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Microstructural evolution and high‐temperature ablative damage mechanism of Csf/SiC‐BN composites with and without polyborosilazane (PBSZ) as sintering aids that can convert to polymer‐derived SiBCN ceramics (PDCs‐SiBCN) by cross‐linking and pyrolysis during sintering were studied. The density of Csf/SiC‐BN composites increases from 2.17 to 2.25 g/cm3 with 10 wt.% sintering aids because PBSZ promotes rearrangement and redistribution of raw materials by pyrolysis during the sintering process. With the increase of carbon fiber content, ablation performance (mass ablation rate: 15.25 mg/s; linear ablation rate: 0.100 mm/s) negligibly decreases, compared to counterparts with lower fiber content (mass ablation rate: 16.53 mg/s; linear ablation rate: 0.072 mm/s) because the presence of PDCs‐SiBCN partially compensates for the detrimental effect of carbon fibers on ablation performance. Csf/SiC‐BN composites with and without PBSZ show similar phase composition mainly containing quartz, amorphous SiO2, and matrix crystallization products and morphology after ablation because of similar phase composition before ablation. The ablation mechanism is oxidative corrosion and mechanical scouring and stripping. [ABSTRACT FROM AUTHOR]

This paper explores the predictive role of speculative sentiment on gold market volatility and its economic implications. Utilizing high frequency data for gold futures and speculative sentiment proxies for the gold and stock markets, we show that incorporating speculative sentiment in volatility models can improve volatility forecasts both in- and out-of-sample. While gold market sentiment helps improve the predictive accuracy of volatility models primarily in the short run, we show that cross-market speculative sentiment from the stock market has predictive ability at relatively longer forecast horizons. More importantly, incorporating speculative sentiment proxies in volatility models yields improved cumulative returns as well as Sharpe ratios for investors who allocate part of their portfolios in this traditional safe haven asset. Finally, examining the predictability of asymmetric risk patterns, we show that speculative sentiment in the gold market matters relatively more for good volatility forecasts and in the short and intermediate forecast horizons, while cross-market sentiment effect from the stock market generally applies to longer term forecasts for both the good and bad volatility metrics. Overall, our findings show that speculative sentiment contains significant incremental information over subsequent volatility patterns in gold, offering sizeable economic gains for a mean-variance investor in a practical investment setting. [ABSTRACT FROM AUTHOR]

This paper investigates whether the cross‐sectional variance (CSV) of stock returns and its asymmetric components contain incremental information to predict stock market volatility under a high‐frequency, heterogeneous autoregressive (HAR) model framework. We present novel evidence that CSV is a powerful predictor of future realized volatility, both in‐ and out‐of‐sample, even after controlling for the well‐established predictors obtained from intraday data. Further analysis suggests that distinguishing between positive and negative CSV components in the forecasting model enhances the predictive capability of volatility models at all out‐of‐sample forecasting horizons, with the asymmetric HAR‐type‐ACSV model consistently outperforming all alternative HAR‐type variations. We argue that the asymmetries in the predictive relation between CSV and volatility are largely driven by the disagreement among market participants that spikes during bad times. Finally, economic analysis shows that incorporating CSV in the forecasting model can generate sizeable economic gains for a mean–variance investor, suggesting that out‐of‐sample predictive ability of CSV can be exploited in forward looking investment strategies to enhance investment returns. [ABSTRACT FROM AUTHOR]

In this study, the amorphous C, ZrB2, and BN single‐layer coatings as well as C/BN, C/ZrB2, ZrB2/BN, and C/ZrB2/BN composite coatings were prepared on SiC fibers (SiCf) by an in situ synthesis and solution impregnation–pyrolysis method. Subsequently, SiCf/SiBCN composites were fabricated by hot‐pressing sintering at 1900℃/60 MPa/30 min to explore the influence of different coatings on the microstructure and mechanical performance of resulting composites. After the preparation of single‐layer‐coated SiCf, the SiCf(BN) or SiCf(ZrB2) tended to be overlapped with each other, whereas the dispersion of amorphous C–coated SiCf was satisfying. Besides, some uneven areas and attached particles have appeared on fiber surfaces of the SiCf(BN) or SiCf(ZrB2), whereas smooth and dense surfaces of amorphous C–coated SiCf were observed. Because the uniformity of ZrB2 coatings can be partially damaged by the subsequent coating process of BN, the composite coatings of ZrB2/BN and C/ZrB2/BN were thereby not suitable for strengthening SiBCN matrix. The SiCf/SiBCN composites with C/ZrB2 coatings have desirable comprehensive mechanical properties. Nevertheless, the conventional toughening mechanisms such as fiber pull‐out and bridging, and crack deflection are not available for these composites because the serious crystallization of SiCf leading to great strength loss, resulting in catastrophic brittle fracture. [ABSTRACT FROM AUTHOR]

In this work, hexagonal boron nitride powders (h-BN) and heat-treatment (HT) were used to tailor microstructures and tribological properties for Carbon/Carbon composites. The microstructures and tribological behaviors of virgin Carbon/Carbon, h-BN modified Carbon/Carbon, Carbon/Carbon after HT, and Carbon/Carbon modified by h-BN with HT were investigated. Results indicate that h-BN powdered addition as matrix leads to PyC matrix with higher texture degree, graphitization and micro-hardness, compared to virgin Carbon/Carbon composites. Moreover, compared to virgin Carbon/Carbon, the coefficient of friction (COF) of h-BN modified Carbon/Carbon composites are increased to 0.29 while wear rates (WR) are decreased to 1.65 μm·side−1·circle−1. However, the Carbon/Carbon composites modified by h-BN and HT presented quite lower COF and WR, in comparison to virgin Carbon/Carbon and Carbon/Carbon after HT. In summary, flexibly tailored COF and WR could be achieved via BN powdered addition and/or HT. [ABSTRACT FROM AUTHOR]