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After a coal seam is mined, the coal remaining in the goaf is prone to flooding and spontaneous combustion accidents. To explore the reignition (secondary oxidation) characteristics of long-flame coal after oxidation and water immersion, the experimental methods of thermogravimetric analysis and infrared spectroscopy were used to analyze coal samples of oxidation first and then water immersion (FO) and samples of water immersion first and then oxidization (FI) at different pre-oxidation temperatures. The results showed that the content of main oxygen-containing functional groups (hydroxyl, carbonyl, and carboxyl groups) of the FO120 (oxidation 120 °C first and then water immersion) coal sample increased, and the FI 90 (water immersion first and then oxidization 90 °C) coal sample decreased. Pre-oxidation at 120 °C will slow down the decrease in the extent of low-temperature secondary oxidation TG, as the pre-oxidation temperature increases, the total heat release of the FO coal samples first increase and then decrease, and the heat released is high at 120 °C. The FI coal samples transfer active sites during the water immersion process, and the high pre-oxidation temperature leads to the rapid increase of the speed of the primary active site, which leads to the transformation between the secondary active site and the oxygen-containing group, resulting in the cleavage of the oxygen-containing group and increasing the heat production. Water immersion pre-oxidation performed under different conditions has the dual effects of promoting and inhibiting spontaneous coal combustion. This result provides a theoretical basis for preventing spontaneous combustion in coal-mined areas in shallow coal seams after soaking in water.

The effect of electrostatic spraying on the formation of CO2 hydrate was experimentally studied. The effects of voltage (0, 0.5, 1.0, 1.5, 2.0 KV) and initial temperature (285.15, 282.15, 279.15 K) on the morphology, phase equilibrium temperature, induction time, rapid growth time, gas consumption, water to hydrate conversion and gas uptake rate of CO2 hydrate were analyzed. The CO2 hydrate primarily took the wall of the vessel as the nucleation site and grew from wall to the center of the reactor. The main morphological structure of CO2 hydrate were white filaments stacked in arcs, branches, and clusters. Low voltage-high initial temperature and high voltage-low initial temperature were conducive to improving the phase equilibrium temperature of CO2 hydrate. The application of voltage would prolong the induction time and shorten the rapid growth time of CO2 hydrate, and the effect would be more apparent with the increase of voltage. Lowering the initial temperature would shorten the induction time, 23 min was observed at 1.0 KV and the initial temperature of 279.15 K. The gas consumption and water to hydrate conversion change with voltage were in an order of 1.0 > 0>2.0 > 1.5>0.5 KV at the same temperature. Only 1.0 KV could enhance CO2 gas consumption, water to hydrate conversion and gas consumption rate.

During coal-mining processes underground, the coal spontaneous combustion caused by air leakage of narrow coal pillars and residual coal in goaf under pressure not only threatens the safety of mining of coal resources, but also leads to serious casualties. Therefore, the research of multi-field coupling for broken coal could be helpful to reveal the spontaneous combustion law of narrow coal pillars and residual coal in goaf and find the key for coal-fire prevention and control. Therefore, the uniaxial compression equipped with a temperature-programmed device is applied to conduct stress-heating tests on five types of coal particle sizes. Moreover, based on the Hertz contact deformation principle under the three-particle model, the thermodynamic hysteresis characteristics of broken coal under uniaxial stresses are theoretically analyzed. The results are shown as follows: (i) Uniaxial stress leads to significant hysteresis effects on the critical temperature of coal spontaneous combustion, heating rate, Graham’s Ratio-R2 temperature coefficient and pyrolysis temperature. (ii) As the particle radius increases, the growth rate of the exothermic intensity is suppressed, while it has no inhibitory effect on the critical temperature of the minimum exothermic intensity. (iii) The extreme point for the limit parameter of coal spontaneous combustion and the critical temperature shows the same corresponding hysteresis under uniaxial stress. The particle size ratio changes the limit parameter of coal spontaneous combustion obviously, and coal with a large particle size would reduce the risk of spontaneous combustion. (iv) The stress–strain curves and the stress-porosity curves are divided into stages, and it is found that the critical uniaxial stress of the porosity and strain of each coal sample is up to 4 MPa. Moreover, the temperature is the main reason for the hysteresis of permeability. The stress sensitivity of permeability declines with the effective stress rising, increases with the temperature going up, and decreases with the coal particle size reducing. (v) The hysteresis effect of uniaxial stress on coal spontaneous combustion first increases, then decreases, and finally increases.

The structural and electronic properties of individual titanium oxide nanotubes have been studied using both empirical and ab initio calculations. Two different types of titanium oxide nanotubes (A-nanotube and B-nanotube) have been constructed and energy-minimized by molecular mechanics calculations. We found that the A-nanotubes are energetically more favorable than the B-nanotubes. The electronic band structure of the titanium oxide nanotubes was also calculated with respect to the tubule diameter and the tubule type using the ab initio method. The band gap of the A-nanotube was reduced by up to 60% as the tubule diameter decreases from 1.2 nm to 0.5 nm.

The extinction and orientational dependence of electron diffraction from single-walled carbon nanotubes have been observed experimentally and investigated in detail theoretically using both algebraic analysis and numerical simulations. Electron diffraction from only achiral carbon nanotubes of zigzag or armchair structure shows observable orientational dependence and extinction of certain layer lines in experiment due to the interference of two primary Bessel functions of the same order that contribute to the scattering intensities on these layer lines.

A direct method is described to determine the chiral indices ( u, v ) of a carbon nanotube from a nanobeam electron diffraction pattern of the nanotube with high accuracy. This method is a one-step procedure and allows rapid and precise assignment of the chiral indices of carbon nanotubes. Using a contemporary electron microscope, we have been able to achieve an accuracy that enables us to assign the chiral indices unambiguously up to the range of (30, 30), corresponding to a diameter of 4.0 nm. Examples of determination of the chiral indices for two single-walled carbon nanotubes are presented as an illustration of the method.

A quantitative method for the structural determination by electron diffraction of nanotubes of elliptical cross-section is developed as a general case while the cylindrical nanotubes are treated as a special class. We found that the chiral indices of a carbon nanotube can always be measured from the electron diffraction pattern regardless if the nanotube is circular or elliptical. An experimental electron diffraction pattern from a partly-deformed carbon nanotube is also analyzed. Assisted with numerical simulations, it is determined that the observed carbon nanotube has chiral indices (15,7) with 8° tilt relative to the horizontal plane and eccentricity of 0.553.

A practical procedure has been proposed to determine the handedness of a carbon nanotube from its electron diffraction patterns. When a nanotube is twisted, the diffraction layer line spacings change, although the intensity distribution on the layer lines stays unchanged. The handedness can be assigned by observing the directions in which the principal layer lines shift under known direction of twisting and the twisting angle can be derived directly from the ratio of the diffraction layer line spacings. Simulated electron diffraction patterns of a carbon nanotube twisted both clockwise and counterclockwise are also presented as an example to corroborate the theoretical analysis.

Electron diffraction patterns of single-walled carbon nanotubes always have 2mm symmetry regardless if the nanotubes themselves have such symmetry. We here show that, for the case of multiwalled carbon nanotubes, the 2mm symmetry can break down when coherent interferences of the electron waves from two different shells take place such as when two shells of the same helicity but their chiral indices (u,v) have opposite evenness/oddity. Both an experimental electron diffraction pattern and analytic analysis are presented to demonstrate the conditions under which the 2mm symmetry breaks down in the electron diffraction patterns of multiwalled carbon nanotubes.

We show that, even when the atomic structure of a single-walled carbon nanotube does not have mirror symmetry perpendicular to the tubule axis, the electron diffraction patterns of the single-walled carbon nanotube always possess 2mm symmetry. We have also analyzed the contributions of higher order Bessel functions to the intensity distribution on the layer lines and found that (a) their contributions are negligible and (b) they would not affect the symmetry of the diffraction pattern. Both experimental and simulated electron diffraction patterns of a single-walled carbon nanotube of indices (14, 9) are presented to illustrate the 2mm symmetry and to corroborate the theoretical analysis.

Current commercial micro-CT scanners have the capability of imaging objects ex vivo with high spatial resolution, but performing in vivo micro-CT on free-breathing small animals is still challenging because their physiological motions are non-periodic and much faster than those of humans. In this paper, we present a prototype physiologically gated micro-computed tomography (micro-CT) scanner based on a carbon nanotube field emission micro-focus x-ray source. The novel x-ray source allows x-ray pulses and imaging sequences to be readily synchronized and gated to non-periodic physiological signals from small animals. The system performance is evaluated using phantoms and sacrificed and anesthetized mice. Prospective respiratory-gated micro-CT images of anesthetized free-breathing mice were collected using this scanner at 50 ms temporal resolution and 6.2 lp mm(-1) at 10% system MTF. The high spatial and temporal resolutions of the micro-CT scanner make it well suited for high-resolution imaging of free-breathing small animals.

A prototype physiologically gated micro-computed tomography (micro-CT) system based on a “eld emissionmicro-focus x-ray sour ce has been developed for in vivo imaging of small animal models. The novel x-raysource can generate radiation with a programmable waveform that can be readily synchronized and gated withnon-periodic physiological signals. The system performance is evaluated using phantoms and sacri“ced andanesthetized mouse models. Prospect ive respiratory-gated CT images of a nesthetized free-breathing mice arecollected using this scanner at 100msec temporal resolution and 10lp/mm of 10% system MTF.Keywords: micro-CT, in vivo imaging, respiratory gating, x-ray source, carbon nanotube 1. INTRODUCTION Micro-computed tomography (micro-CT) is an important tool in non-invasive screening of small animals suchas mice and rats for pre-c linical cancer studies. 1–3 Compared to clinical CT scanners, a much higher spatialresolution is needed in order to image the anatomy of small animals with research interest. Current commercialmicro-CT scanners oer the capability of imaging objects ex-vivo with high spatial resolution. Studies on lungand colon cancer models have shown that micro-CT is capable of sequentially tracking tumors of larger than2mm in diameter with high “delity. Small tumors are still hardly distinguishable from the normal tissue, whichis partially attributed to the motion-induced artifacts created by peristalsis, respiration and cardiac motion.The motion-induced artifacts can in principle be minimized by prospective gating where the image acquisition issynchronizedwith the physiological signals, as commonly done in clinical CT scanners. However, performing suchmeasurements on small animals is challenging because their physiological motions are at least ten times fasterthan those of human. Motion-induced artifacts blur the micro-CT image resulting in signi“cantly deterioratedspatial resolution than the nominal values.Several new designs have been considered for high-resolution CT imaging of live animals. One is to use ahigh power rotating anode x-ray source instead of a “xed-anode micro-focus x-ray tube.

We report a detailed study on the design of carbon nanotube based microfocus x-ray tube with one electrostatic focusing electrode. Based on the electron optics simulations, such parameters as geometrical distances and applied voltages among all the electrodes are considered, respectively, in relation to the size of x-ray focal spot. The stability of the x-ray focal spot size is also examined with respect to the variation of gate and anode voltages. Experimental results that agree well with the simulated data are also provided to corroborate the design method. We also discuss the operating stability and limitations when designing a carbon nanotube based microfocus x-ray tube with only one electrostatic focusing electrode. The designed x-ray tube with an isotropic focal spot sees wide applications in in vivo medical imaging studies.

The atomic structures of 124 single-walled carbon nanotubes, described by their diameter and helicity or equivalently by the two chiral indices $(u,v)$ that define the perimeter of each nanotube, have been determined unambiguously by nanobeam electron diffraction. A mapping of 70 possible nanotubes in the range of 1.20--1.65 nm in diameter with all possible helicities has been constructed experimentally for a carbon nanotube sample produced by arc discharge. Among the total 124 nanotubes characterized experimentally, 58 nanotubes of different structure have been identified. By examining the histogram of occurring helicities, we find that, while certain nanotubes were observed slightly more often than others, the overall feature showed a rather uniform distribution in occurrence. Basing on a nucleation-and-growth model, we suggest that the uniform distribution of helicity be originated from the weak dependence on helicity of the formation energy of carbon nanotubes, while the growth prefers slightly the structure with helicity 15\ifmmode^\circ\else\textdegree\fi{}--30\ifmmode^\circ\else\textdegree\fi{} for which the addition of carbon dimers would facilitate the growth of carbon nanotubes.

We report a method that allows direct, systematic, and accurate determination of the atomic structure of multiwalled carbon nanotubes by analyzing the scattering intensities on the nonequatorial layer lines in the electron diffraction pattern. Complete structure determination of a quadruple-walled carbon nanotube is described as an example, and it was found that the intertubular distance varied from 0.36 nm to 0.5 nm with a mean value of 0.42 nm.

Microcomputed tomography is now widely used for in vivo small animal imaging for cancer studies. Achieving high imaging quality of live objects requires the x-ray source to have both high spatial and temporal resolutions. Preliminary studies have shown that carbon nanotube (CNT) based field emission x-ray source has significant intrinsic advantages over the conventional thermionic x-ray tube including better temporal resolution and programmability. Here we report the design and characterization of a CNT based field emission x-ray source that also affords a high spatial resolution. The device uses modified asymmetric Einzel lenses for electron focusing and an elliptical shaped CNT cathode patterned by photolithography. Stable and small isotropic x-ray focal spot sizes were obtained.