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  In this paper, the chemical microstructure of coal samples is quantitatively analyzed experimentally before and after liquid nitrogen cold soaking, by using elemental analyzer, X-ray diffractometer, and Fourier infrared spectrometer, including the reverse side of chemical composition of elements, organic matter, and functional groups. It was found that with the increase of coal metamorphism, the contents of carbon, nitrogen, and sulfur elements gradually increase, while those of hydrogen and oxygen elements gradually decrease. In addition, as the degree of metamorphism increases, the graphitization phenomenon of coal becomes weaker, the interlayer spacing of aromatic rings ( d 002 ) increases, the structure of coal crystal nucleus is loose, its order is weakened, the crystal volume becomes smaller, and the void structure unit increases. The FTIR spectra of each coal sample could be divided into four absorption bands, i.e., the aromatic structure, oxygen-containing functional group, aliphatic group, and hydroxyl absorption band. After cold soaking of liquid nitrogen, the peak intensity areas of aromatic and aliphatic structures decrease, while those of oxygenated functional groups and hydroxyl groups increase, and the values of A(C = O)/A(C-O) increase and those of A(CH 3 )/A(CH 2 ) decrease, mainly due to the gradual decrease of methylene side chains and increase of methylene straight chains. The present results are helpful to further reveal the mechanism of adsorption-resolution deformation of coal body due to cold immersion of liquid nitrogen.

Liquid nitrogen freeze–thaw has been used in oil, shale gas and coalbed methane exploitation as an efficient fracturing technology. This paper aimed to study the effect of different coal ranks and liquid nitrogen soaking times on the temperature distribution of coal samples, and to explore the temperature evolution mechanism of different coal ranks during liquid nitrogen soaking. For these objectives, the temperature change process, thermophysical parameters and infrared spectrum of different coal ranks under liquid nitrogen soaking were tested using, respectively, (a) liquid nitrogen soaking temperature measurement, (b) laser thermal instrument and (c) Fourier transform infrared spectrometer. The results showed that the temperature curves of coal samples under liquid nitrogen soaking were divided into an accelerated cooling stage I, a decelerated cooling stage II, and a maintained low-temperature stage III. As the number of liquid nitrogen soaking increased, the time required to reach low-temperature Stage III gradually shortened. During the rise in coal sample temperature, it increased with time in accordance with a logarithmic function. The order of absolute values of maximum heating/cooling speed was lignite > bituminite > anthracite. The higher coal rank is, more oxygen-containing functional groups were removed by coalification. The less content of oxygen-containing functional groups led to closer molecular structure, which resulted in smaller thermal conductivity and ultimately caused slower temperature transfer. The study results are of important guides to understand further the action process and mechanism of liquid nitrogen soaking on coal.

Liquid nitrogen freeze–thaw (LNFT) cycles can realize the high-volume fracturing of coalbed methane (CBM) reservoirs, because they can effectively improve the distribution of pores and cracks inside coal. To quantitatively evaluate the mechanical behaviors and permeability evolution law of coal with LNFT cycles, the mechanical and permeability tests of the coal subjected to LNFT cycles under different geological conditions are performed. The structural damage of the coal under different LNFT cycles is analyzed from both macro- and micro-perspectives. The results indicate that under a low confining pressure, the LNFT cycle treatment can significantly aggravate the deterioration of mechanical properties of coal; whereas this deterioration effect will be weakened to some extent under a high confining pressure. The coal treated with LNFT cycles tends to shear failure under no confining pressure. However, under confining pressure, the coal treated with LNFT cycles is more prone to tensile failure, especially under relatively high confining pressure. The permeability of the coal presents a natural logarithm increase with LNFT cycles. The changes in the coal permeability are controlled by the internal structure of coal and the gas rarefaction effect. With the increase of LNFT cycles, the coal permeability change becomes more sensitive to confining pressure and gas pressure due to the gas rarefaction effect. During the LNFT of the coal, the temperature gradient mainly produces macro-main cracks, while the anisotropy of the thermal expansion coefficient of mineral grains, water–ice phase transition, and liquid nitrogen vaporization mainly induce internal micro-cracks including intergranular cracks and intragranular cracks inside the coal. The ideal number of LNFT cycles for CBM reservoirs is closely related to the geological condition including in situ stress and pore pressure.HighlightsThe mechanical parameters and failure behavior of coal treated with liquid nitrogen freeze–thaw cycles have an obvious confining pressure effect.The gas rarefaction effect inside coal is significantly affected by liquid nitrogen freeze–thaw cycles and confining pressure.The ideal number of liquid nitrogen freeze–thaw cycles for CBM reservoirs is closely related to in-situ stress and pore pressure.The structural damage of coal under different liquid nitrogen freeze–thaw cycles is analyzed from both macro and micro scales.

Microwave-liquid nitrogen treatment has attracted huge attention due to its significant damage to rock and broad application prospect in assisting mechanical rock-breaking. Meanwhile, the mode I fracture of hard rock plays a pivotal role in the deep drilling of tunneling boring machines (TBM). Therefore, the structure damage and mode I fracture behaviors of the granite treated with different microwave and liquid nitrogen conditions are analyzed experimentally. The damage mechanism of the microwave-liquid nitrogen treatment is deeply explored and the fracture propagation mechanism of the treated granite under the three-point bending (TPB) loading is given. The results show that the microwave-liquid nitrogen treatment can result in more microcracks and a decrease in the fracture toughness of the granite. The decrease in the fracture toughness of the granite is more obvious after the multiple cycle treatment (after 20 cycles, the decline rate reaches 82.2%). At the same power, the effect of the microwave-liquid nitrogen treatment on the mechanical degradation of the granite is more obvious than that of a single microwave treatment. In the TPB test, the microwave-liquid nitrogen treatment can significantly promote the propagation of shear cracks, especially under the high-cycle microwave-liquid nitrogen treatment. With the increase in the microwave-liquid nitrogen cycle, the required fracture energy decreases, and the induced crack surface is coarser. Under the high-cycle microwave-liquid nitrogen treatment, the granite exhibits ductility and the top loading point is prone to disintegration. Therefore, the microwave-liquid nitrogen treatment can improve the TBM rock-breaking efficiency by decreasing the fracture resistance and fracture energy of hard rock. The results will help to deepen the understanding of the application of microwave-liquid nitrogen treatment in assisting mechanical rock-breaking. Highlights Mode I fracture properties of the granite treated with microwave and liquid nitrogen are analyzed experimentally. Microwave-liquid nitrogen treatment is more obvious in the mechanical deterioration of the granite than single microwave treatment at the same power. Microwave-liquid nitrogen treatment can significantly promote the propagation of shear cracks in the three-point bending experiment of the granite. Crack propagation mechanism of the microwave-liquid nitrogen treated granite under the three-point bending load is revealed.

Spray cooling has been considered one of the most promising thermal control methods of high-heat flux devices. Most of the spray cooling research focuses on electronic components as the main application object to achieve higher heat dissipation heat flow in ambient temperature regions for small areas. Water is the most common cooling medium. This paper investigates the application of spray cooling thermal control over large areas. In this study, the heat-transfer characteristics of liquid nitrogen (LN2) for large areas was investigated by conducting experiments. The test surface is 500 mm × 500 mm, which was cooled by a nine-nozzle array. The spray nozzles used in the experiment were conical nozzles with an orifice diameter of 1.6 mm, a spray angle of 120°, and a spray height of 42 mm. Liquid nitrogen was forcefully ejected from nozzles by the high pressure of a liquid storage tank to cool the test surface. According to the cooled surfaces, spray directions, and spray pressures, three groups of experiments were conducted. The results showed that the smooth flat surface has the best heat-transfer performance in three kinds of surface structures, which are macro surface, porous surface, and smooth flat surface. The heat-transfer coefficient varied by ±20% with different spray directions, and the surface heat-transfer coefficient increased linearly with increasing spray pressure. Most of the spray cooling research focuses on heat dissipation in the ambient temperature region for equipment over small areas. The results can benefit thermal control application in various fields. The research in this paper can provide a reference for the application of large-area spray cooling, and the application areas mainly include metal manufacturing processing cooling, aircraft skin infrared radiation characteristics modulation, and laser weapon equipment cooling.

Liquid nitrogen (LN2) cooling can induce significant cracking damage in coalbed methane (CBM) reservoirs. To investigate the combined effects of LN2 cooling and bedding angles on the mechanical behaviors of bedded coal, coal specimens with different bedding angles (0°, 30°, 60° and 90°) were cooled. The variations in the surface features and P-wave velocity of the specimens were analyzed. The mechanical properties (tensile strength, peak strain and absorbed energy) were measured under Brazilian test conditions and compared. Furthermore, the crack propagation processes and the failure mechanisms were observed and compared. The results showed that LN2 cooling can induce considerable thermal damage inside the coal, resulting in the deterioration of the coal’s mechanical properties. This deterioration led to easier cracking and a higher fragmentation degree under splitting load. In addition, the reductions in tensile strength, peak strain and absorbed energy exhibited obvious anisotropic characteristics. The thermal cracks induced by LN2 cooling participated in the formation of macrocracks. After LN2 cooling, the failure mode remained unchanged from that before cooling at bedding angles of 0°, 30° and 90°. Notably, a transition from mixed-mode failure (primarily caused by tensile failure) to tensile failure occurred at the 60° bedding angle. This transition caused the specimen to fail at a relatively lower load, resulting in maximum reductions in tensile strength (34.8%), peak strain (27.6%) and absorbed energy (55.0%) at the 60° bedding angle. This paper provides new insights into the LN2 fracturing of bedded coal seams.

Minimizing thermal deformation in X-ray crystal optics is crucial for preserving coherence and wavefront in high-repetition-rate free-electron lasers (FELs). This study presents two approaches to reduce pulse-by-pulse transient thermal deformation in diamond crystals used in cavity-based X-ray FELs (CBXFELs): (i) cryogenic cooling with liquid nitrogen (LN 2 ), and (ii) second-order correction via focusing optics. We revisit the temperature-dependent thermal-mechanical properties of diamond and silicon, and implement a finite-element analysis method to accelerate convergence to a quasi-steady-state regime. Results show that LN 2 -cooled diamond crystals meet the stringent deformation requirement of less than 15 pm RMS for the pulse at the mJ scale at 1 MHz repetition frequency, and up to 1.5 mJ for 100 kHz. Second-order correction by using focusing elements within the cavity can reduce the impact of thermal deformation for both LN 2 and water cooling.

In recent years, liquid nitrogen (LN2) fracturing technology has been applied in coalbed methane (CBM) development. However, the impact of the liquid nitrogen freeze‐thaw process on the pore structure of anthracite coal has not yet been systematically investigated. In this study, nuclear magnetic resonance (NMR) analysis and scanning electron microscopy (SEM) imaging of coal samples after treatment with liquid nitrogen for different freezing times and freeze‐thaw cycles were performed to study the pore structure damage of anthracite subjected to the liquid nitrogen freeze‐thaw process as well as the variation in the porosity, fracture evolution, and permeability. The results show that the LN2 freeze‐thaw process can enlarge the pore size, enhance the pore connectivity, and form a fracture network on the surface of coal samples, which increases the total porosity, residual porosity, effective porosity, and permeability. This study provides a theoretical value for LN2 fracturing in the development of CBM. Classification and characterization of pore structure of coal reservoirs.

In the present research, the effect of ageing conditions such as time and temperature as well as the use of liquid nitrogen as a cooling agent on creep behaviour and relevant creep parameters such as steady creep strain rate was investigated by nanoindentation tests. Also, the creep behaviour of the material during the nanoindentation tests has been studied with the aid of measurements that have been collected during the holding stage of the experiment. The results indicate that ageing conditions influence the hardness of the material. Specifically, when using liquid nitrogen as a cooling agent during the extrusion process at ageing temperatures of 160 °C and 180 °C, a decrease in material hardness has been observed. Also, the results revealed that creep displacement is strongly affected by the ageing conditions. Additionally, no important change was found regarding the creep strain rate over the creep time both for the ageing conditions and the use of liquid nitrogen as a cooling agent. Finally, the creep stress exponent ( n ) increases for the ageing temperature of 160 °C, 180 °C and 200 °C, with increasing the ageing time.

HighlightsPresenting the first investigation into the structurally bubbling-failure mechanism of graphitic film during cyclic liquid nitrogen shocks.Proposing an innovative design about seamless heterointerface constructing a Cu-modified structure.Inventing a new ultra-stable species of highly thermally conductive films to inspire new techniques for efficient and extreme thermal management.Highly thermally conductive graphitic film (GF) materials have become a competitive solution for the thermal management of high-power electronic devices. However, their catastrophic structural failure under extreme alternating thermal/cold shock poses a significant challenge to reliability and safety. Here, we present the first investigation into the structural failure mechanism of GF during cyclic liquid nitrogen shocks (LNS), which reveals a bubbling process characterized by “permeation-diffusion-deformation” phenomenon. To overcome this long-standing structural weakness, a novel metal-nanoarmor strategy is proposed to construct a Cu-modified graphitic film (GF@Cu) with seamless heterointerface. This well-designed interface ensures superior structural stability for GF@Cu after hundreds of LNS cycles from 77 to 300 K. Moreover, GF@Cu maintains high thermal conductivity up to 1088 W m−1 K−1 with degradation of less than 5% even after 150 LNS cycles, superior to that of pure GF (50% degradation). Our work not only offers an opportunity to improve the robustness of graphitic films by the rational structural design but also facilitates the applications of thermally conductive carbon-based materials for future extreme thermal management in complex aerospace electronics.