Second-stage Phase Research Articles

Effects of second-stage active phase shifter on the inter-stage refrigeration capacity of a Stirling/pulse tube cryocooler

The Stirling/pulse tube cryocooler (SPC) is one of the most promising cryocoolers for space applications due to its advantages like high efficiency and long lifetime. The second pulse tube stage is coaxial pneumatically coupled in series with the first Stirling stage. The SPC possesses a distinctive capability to dynamically adjust the inter-stage refrigeration capacity by actively controlling the Stirling displacer in the first stage. This enables the SPC to effectively cater to varying heat loads over time. Nevertheless, the investigation into the regulation mechanism of the second-stage phase shifter on inter-stage refrigeration capacity shift is hindered by the presence of the second-stage passive phase shifter. This manuscript proposes an SPC with a second-stage active phase shifter (APS), which can actively control the phase difference between the pressure wave and mass flow at the second-stage hot end of the pulse tube in real time. The effect of the APS phase and amplitude on the inter-stage refrigeration capacity shift is analyzed using the theoretical and numerical model proven by the experiment. The first experimentally reported SPC with a second-stage APS demonstrates a refrigeration capacity of 0.22 W at 20 K plus 3.11 W at 70 K, requiring a compressor electrical power of 220 W. The SPC achieves a no-load minimum temperature of 13.5 K.

Transition-metal hydroxide nanosheets with peculiar double-layer structures as efficient electrocatalysts

Single- or few-layer nanosheets with a molecular-level thickness may provide an ideal model for the fundamental investigation of layer-number-dependent properties. Herein, we report a new strategy for a selective synthesis of transition-metal-containing Co-Fe and Co-Ni-Fe hydroxide nanosheets with peculiar double-layer structures. A unique second-stage phase was obtained from topochemical oxidative intercalation of brucite-type metal hydroxides, e.g., Co5/6Fe1/6(OH)2 and Co2/3Ni1/6Fe1/6(OH)2, with a molar ratio of Fe fixed at 1/6. After anion exchange, the second-stage phase was exfoliated into double-layer hydroxide nanosheets with an average thickness of ∼1.9 nm, which was approximately twice that of their single-layer counterparts (∼1.0 nm). Electrocatalytic measurements combined with theoretic calculations revealed that the double-layer hydroxide nanosheets exhibited excellent oxygen evolution reaction (OER) catalytic activity and kinetics, outperforming their single-layer counterparts, which validates the great potential of transition-metal hydroxide nanosheets with both tunable composition and layer numbers for efficient electrocatalysis.

Robust Adaptive Pulse Compression Method Based on Two-Stage Phase Compensation

The conventional pulse compression method [i.e., matched filter (MF)] and adaptive pulse compression (APC) methods are based on the assumption that the echo sampling point is located in the target point which is in a range cell. If the echo sampling point is not located in the target point, it will result in sampling mismatch problem and the performance of MF and APC methods will be significantly reduced when using the continuous-time phase-modulated waveforms, such as linear frequency modulation (LFM) signal. Aiming at solving the sampling mismatch problem, an APC based on two-stage phase compensation (TPC-APC) and its dimensionality-reduced version-contiguous fast APC based on two-stage phase compensation (TPC-CFAPC) are proposed in this article. In TPC-APC/TPC-CFAPC method, the mismatch phase caused by the sampling mismatch is first compensated through the first-stage phase compensation, which suppresses the sampling-mismatch-induced range sidelobes; then the mismatch phase caused by the target’s Doppler frequency is compensated through the second-stage phase compensation to suppress the Doppler-mismatch-induced range sidelobes; finally, the APC is applied to suppress the range sidelobe. To further compensate the mismatch phases of different targets, this article proposes a reiterative TPC-APC method and its dimensionality-reduced version, which iteratively use TPC-APC or TPC-CFAPC to suppress the range sidelobes caused by the mismatch phases of different targets. The results of the experiments have shown that the proposed methods are more robust compared with MF and APC methods.

Effect of substrate on phase-change characteristics of GeSb thin films and its potential application in three-level electrical storage

Phase-change Ge8Sb92 films were deposited on the varied thermal-conductivity substrates by radio frequency sputtering and their crystallization behavior was investigated. Situ sheet resistance measurement and the X-ray diffraction spectra show a double stage phase transitions of Ge8Sb92 films on the low thermal-conductivity substrates while a single stage phase transition on the high ones with the increasing heating temperature. The first-stage phase transition is amorphous-to-crystalline transition and the second-stage phase transition is partial-to-complete crystalline transition. The results provide experimental basis for the optimization of Ge8Sb92 phase-change memory and the possibility application in three-level electrical storage with single layer GeSb-type phase-change materials.

Silver intercalation in titanium disulphide

In order to study the intercalation process of Ag into the layered crystal TiS2, Rutherford backscattering spectrometry (RBS) measurements have been performed using the Nuclear Microprobe at the Microanalytical Research Centre, University of Melbourne. The focused ion beam allows the three-dimensional distribution of the intercalate in the bulk material to be determined with micron lateral resolution. The data exhibit a sharp intercalation front. Silver was found to be present over the whole investigated area of the crystal but only over the first 100 nm in depth. In deeper layers the intercalation was restricted to the border zone of the crystal edge. After 10 min intercalation time, the intercalation front has moved 70 μm laterally from the crystal edge and after 2.5 h it has moved 160 μm. The stoichiometry of the Ag changes from 0.38–0.42 indicating a first-stage phase in the near-surface region to 0.14–0.2 (second-stage phase) in deeper layers.

The thermodynamic and kinetic properties of silver intercalated niobium disulfide

A study of structures, thermodynamic and kinetic properties of Ag intercalated 2H-NbS 2 is reported. The observed phases Ag xNbS 2, being a dilute gasphase (x<0.22) and a second- and first-stage phase (0.22≦×≦0.30 and 0.55 ≦×≦0.76 respectively) are structurally related. Silver atoms reside in tetrahedral holes between neighbouring sandwiches of NbS 2. Kinetic parameters of the mobile species (Ag) were obtained from ac response and the Galvanostatic Intermittent current Titration Technique (GITT), using samples of Ag xNbS 2 as solid solution electrode in an AgI electrochemical cell above 425 K. Both first- and second-stage Ag xNbS 2 exhibit fast ionic conduction. The relatively low chemical diffusion coefficient at 450 K, D̃ ≈ 10 −6.5 cm 2s −1, in the second-stage phase, as compared to that in the first-stage, D̃≈ 10 −4.5 cm 2s −1, is attributed to the coupling of the diffusion kinetics with stage conversion in islands type domains.

A kinetic neutron diffraction study on the intercalation of deuterobenzene into KC 24

The reaction of KC 24 with C 6D 6 vapour was studied by neutron diffraction on a time scale of 5 min/spectrum. The reaction is initiated by the formation of a second-stage phase KC 24(C 6D 6) y. The CC spacing increases during the reaction which is explained by a charge transfer towards the graphite planes.