Freezing Method for Rock Cross-Cut Coal Uncovering: Aging Characteristic of Effective Freezing Distance on Injecting Liquid Nitrogen into Coal Seam

Based on artificial freezing engineering practice, the comprehensive technology is suggested to realize safe and fast rock cross-cut coal uncovering, which mainly includes four steps of drilling, water injection wetting coal, gas drainage, and injection liquid nitrogen into coal seam. Freezing test of liquid nitrogen injection into coal is carried out to obtain the cooling curves, and comparing the test results, the numerical inverse method is applied to determine the thermal conductivity of coal seam. Then, the model of injecting liquid nitrogen into coal seam is established to simulate and analyze the aging characteristic of effective freezing radius. The results show that the thermal conductivity of wetting coal increases linearly with temperature decreasing. The periodic method with 8h intervals can be adopted to inject liquid nitrogen into coal seam, and the freezing wall is formed around the injection hole. With the increase of freezing time, the effective freezing radius (below 273.15 K) increases by power exponent, and the freezing speed in coal seam decreases gradually. This result will provide a theoretical basis for layout optimization of injection holes in rock cross-cut coal uncovering.

Research on the Thermal Aging Behaviors of LDPE/TiO2 Nanocomposites

The ability of antithermal aging of LDPE/TiO2 nanocomposites was investigated through SEM, FTIR, DSC, and dielectric properties in this paper. The results of SEM images showed that the thermal aging had a significant influence on the structure of Pure-LDPE and LDPE/TiO2 samples. The measurement of FTIR showed that the content of hydroxyl and carboxyl increased with thermal aging, but the time of emerging aging characteristic peaks for the LDPE/TiO2 samples was delayed. The DSC measurement indicated that filling TiO2 nanoparticles changed the crystallization behavior of LDPE, played a role of heterogeneous nucleation during the process of recrystallization, and improved the crystallinity of LDPE/TiO2. Similarly, the aged LDPE/TiO2 samples had lower permittivity and dissipation factor compared to the aged Pure-LDPE samples. All the results had indicated the LDPE/TiO2 samples had the significant ability of antithermal aging, especially the LDPE/TiO2-0.5 samples with good dispersion of nanoparticles. A new model was proposed to illustrate the antithermal aging behaviors of LDPE/TiO2 samples, which shows that the TiO2 nanoparticles play a role of “crosslinking points” between LDPE molecular chains, increasing the density of crystal structure and reducing oxygen diffusion into materials to break molecular structure.