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[en] We have measured positronium formation cross sections, QPs, for 1 to 100 eV positrons scattered by K atoms. Our experimental approach involves setting lower and upper limits on QPs using (1) the detection of the coincidences of 511 keV annihilation gamma rays produced by the decay of para-Ps and (2) measurement of the transmission of the positron beam through the K vapor in our scattering cell with the angular discrimination of our apparatus deliberately made as poor as possible. Our present QPs results between 1 and 10 eV are substantially higher than those predicted by Guha and Mandal using destorted wave approximation calculation, and suggest, along with our recent total scattering cross section measurements, that cross-channel coupling effects between the Ps formation channel and other scattering channels may be playing an important role in positron-K scattering as has been shown to be the case for positron-Li scattering
[en] Rare earth ions doped CaMgSi2O6 photo-luminescent materials with long afterglow were prepared by solid state reaction in a reductive atmosphere. The influence of co-doping rare earth ions (Dy and Nd) on the luminescence of CaMgSi2O6 host was studied. The 438 nm emission of the phosphors can be ascribed to 4f7-4f65d interconfiguration transitions of Eu2+ ions, which is caused by the [EuO6] emitting center formed in CaMgSi2O6 host. All of these phosphors have long afterglow phenomenon. Both afterglow intensity and persistent afterglow time of CaMgSi2O6:Eu, Dy are better than those of CaMgSi2O6:Eu and CaMgSi2O6:Eu, Nd, which is in agreement with the results of thermally stimulated luminescence curves
[en] Fatigue analysis is an important aspect of understanding mechanical properties of metals and alloys. It is well known that the temperature of a specimen during cyclic fatigue testing increases as a result of internal friction. The temperature changes of a specimen are usually obtained using a thermocouple, but such measurements only provide temperature readings at a specific location. On the other hand, a detailed temperature distribution map of the specimen obtained during fatigue tests could be used as a new dynamic method of monitoring crack initiation and propagation behavior. Temperature changes during fatigue tests are expected to be local and in the form of thermal transients. Capturing thermal signatures associated with fatigue cracks requires a high-speed, high-sensitivity infrared imaging system. Relevant studies using a scanning infrared camera (IR) with temperature resolution of 0.1 K at 293 K can be found in the literature. In the present study, a state-of-the-art IR camera was employed to monitor temperature changes of ULTIMET* alloy during high-cycle fatigue tests. Using the snap-shot and the sequence modes of the IR camera, temperature distribution maps of the specimen as a function of time (cycles) have been obtained. This work reports temperature changes during high-cycle fatigue tests and infrared imaging of fatigue cracks as a heat source before the final failure
[en] Highlights: • A novel resorption thermal energy storage system is established. • Working pair of MnCl_2−CaCl_2−NH_3 is chosen for heat and refrigeration cogeneration. • The largest energy storage density reaches 1706 kJ/kg. • The maximum average cooling power is 1.07 kW during discharging phase. • The largest energy and exergy efficiency are 0.87 and 0.35 respectively. - Abstract: Sorption thermal energy storage (STES) is perceived as one prospective way of thermal energy storage (TES) owing to the advantages of high energy density, negligible heat loss, flexible working modes and long-term storage capability. Based on STES, this paper exhibits an innovative resorption thermal energy storage (RTES) system, and the experimental prototype is established for heat and refrigeration cogeneration. Working pair of MnCl_2−CaCl_2−NH_3 is chosen, 4.8 kg MnCl_2 and 3.9 kg CaCl_2 impregnated in expanded natural graphite treated with sulfuric acid are filled in two sorption beds respectively. Experimental results indicate that the largest energy storage density reaches 1706 kJ/kg when charging and discharging temperature are 160 °C and 30 °C, respectively. The maximum average cooling power achieves 1.07 kW during discharging phase and corresponding SCP is 27.33 W/kg within the overall cycle period. When charging temperature increases from 135 °C to 160 °C, the energy efficiency of the resorption system for heat and refrigeration cogeneration augments steadily from 0.72 to 0.87 while the exergy efficiency rises slowly from 0.29 to 0.35.
[en] The growth and thermal stability of ultrathin (<50-A-thick) Ag and Au films on Nb(110) was investigated with use of photoemission spectroscopy and low-energy electron-diffraction (LEED) patterns. The LEED results show that Ag or Au forms a monolayer which is commensurate with the substrate, but also show that the structure of this overlayer reverts to a more close-packed structure after three monolayers (ML) have been deposited. Photoemission spectra show trends that suggest the evolution of the Ag or Au band structure with coverage and also show that a surface deposit of at least 2 ML of Ag or Au is required to observe valence-band states characteristic of the bulk metal. Further examination of the photoemission spectra suggests that the Ag or Au d-band states play only a small role in the ing that occurs between the noble metals and Nb(110), and this can be contrasted with the extensive d-band involvement in chemisorption bonds formed between Pd or Pt and Nb(110). Heating does not cause substantial change in the electric structure of 12.8-ML-thick Ag films and they thermally desorb from Nb when heated above 830 K. Similar Au films intermix with Nb to form a disordered surface phase when heated beyond 750 K. Desorption or complete in-diffusion of Au occurs when the substrate is heated beyond 1100 K. Implications for the formation of strained surface layers and surface chemical properties of noble metals on Nb(110) are discussed
[en] Energy conversion technologies, especially for power generation and refrigeration technologies driven by the low temperature heat, are gathering the momentum recently. This paper presents a novel resorption system for electricity and refrigeration cogeneraion. Compared with adsorption refrigeration system, resorption refrigeration is characterized as safety and simple structure since there is no ammonia liquid in the system. The cogeneration system is mainly composed of three HTS (high temperature salt) unit beds; three LTS (low temperature salts) unit beds, one expander, three ammonia valves, two oil valves, four water valves and connection pipes. Chemical working pair of MnCl_2–CaCl_2–NH_3 is selected. Since scroll expander is suitable for small type power generation system, it is chosen for expansion process. 4.8 kg MnCl_2 and 3.9 kg CaCl_2 impregnated in expanded natural graphite treated with sulfuric acid (ENG-TSA) are filled in the cogeneration system. Experimental results show that maximum cooling power 2.98 kW is able to be obtained while maximum shaft power is about 253 W with 82.3 W average value. The cogeneration system can be utilized for the heat source temperature lower than 170 °C. Total energy efficiency increases from 0.293 to 0.417 then decreases to 0.407 while exergy efficiency increases from 0.12 to 0.16. - Highlights: • A resorption system for power and refrigeration cogeneration is established and investigated. • ENG-TSA as the additive improves the heat and mass performance of composite adsorbent. • The highest shaft power and refrigeration power are 253 W and 2.98 kW, respectively. • Total energy efficiency of the system increases from 0.293 to 0.417 then decreases to 0.407.
[en] VUV photoemission spectra of the valence bands of the high-T/sub c/ superconductor YBa2Cu3O/sub 9-δ/ δ = 2.1, have been measured using the UV undulator (beam line U5U). Photon energies h nu = 45.0 eV and 52.0 eV lie within the first harmonic flux output from the undulator, while h nu = 105.0 lies within the second harmonic. the spectra at h nu = 21.2 eV and 40.8 were taken using a He resonance lamp
[en] An Organic Rankine Cycle (ORC) with a scroll expander with varying displacement is studied. First, to obtain the isentropic efficiency of the scroll expander modified from an automobile air-condition compressor with a displacement of 66 ml/r, the performance is investigated by experiments on a test rig driven by the compressed air. Second, based on the experimentally obtained isentropic efficiency, thermodynamic and heat transfer models of ORC are established on the basis of sub-models of the main apparatuses. Consequently, energy and exergy efficiencies are analyzed. Based on the simulation, an ORC system is constructed and investigated. Experiments show that for a given heat source temperature of 105 °C, the energy efficiency of the system ranges from 1.7% to 3.2% and the exergy efficiency of the system is 8.6%–16.9%. Additionally, another scroll expander with a displacement of 86 ml/r is utilized to investigate how displacement of the scroll expander influences performance of the ORC system. Finally, experiments also show that the IHX deteriorates the performance of ORC system, which is significantly different from simulation results. - Highlights: • The isentropic efficiency of the scroll expander is investigated by compressed air. • An ORC system with R245fa fluid is designed and simulated. • An ORC system with scroll expanders of different displacements is established. • Reasonable exergy and energy efficiencies of ORC are achieved by experiments. • Experiments show an IHX requires a special design for getting optimal performance
[en] Highlights: • Solid sorption heat pipe (SSHP) with composite NaBr-NH_3 is proposed for continuous heat transfer. • Both vertical and horizontal SSHPs are investigated. • SSHP features non-isothermal heat transfer performance at sorbent and condenser sections. • The highest radial heat flux in vertical and horizontal SSHPs is 22.1 and 12.4 kW/m"2, respectively. • Both SSHPs have axial heat flux higher than 400 kW/m"2. - Abstract: A novel type solid sorption heat pipe (SSHP) is developed for continuous heat transfer. In contrast to conventional heat pipe (HP), SSHP utilizes the composite sorbent-sorbate as working media to replace the wick structure inside HP. Such a technology is expected to alleviate the heat transfer limits of conventional HP. NaBr is chosen as the sorbent, and the expanded natural graphite treated with sulfuric acid serves as the matrix. A certain molar amount of the sorbate (NH_3) is complexed with the composite sorbent. The desorption, condensation and chemisorption processes of NaBr-NH_3 working pairs are investigated for both vertical and horizontal placed SSHP. The results show that the desorption process of NaBr-NH_3 solid-gas reaction can be carried out while the heating temperature reaches up to 60 °C or above. The highest radial heat flux in both vertical and horizontal placed SSHP is around 22.1 and 12.4 kW/m"2, respectively, while the axial heat flux for both SSHPs is not less than 400 kW/m"2. It can be concluded that the SSHP is characterized by the non-isothermal heat transfer performance and verified to be available for continuous heat transfer. The vertical SSHP has a better overall heat transfer performance than horizontal SSHP under the same condition and NaBr-NH_3 working pairs applied in SSHP is suitable for low-grade thermal energy transfer above 60 °C.
[en] A small pumpless ORC (organic rankine cycle) system with different scroll expanders modified from compressors of the automobile air-conditioner is established, and the refrigerant R245fa is chosen as the working fluid. Different hot water temperatures of 80, 85, 90 and 95 °C are employed to drive the pumpless ORC system. Experimental results show that a maximum shaft power of 361.0 W is obtained under the hot water temperature of 95 °C, whereas the average shaft power is 155.8 W. The maximum energy efficiency of 2.3% and the maximum exergy efficiency of 12.8% are obtained at the hot water temperature of 90 °C. Meanwhile a test rig for investigating the mechanical loss of the scroll expander is established. The torque caused by the internal mechanical friction of the expander is about 0.4 N m. Additionally, another scroll expander with a displacement of 86 ml/r is also employed to investigate how scroll expander displacement influences the performance of the pumpless ORC system. Finally, the performance of the pumpless ORC system is compared with that of the conventional ORC system, and experimental results show that the small pumpless ORC system has more advantages for the low-grade heat recovery. - Highlights: • A small pumpless ORC (organic rankine cycle) system is established, and different scroll expanders are tested. • The maximum energy and exergy efficiency are 2.3% and 12.8% respectively. • A maximum shaft power of 361.0 W is obtained under the heat source temperature of 95 °C. • The small pumpless ORC system has characteristics of the high efficiency.