically investigated. Further, SMA-based micro-grippers were developed and tested

Fretting friction and wear of polycrystalline diamond coatings

The tribological behaviour of diamond coatings deposited by widely different processes has been investigated. Hot-flame and microwave CVD diamond coatings show a homogeneous structure and morphology, whereas plasma jet diamond deposits display a laterally changing structure and morphology. The friction and wear behaviour of these three coating types sliding against a corundum counterbody showed a marked difference in a fretting test. A single-crystal diamond with a polished (100) surface was involved for comparative tests. An attempt is made to clarify the tribological behaviour of diamond coatings sliding against corundum in terms of the purity of diamond coating, its surface roughness and crystal orientation.status: publishe

Process parameter optimization and mechanical properties for additively manufactured stainless steel 316L parts by selective electron beam melting

This work presents an experimental study of process optimization of the pair of critical parameters (speed function (SF) and focus offset (FO)) for stainless steel 316 L (SS316L) parts additively manufactured by selective electron beam melting (SEBM). Here, there are two sets of optimized SF-FO parameters that could build SS316L parts with high relative densities (>99%) and well-melted top build surfaces. Tensile test results show that most of the SEBM-built SS316L samples exhibit higher tensile strengths than the conventional cast and wrought counterparts, whereas their ductility is lower. In addition, strong anisotropic tensile properties are observed for the SEBM-built SS316L samples, e.g. they generally have better tensile properties when loaded parallel to the build direction as compared to the horizontal direction. However, a large number of σ phase was found to precipitate at grain boundaries in the SS316L samples fabricated under lower SF and larger FO with a higher build temperature, which evidently deteriorates their tensile properties particularly for the horizontal direction. It is suggested that SEBM process parameters for SS316L must be optimized to avoid σ phase precipitation at elevated temperatures apart from a well-melted top build surface and a high relative density.NRF (Natl Research Foundation, S’pore

Study on changes in hardness and wear resistance of 3D printed Ti6Al4V with heat treatment temperature

The hardness and specific wear rate of 3 dimensionally (3D) printed Ti6Al4V specimens were studied with respect to heat treatment temperature (HTT) in a range of 400-800 °C. Increasing the HTT from 400 °C to 500 °C promoted the hardness of the printed specimen while its decreased hardness associated with a further increase in the HTT to 800 °C was observed. Since higher wear resistance was the result of higher hardness, the trend of specific wear rate versus HTT of the printed specimen was opposite to that of its hardness versus HTT. The highest hardness and lowest wear of the printed specimen were therefore found at the HTT of 500 °C.Published versio

Wear performance of Y-doped nanolayered CrN/AlN coatings

The incorporation of yttrium (Y) in CrAlN based hard coatings has been known to be able to improve the oxidation resistance and thermal stability of these coatings, which is an important characteristic for protective coatings at high temperatures. However, there has been limited information available for the effects of Y doping on the tribological properties of such coatings, especially the wear performance. In this paper, a systematic investigation on the triblogical properties of CrAlN coatings doped with different levels of Y was conducted to correlate the coatings' microstructural, mechanical and tribological properties with their Y contents. The results showed that the wear performance of the coatings decreased with increasing Y doping, though the coatings doped with low to moderate Y contents had comparable wear performance with the pristine CrAlN coating