Microstructure Study of High-Rank Coal in an Alkaline Solution at the Chengzhuang Mine.

To study the adsorption performance of coal bodies after alkaline solution erosion and the microscopic mechanism of alkali erosion on coal bodies, isothermal adsorption experiments at different pH values and with different numbers of soaking days were conducted on high-order coal bodies from the Chengzhuang mine. The results showed that the adsorption capacity of the coal bodies after alkali leaching was improved compared to that of the original coal, all of which was in accordance with the Langmuir equation. The unit adsorption capacity of coal samples increased gradually with an increase in the number of soaking days and solution pH, reaching the maximum at pH 13 and eight soaking days. The adsorption constant a of the coal sample was positively correlated with the pH, and the number of soaking days was a power exponential function; the adsorption constant b increased gradually with an increase in the pH of the solution and increased first and then decreased with an increase in the number of soaking days. The change in the adsorption of coal samples occurs because the alkaline solution reacts with the minerals in the coal as well as the mineral ions, and the resulting complex gels and precipitates block the pore channels of the coal body, which in turn inhibits the adsorption of gases. The presence of Na, Mg, Al, Si, Ca, Fe, and other elemental compounds detected in the generated sediments verified the mechanism of alkaline solution erosion. The changes in the microscopic pore structure of the coal body were quantified by low-temperature liquid nitrogen adsorption experiments. The small and medium pore volumes of the coal samples reached the maximum values at pH 13 and with eight soaking days, which is in agreement with the conclusion of optimal alkali modification.

Study on the Critical Value of Residual Gas Content Based on the Difference of Adsorption Structure between Soft and Hard Coal.

In view of the difference of the adsorption structure between soft and hard coal, there is a big difference in the critical value of the inspection index for the regional outburst risk caused by the gas content. For the coal seams with soft and hard coal stratification, the model of gas content in the equilibrium state was established first, and the microscopic parameters of different rank coals were determined by the low-temperature liquid nitrogen adsorption test and mercury intrusion test. Then, the adsorption capacity of coal samples was determined by the adsorption test. Finally, the residual gas content of the coal seam in the equilibrium state was calculated based on the adsorbed gas content, and the critical value of prediction indexes of regional outburst based on the residual gas content was studied. The results show that for the same metamorphic degree, the specific surface area of soft coal is larger than that of hard coal. However, under the same gas pressure, the residual gas content of hard coal of anthracite and lean coal is greater than that of soft coal with the same metamorphic degree, while that of meager-lean coal and gas-fat coal is opposite. It is suggested to adopt the small value (rounded) of the measured gas content of soft and hard coal at 0.74 MPa as the critical value of the residual gas content in the regional effect test from the economic perspective. It is of great significance to determine the critical standard of the residual gas content in the regional effect test according to local conditions for reducing the cost of outburst prevention work.

Loaded Microwave Cavity for Compact Vapor-Cell Clocks.

Vapor-cell devices based on microwave interrogation provide a stable frequency reference with a compact and robust setup. Further miniaturization must focus on optimizing the physics package, containing the microwave cavity and atomic reservoir. In this article, we present a compact cavity-cell assembly based on a dielectric-loaded cylindrical resonator. The loaded cavity resonating at 6.83 GHz has an external volume of only 35 cm3 and accommodates a vapor cell with 0.9-cm3 inner volume. The proposed design aims at strongly reducing the core of the atomic clock, maintaining, at the same time, high-performing short-term stability ( σy(τ) ≤ 5×10-13 τ-1/2 standard Allan deviation). The proposed structure is characterized in terms of microwave field uniformity and atom-field coupling with the aid of finite-element calculations. The thermal sensitivity is also analyzed and experimentally characterized. We present preliminary spectroscopy results by integrating the compact cavity within a rubidium clock setup based on the pulsed optically pumping technique. The obtained clock signals are compatible with the targeted performances. The loaded-cavity approach is, thus, a viable design option for miniaturized microwave clocks.

Pulsed optically pumped atomic clock with a medium- to long-term frequency stability of 10-15.

Herein, we report a significant improvement in the medium- to long-term frequency stability of our pulsed optically pumped (POP) vapor-cell rubidium clock. Such an achievement is established with the better control of our system and the environment. An integrated optical module, including a distributed Bragg reflector laser and an acousto-optic modulator, is developed to improve the stability of the laser. The physics package is sealed in a vacuum chamber with a vacuum of 4 × 10-4 Pa to significantly reduce the impacts of the barometric effect. An AC-driven heater is placed much closer to the cell to enable a better temperature control. The resolution of the servo control voltage is also optimized. With all these improvements, a frequency stability of 4.7 × 10-15 at 104 s in terms of the Allan deviation is obtained. We also estimate the main noise sources that limit the frequency stability of the POP atomic clock.

Intensity Detection Noise in Pulsed Vapor-Cell Frequency Standards.

Laser intensity noise is currently recognized as one of the main factors limiting the short-term stability of vapor-cell clocks. In this article, we propose a signal theory approach to estimate the contribution of the laser intensity fluctuations to the short-term stability of vapor-cell clocks working in a pulsed regime. Specifically, given the laser intensity noise spectrum, an analytical expression is derived to evaluate its impact on the clock Allan deviation (ADEV). The theory has been tested for two intensity noise spectra of interest in clock applications: white frequency noise and flicker noise. The predicted results turn out to be in good agreement with experiments performed with a prototype of pulsed optically pumped Rb-cell clock, and can be extended to other compact clocks.

Note: Pulsed optically pumped atomic clock based on a paraffin-coated cell.

We report on the implementation of a pulsed optically pumped atomic clock based on a paraffin-coated cell. The relaxation times are measured, with the longitudinal relaxation time, T1 = 9.7 ± 0.4 ms, and the transversal relaxation time, T2 = 0.40 ± 0.03 ms. We demonstrated that the measured frequency stability of the clock is 3.9 × 10-13 τ-1/2 (1 s ≤ τ ≤ 100 s) and reaches a value of 3.1 × 10-14 for τ = 1000 s, where τ is the averaging time. This is an unprecedented result for a paraffin-coated vapor cell clock, and it makes significant contributions toward improving the performance of the wall-coated vapor cell atomic clock.

Frequency stability of a pulsed optically pumped atomic clock with narrow Ramsey linewidth.

In general, the linewidth of the Ramsey central fringe (RCF) is equal to 1/(2T), where T is the Ramsey free-evolution time. We demonstrate that the RCF linewidth of a pulsed optically pumped (POP) atomic clock with orthogonal polarization detection based on the magneto-optical rotation effect can be narrowed down to 1/(4T). The Allan deviation of the POP atomic clock decreases from 2.4×10-13τ-1/2 to 1.4×10-13τ-1/2. This corresponds to an improvement in the frequency stability by about 60%. We also estimate the main noise sources that limit the short-term frequency stability of the POP atomic clock.

Pulsed optically pumped atomic clock with zero-dead-time.

By alternatively operating two pulsed optically pumped (POP) atomic clocks, the dead time in a single clock can be eliminated, and the local oscillator can be discriminated continuously. A POP atomic clock with a zero-dead-time (ZDT) method is then insensitive to the microwave phase noise. From τ = 0.01 to 1 s, the Allan deviation of the ZDT-POP clock is reduced as nearly τ-1, which is significantly faster than τ-1/2 of a conventional clock. During 1-40 s, the Allan deviation returns to τ-1/2. Moreover, the frequency stability of the ZDT-POP clock is improved by one order of magnitude compared with that of the conventional POP clock. We also analyze the main factors that limit the short-term frequency stability of the POP atomic clock.

Continuously tuning effective refractive index based on thermally controllable magnetic metamaterials.

By employing a thermally active magnetic material, we theoretically design a kind of electromagnetic metamaterial with intrinsic magnetic response, termed magnetic metamaterial (MM). The retrieved effective electric permittivity ε(eff) and magnetic permeability µ(eff) exhibit a nearly continuous transition from double negative to double zero, and then to double positive by controlling the temperature, indicating a flexible tunability of the effective refractive index. The beam splitting, collimation, focusing, and total reflection are achieved at different typical temperatures. Most importantly, with the MM implemented under a gradient temperature, a gradient negative-zero-positive index metamaterial (NZPIM) can possibly be realized, thus providing a new platform to study wave features in NZPIM.

Color-encoded microcarriers based on nano-silicon dioxide film for multiplexed immunoassays.

Multiplexed analysis allows researchers to obtain high-density information with minimal assay time, sample volume and cost. Currently, microcarrier or particle-based approaches for multiplexed analysis involve complicated or expensive encoding and decoding processes. In this paper, a novel optical encoding technique based on nano-silicon dioxide film is presented. Microcarriers composed of thermally grown silicon dioxide (SiO(2)) film and monocrystalline silicon (Si) substrate were fabricated. The nano-silicon dioxide film exhibited unique surface color by low-coherence interference. Hence the colors can be used for encoding at least 100 microcarriers loaded with films of different thickness. We demonstrated that color-encoded microcarriers loaded with antigens could be used for multiplexed immunoassays to detect goat anti-human IgG, goat anti-mouse IgG and goat anti-rabbit IgG, with fluorescent detection as the interrogating approach. This microcarrier-based method also exhibited improved analytical performance compared with a microarray technique. This approach will provide new opportunities for multiplexed target assay development.