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Liquid CO2 phase transition fracturing (LCO2-PTF) is an effective and economical technology used to improve the permeability of rock and coal. In this study, the working mechanisms of LCO2-PTF were analysed and relevant equipment was designed to develop and promote the application of this technology. It utilized phase transition equipment (PTE) consisting of a liquid gas container, control unit for the current and gas volume, heating tube, and other components. LCO2 blasting experiments were conducted in an airtight container to investigate the released energy, pressure, and other technical parameters. The application of LCO2-PTF for enhancing gas drainage in coal mines was then evaluated. The results of a blasting experiment showed a maximum energy of 947.12 kJ and revealed that the releasing pressure could be easily changed by varying the plate and heat tube. The releasing pressure remained unchanged within an initial distance and then decreased exponentially. The blasting products were gaseous CO2 and water vapor, with no sparks or flames. The surface temperature of the PTE ranged from 269.32 to 277.96 K. Application of LCO2-PTF in coal mines with low permeability showed gas drainage 3.38 times higher than that achieved with conventional technologies, with methane concentration increasing from 55 to 89%. The attenuation coefficient of gas emissions dropped by 94% and the gas permeability coefficient of the coal body increased more than 23 times after fracturing. The shockwave of the high-pressure gas promoted the development and extension of cracks. The influence radius of the pre-cracking coal seam was 8.1 m, which is 6.75 times that of the original coal seam. The experimental results and the engineering applications indicate that LCO2-PTF is a safe and effective technology to enhance gas drainage in coal mines. [ABSTRACT FROM AUTHOR]

In this paper, a highly sub-wavelength double negative (DNG) metamaterial for operation at high frequencies is designed, fabricated, and measured. A dual layer design that consists of a planar spiral on one side and a meander line touching with narrow metallic strips on the other side produces the properties of effective negative permittivity and permeability simultaneously, i.e., negative refractive index (NRI). The size of the unit cell of DNG metamaterials is about 253 times smaller than the operation wavelength at 15.2 MHz. The strong coupling between the topological structures on two sides makes a contribution to the miniaturization of the proposed metamaterials. A simple probe method is used to measure the transmission and reflection coefficients, and the comparison of effective medium parameters retrieved from transverse electromagnetic waveguide simulation and the probe method is given to verify the effectiveness and correctness. The measured results from one cell show a good agreement with simulated results. The measured effective real index of refraction n = -6.9 with a total loss that is less than 0.45 dB/cm at 15.2 MHz. The NRI bandwidth was > 10% in the neighborhood of 15.2 MHz. [ABSTRACT FROM AUTHOR]

Fe 3 O 4 is considered as an appealing anode material for constructing high-energy-density asymmetric supercapacitors owing to its high theoretical capacitance, large potential window, natural abundance and eco-friendliness. However, the dilemma of inadequate practical capacitance, low rate performance and unsatisfactory cycling stability of Fe 3 O 4 -based electrode materials significantly plagues their progress in supercapacitor applications. To well utilize the advantage of Fe 3 O 4 , herein, Fe 3 O 4 @onion-like carbons (OLCs) with covalently interconnected and few-shelled graphitic coatings were synthesized in a size-controllable manner from uniform-sized monodispersed nanoparticles of Fe 3 O 4 @oleic acid ligands. The ultrasmall-sized Fe 3 O 4 nanoparticles with a good size uniformity enable a high electrochemical activity and a pseudocapacitive-dominated electrochemical behavior. Meanwhile, the efficient ion/electron transportations are facilitated by the structural advantages including covalently interconnected graphitic layers, conformal and seamless graphitic coating with ultrathin thickness, and hierarchical pores. As a result of the synergistic cooperation of these merits, the Fe 3 O 4 @OLCs hybrid structure exhibits a high specific capacitance of 686.1 F g−1 at 1 A g−1 and a high rate capability with 71.3% capacitance retention at 10 A g−1, superior to most of the reported Fe 3 O 4 -based electrode materials. An asymmetric supercapacitor device assembled based on Fe 3 O 4 @ OLCs shows a high energy density of up to 63.1 Wh kg−1, which is still maintained as high as 39.1 Wh kg−1 at a high power density of 1.49 kW kg−1. Moreover, a high cycling stability (more than 80% of initial capacitance remained after 10,000 cycles) is obtained. This study offers a novel strategy for preparing high-performance Fe 3 O 4 /carbon hybrid structures for asymmetric supercapacitor applications. [Display omitted] • A size-controllable synthesis of covalently interconnected Fe 3 O 4 @OLCs hybrid structures was developed. • The ultrasmall Fe 3 O 4 size and graphitic structural advantages result in high capacitance and rapid ion/electron transfer. • The hybrid structure exhibits a high specific capacitance of 686.1 F g−1 with a high rate performance. • The asymmetric supercapacitor shows a high energy density of 63.1 Wh kg−1 and a good cycling stability. [ABSTRACT FROM AUTHOR]

Developing high-performance carbon-based materials for environmental and energy-related applications produces solid waste with secondary pollution to the environment at the end of their service lives. It is still challenging to utilize these functional materials in a sustainable manner in different fields. In this study, we demonstrate a cascaded utilization of an Fe 3 O 4 @onion-like carbon (Fe 3 O 4 @OLC) structure from wastewater adsorbents to a supercapacitor electrode. The structure was formed by carbonizing Fe 3 O 4 @oleic acid monodisperse nanoparticles into interconnected Fe 3 O 4 @OLCs and subsequent insufficient acid etching. The hollow OLCs in the outside region of the hybrid structure provide high surface area and the encapsulated Fe 3 O 4 nanoparticles in the inside region offer high ferromagnetism. The three-dimensionally interconnected graphitic layers are advantageous for efficient separation and high conductivity. As a result, the maximum saturation adsorption capacity of insufficiently etched interconnected Fe 3 O 4 @OLCs can reach up to 90.2 mg g -1 and they can be efficiently separated under a magnetic field. Furthermore, the hybrid structure is thermally transformed into N-doped HOLCs, which are demonstrated to be a high-performance supercapacitor electrode with high specific capacitance and high electrochemical stability. The cascaded utilization of the hybrid structure in this study is meaningful for eco-friendly development of functional materials for environmental and energy storage applications.