Porous α-Si 3 N 4 whiskers bonded/reinforced SiC (Si 3 N 4( w ) /SiC) ceramics were successfully prepared at as low as 1473 K for 2 h, via a combined foam-gelcasting and catalytic nitridation route using commercial Si and SiC powders containing some Fe impurity as the main raw materials. Small pores (0.03–5 μm) left by the packing of raw material particles and interlocking of in-situ formed Si 3 N 4 whiskers coexisted with large ones (8–400 μm) resultant mainly from the foaming process. The impurity Fe from the raw materials Si and SiC acted as an internal catalyst, accelerating the nitridation of Si by increasing the bond length and weakening the bond strength in the N 2 molecules adsorbed on it. As-prepared Si 3 N 4( w ) /SiC porous ceramics contained 71.53% porosity and had flexural and compressive strengths of 5.60 ± 0.69 MPa and 12.37 ± 1.05 MPa, respectively.

Low-temperature preparation of Si3N4 whiskers bonded/reinforced SiC porous ceramics via foam-gelcasting combined with catalytic nitridation

Silicon nitride (Si3N4) powders were prepared by the direct nitridation of silicon powders diluted with α-Si3N4 at normal pressure. Silicon powders of 2.2 μm in average diameter were used as the raw materials. The nitriding temperature was from 1623 to 1823 K, and the reaction time ranged from 0 to 20 min. The phase compositions and morphologies of the products were analyzed by X-ray diffraction and scanning electron microscopy, respectively. The effects of nitriding temperature and reaction time on the conversion rate of silicon were determined. Based on the shrinking core model as well as the relationship between the conversion rate of silicon and the reaction time at different temperatures, a simple model was derived to describe the reaction between silicon and nitrogen. The model revealed an asymptotic exponential trend of the silicon conversion rate with time. Three kinetic parameters of silicon nitridation at atmospheric pressure were calculated, including the pre-exponential factor (2.27 cm·s−1) in the Arrhenius equation, activation energy (114 kJ·mol−1), and effective diffusion coefficient (6.2×10−8 cm2·s−1). A formula was also derived to calculate the reaction rate constant.

Kinetic study on the direct nitridation of silicon powders diluted with α-Si 3 N 4 at normal pressure

Low-temperature preparation of Si3N4/SiC porous ceramics via foam-gelcasting and microwave-assisted catalytic nitridation

Liquid air has recently emerged as a new energy vector that has the ability to reserve considerable amounts of renewable energy as both cold and power. Liquid air used for energy storage and transportation has gained increasing attention in both academia and industry, owing to its high flexibility, free availability, and potential high cost-efficiency. Supported by an existing established cryogenic transportation network, liquid air can be an intermediate between conventional fossil fuels and green hydrogen, allowing a smooth and reliable transition towards a 100% renewable energy system. This review offers an overview of previous and emerging liquid air technologies and highlights the potential of liquid air for energy recovery and production from a multi-scale systems perspective (molecular, process, system, and supply chain). Emphasis is placed on how liquid air participates in the current energy system transition and assists in the efficient use and management of both renewable energy and waste energy derived from industries. The review covers a range of technologies, such as air liquefaction and liquid air energy extraction cycles, liquid air energy storage, air separation units, and liquid air supply chains, with a focus on identifying and organizing influential factors to construct energy- and cost-efficient liquid air energy systems. To establish a future liquid air economy, challenges and opportunities are outlined in terms of multi-scale systems design and optimization, liquid air flexibility, and system real-time scheduling and planning to balance power supply and demand while improving the techno-economics for feasible industrial implementation. 

Liquid air as an emerging energy vector towards carbon neutrality: A multi-scale systems perspective

The effect of air purification on liquid air energy storage – An analysis from molecular to systematic modelling

Waste heat recovery method for the air pre-purification system of an air separation unit

Effect of the addition of α-Si3N4 on the nitridation of silicon powder was studied at 1550 and atmospheric pressure. Silicon powder of 2.2μm in size and 99.99% in purity was adopted as experimental materials, α-Si3N4 powder of 2.2μm in size and 99% in purity was used as diluent, and nitrogen of 99.9993% in purity was used as reaction gas. The morphologies and phase compositions of the products were analyzed by SEM and XRD respectively. The results show that the conversion of silicon is improved considerably by increasing the adding proportion of α-Si3N4 diluent. When the adding proportion of α-Si3N4 increases from 10% to 50%, the conversion of silicon powder increases from 38% to 92.55%. The content of β-Si3N4 in product decreases, after an initial increase, with the adding proportion of α-Si3N4 increases, and it reaches the peak point when the adding proportion of α-Si3N4 was 30%. The sinterability of silicon nitride decreases as the content of β phase increases, so the adding proportion of α-Si3N4 should be kept away from 30%.

Effect of the Addition of α-Si3N4 on the Direct Nitridation of Silicon Powder at Atmospheric Pressure

Combustion reaction between silicon powders and nitrogen in transport bed was studied. The reaction temperature ranged from 1523 to 1653 K, and the reaction time ranged from 0 to 2.7 min. The phase compositions, morphologies and chemical composition of the products were analyzed by X-ray diffraction, scanning electron microscopy and O/N determinater, respectively. The experimental results showed, in the case of silicon powders with particle size of 2.2 μm, the conversion rate of silicon was 61.9% at reaction temperature of 1653 K and reaction time of 2.7min, and the products mainly comprised amorphous silicon nitride powders.

Chuan Ping Liu

Papers

Waste heat recovery method for the air pre-purification system of an air separation unit

Effect of the Addition of α-Si3N4 on the Direct Nitridation of Silicon Powder at Atmospheric Pressure

Experimental Study on Combustion Reaction between Silicon and Nitrogen in Transport Bed