Continuous adsorption refrigerator for producing temperatures below 1 K

This paper describes an evaporation refrigerator in which the pumping of the vapour above liquid He³ or He⁴ was accomplished with two alternately operating adsorption pumps. In this way temperature can be maintained constant with an accuracy to ～0.003 K for a long time; ～ 10 l helium vapour being used. The refrigeration capacity of the apparatus has been determined.The characteristics of the dilution refrigerator (namely, the rate of He³ circulation, the starting and operating temperatures) have been calculated for He³ circulation using the adsorption pumps and pumping line.This system is shown to be successful for producing very low temperatures at an He³ circulation rate of ～ 10^?5?10^?4 mole s^?1.     

The project of ultracold neutron sources at the PIK reactor with superfluid helium as a moderator

The project of ultracold neutron sources at the PIK reactor with superfluid helium as a moderator is presented. The rate of producing ultracold neutrons in superfluid helium is 100 cm^?3 s^?1 at neutron flux density Φ(λ = 9 Å) = 10⁹ cm^?2 s^?1 Å^?1. At a moderator temperature of 1 K within the experimental volume of 351, the density of ultracold neutrons may be equal to 1.3 × 10³ cm^?3, which is two orders of magnitude exceeds that the currently existing ultracold neutron sources.     

A highly stable and sensitive temperature regulator for the range 1.5–350 K

The equipment described was designed to control and stabilize temperature in the range of 1.5–350 K in a liquid helium type cryostat. The temperature sensor was a germanium resistor for the range 1.5–100 K and a platinum resistor above 60 K. The long-time temperature stability was 1 × 10^?4 K h^?1 up to 20 K, 2 × 10^?4 K h^?1 up to 60 K, and better than 5 × 10^?3 K h^?1 in the range 60–350 K.     

Nitrogen Vacancies on 2D Layered W2N3: A Stable and Efficient Active Site for Nitrogen Reduction Reaction

Electrochemical nitrogen reduction reaction (NRR) under ambient conditions provides an avenue to produce carbon‐free hydrogen carriers. However, the selectivity and activity of NRR are still hindered by the sluggish reaction kinetics. Nitrogen Vacancies on transition metal nitrides are considered as one of the most ideal active sites for NRR by virtue of their unique vacancy properties such as appropriate adsorption energy to dinitrogen molecule. However, their catalytic performance is usually limited by the unstable feature. Herein, a new 2D layered W₂N₃ nanosheet is prepared and the nitrogen vacancies are demonstrated to be active for electrochemical NRR with a steady ammonia production rate of 11.66 ± 0.98 µg h^?1 mg_cata^?1 (3.80 ± 0.32 × 10^?11 mol cm^?2 s^?1) and Faradaic efficiency of 11.67 ± 0.93% at ?0.2 V versus reversible hydrogen electrode for 12 cycles (24 h). A series of ex situ synchrotron‐based characterizations prove that the nitrogen vacancies on 2D W₂N₃ are stable by virtue of the high valence state of tungsten atoms and 2D confinement effect. Density function theory calculations suggest that nitrogen vacancies on W₂N₃ can provide an electron‐deficient environment which not only facilitates nitrogen adsorption, but also lowers the thermodynamic limiting potential of NRR.     

Stabilization of H and D atoms in Aggregates of Kr Nanoclusters Immersed in Superfluid Helium

Impurity–helium condensates containing krypton atoms and also atoms and molecules of hydrogen isotopes have been studied via an electron spin resonance (ESR) technique. Analysis of the ESR spectra shows that most of the H and D atoms reside in molecular layers (H₂ or D₂) formed on the surfaces of Kr nanoclusters. The thickness of the molecular films was found to determine the rates of recombination of the atoms into molecules, with atoms in the thinner films recombining much more slowly. Very large average concentrations were obtained for H atoms (10¹⁹ cm^?3) and D atoms (3?10¹⁹ cm^?3) in these experiments.     

A continuous flow helium cryostat for optical studies at fixed high pressure

The design of a continuous flow helium cryostat is described for optical studies with an optical fixed high-pressure chamber in the temperature range from 4 to 300 K. The liquid helium consumption for 4 K is 2 l h^?1, and less than 0.5 l h^?1 for temperatures above 20 K.     

High surface area ordered mesoporous carbon for high-level removal of rhodamine B

We report the synthesis of ordered mesoporous carbons (OMCs) with high surface area by the variation of mass ratio of tetraethylorthosilicate (TEOS) to resol, followed by carbonization and removal of silica. The obtained OMCs were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, N₂ adsorption and desorption analysis, Zeta potential and Fourier transform infrared spectroscopy. Results reveal that the OMCs were transformed from ordered to disordered structure at high mass ratio of TEOS/resol. A typical sample of OMCs possesses very high specific surface area of 1906 m² g^?1 and large pore volume of 1.8 cm³ g^?1. The OMCs as adsorbent show an ultrahigh-level adsorption capacity for the removal of toxic dye Rhodamine B (1028 mg g^?1) in the short contact time (60 min). The adsorption follows pseudo second-order kinetics with rate constant 2.5 × 10^?4 g mg^?1 min^?1, showing rapid adsorption properties. The OMCs can be reused; though the adsorption capacity seems to decrease somewhat after each cycle tested over 10 reuse cycles, it might be affected by the chemisorptions. The adsorption mechanism study reveals that the adsorption proceeds with hydrogen bonding between hydrogen atom of carboxylic group at OMCs and electronegative element (nitrogen) of RhB. It is concluded that the surface area and pore volume of the OMCs is tuned by the variation of mass ratio of TEOS to resol which is also demonstrated to have ultrahigh adsorption capacity for the model RhB dye.     

Investigation of electrons localized above solid hydrogen by means of the cyclotron resonance method

The magnetoresistance and cyclotron resonance of a two-dimensional sheet of electrons localized above liquid and solid hydrogen surfaces are investigated at the frequency of 20 GHz. It is found that the effective electron mass relevant to the motion along the surface is close to the free electron mass. The electron mean free path is determined by collisions with defects of the solid hydrogen surface. For 5?T?12 K the electron mobility was ～8×10⁴/T cm²/V sec, with 20% accuracy, for all the specimens investigated. ForT?5 K the mobility follows the law \(\mu \propto 1/\sqrt {{\text{W}}_{\text{e}} } \) , wherew _e is the mean electron energy. The numerical values of the electron mobilities for different specimens differ by factors up to five, and forT?1 K they fall within within the range 2×10⁴ to 10⁵ cm²/V sec. The experimental results indicate that the solid hydrogen surface has a terrace structure with flat sections about 10^?5 cm in size. During the investigation of electrons localized above a saturated helium film wetting the solid hydrogen, we observed a shift of the resonance to weaker magnetic fields, which amounts to ～100% at a pressing field of ～10³ V/cm. It is shown that this shift can be explained by the terrace structure of the substrate.     

On the growth of ZnSe on (100) GaAs by atmospheric pressure movpe

High quality, single crystalline, low resistivity ZnSe has been grown on (100) GaAs by atmospheric pressure metal-organic vapour phase epitaxy (MOVPE) using dimethylzinc and hydrogen selenide as reagents. The highest quality material is grown at 553 K with a growth rate of ≈ 14–26 μ h^?1 corresponding to a dimethylzinc flux (2.2–14.8) × 10^?5molmin^?1. The material is characterised by a low donor carrier concentration of 6.0 × 10¹⁴cm^?3, an electron mobility of 415 ± 10 cm²V^?1s^?1 at 298 K and is not highly compensated. The photoluminescence spectrum at 12 K is dominated by a single peak ≈ 2 meV wide at 2.7956 eV. The emission is retained with high intensity at 300 K in a peak corresponding to 2.690 eV and lower energy emission is virtually absent.     

Porous Cobalt–Nickel Hydroxide Nanosheets with Active Cobalt Ions for Overall Water Splitting

Low‐cost and high‐performance catalysts are of great significance for electrochemical water splitting. Here, it is reported that a laser‐synthesized catalyst, porous Co_0.75Ni_0.25(OH)₂ nanosheets, is highly active for catalyzing overall water splitting. The porous nanosheets exhibit low overpotentials for hydrogen evolution reaction (95 mV@10 mA cm^?2) and oxygen evolution reaction (235 mV@10 mA cm^?2). As both anode and cathode catalysts, the porous nanosheets achieve a current density of 10 mA cm^?2 at an external voltage of 1.56 V, which is much lower than that of commercial Ir/C‐Pt/C couple (1.62 V). Experimental and theoretical investigations reveal that numerous Co³⁺ ions are generated on the pore wall of nanosheets, and the unique atomic structure around Co³⁺ ions leads to appropriate electronic structure and adsorption energy of intermediates, thus accelerating hydrogen and oxygen evolution.     

A circulating cooling system based on a commercial G-3 helium liquifier

A circulating cooling system for a superconducting magnet is described, based on a commercial G-3 (G-45) helium liquifer. A flow of helium in the pressure range 2026-101 kNm^?2 (20 to 1 atm) with a temperature of 4.3 to 4.35 K can be obtained with the system for a maximum consumption of 3.6 Kg h^?1.     

Photothermal Beam Deflection Spectroscopy for the Determination of Thermal Diffusivity of Soils and Soil Aggregates

Photothermal beam deflection spectroscopy (BDS) with a red He–Ne laser (632.8 nm, 35 mW) as an excitation beam source and a green He–Ne laser (543.1 nm, 2 mW) as a probe was used for estimating thermal diffusivity of several types of soil samples and individual soil aggregates with small surfaces (2?×?2 mm). It is shown that BDS can be used on demand for studies of changes in properties of soil entities of different hierarchical levels under the action of agrogenesis. It is presented that BDS clearly distinguishes between thermal diffusivities of different soil types: Sod-podzolic [Umbric Albeluvisols, Abruptic], 29?±?3; Chernozem typical [Voronic Chernozems, Pachic], 9.9?±?0.9; and Light Chestnut [Haplic Kastanozems, Chromic], 9.7?±?0.9 cm²·h^?1. Aggregates of chernozem soil show a significantly higher thermal diffusivity compared to the bulk soil. Thermal diffusivities of aggregates of Chernozem for virgin and bare fallow samples differ, 53?±?4 cm²·h^?1 and 45?±?4 cm²·h^?1, respectively. Micromonoliths of different Sod-podzolic soil horizons within the same profile (topsoil, depth 10–14 cm, and a parent rock with Fe illuviation, depth 180–185 cm) also show a significant difference, thermal diffusivities are 9.5?±?0.8 cm²·h^?1 and 27?±?2 cm²·h^?1, respectively. For soil micromonoliths, BDS is capable to distinguish the difference in thermal diffusivity resulting from the changes in the structure of aggregates.     

Status of the cyclotron vacuum system at TRIUMF

The world's largest cyclotron was built at TRIUMF in 1972 and commissioned to full energy in 1974 [Harwood VJ, Yandon JC. TRIUMF Design Note, TRI-69-7, 1969 [1], Blakely RG, Moore RW, Harwood VJ. TRIUMF Design Note, TRI-69-9, 1969 [2]]. The cyclotron accelerates negatively charged hydrogen ions up to 500 MeV, and protons are produced by inserting a stripping foil in the beam which removes two electrons from each negatively charged hydrogen ion and allows the remaining bare protons to be channeled out of the accelerator. By making these protons strike different kinds of targets, intense beams of neutrons, pions and muons can also be created, thus making possible many different experiments. The volume of the cyclotron vacuum tank is about 100 m³ and operates at 2×10⁻⁸ Torr pressure during beam production. Most of the vacuum is achieved by cryopumping with a B-20 cryogenerator and six cryopumps. The B-20 is a Stirling cycle refrigerator, which supplies helium gas at 16 and 70 K to the cryopanels in the tank. The tank is also equipped with two turbo pumps. The vacuum system went through a few modifications during more than 30 years of operation. This paper presents the status of the cyclotron vacuum system and discusses the latest upgrades.     

A small radio-frequency superconducting resonance circuit

A superconducting continuously tunable resonance circuit is described with a volume not exceeding 6 cm³. Its Q is 2 × 10⁵ and the resonance frequency instability 5 × 10^?7h^?1.     

Stabilization of High Concentrations of Nitrogen Atoms in Impurity-Helium Solids

Impurity-helium (Im-He) solids created by injecting gaseous helium with an admixture of nitrogen atoms and molecules into superfluid ⁴He have been studied via electron spin resonance (ESR). We have studied the efficiency of stabilization of N atoms in Im-He samples prepared from nitrogen-helium gas mixtures with different fractions of nitrogen varying from 0.25% to 4%. Some of the observed ESR spectra of N atoms in the Im-He samples are very broad. The highest local concentration of N atoms determined from dipole-dipole broadening of the ESR line is ～8×10²⁰ cm^?3. The highest average concentrations of N atoms in N-N2-He solids were much lower (of order 10¹⁹ cm^?3). The samples with high concentrations of N atoms were stable in liquid helium, and remained stable even after draining liquid helium from the sample at T≤3.5 K.     

Generating H<Superscript>−</Superscript> ions in a low-voltage xenon-hydrogen discharge

Low-voltage discharge in a xenon-hydrogen (Xe-H₂) mixture has been theoretically studied. It is established that, at the optimum concentrations of hydrogen and xenon (on the order of 10¹⁵ cm^?3), the optimum total pressure of the plasma (about 1 Torr), a sufficiently high concentration of negative hydrogen ions (on the order of 10¹² cm^?3) in discharge plasma can be reached. The necessary emission current density at the cathode is on the order of 10 A/cm².     

A saddle field ion source of spherical configuration for etching and thinning applications

The construction is described of a saddle field ion source with its axis of symmetry coincident with the centre of the ion beam. The source consists of a spherical cathode enclosing an annular anode. Source current-source voltage and source current-ion beam current characteristics are given for argon, nitrogen, helium and hydrogen. A typical energy distribution curve is shown for argon at 6 kV at 2 × 10^?4 torr, the beam energy is 0.85 of the anode potential and the half width is about 250 V. The beam density is about 1 mA cm^?2, and the beam divergence 4°. Applications of cylindrical and spherical sources are described to ion beam machining, thinning and surface analysis.     

Enhanced Photocatalytic H2‐Production Activity of CdS Quantum Dots Using Sn2+ as Cocatalyst under Visible Light Irradiation

Herein, oil‐soluble CdS quantum dots (QDs) are first prepared through a solvent‐thermal process. Then, oil‐soluble CdS QDs are changed into water‐soluble QDs via ligand exchange using mercaptopropionic acid as capping agent at pH 13. The photocatalytic performance is investigated under the visible light irradiation using glycerol as sacrificial agent and Sn²⁺ as cocatalyst. No H₂‐production activity is observed for oil‐soluble CdS QDs. Water‐soluble CdS QDs exhibit significantly enhanced hydrogen evolution rate. When the concentration of cocatalyst Sn²⁺ increases to 0.2 × 10^?3 m , the rate of hydrogen evolution reaches 1.61 mmol g^?1 h^?1, which is 24 times higher than that of the pristine water‐soluble CdS QDs. The enhanced H₂‐production efficiency is attributed to the adsorption of Sn²⁺ ions on the surface of CdS QDs that are further reduced to Sn atoms by photogenerated electrons. The in situ generated Sn atoms serve as photocatalytic cocatalyst for efficient hydrogen generation.     

A new technique for the vapour phase epitaxial growth of ZnO as a guided-wave optical material

A new method is presented for the chemical vapour deposition of single-crystal ZnO films onto sapphire substrates. In this method the gas used for reducing the ZnO source is ammonia instead of the more conventional hydrogen. Because of the relatively weak reducing power of ammonia, the growth rate of the ZnO film is decreased to 2–3 μm h^?1 and the substrate temperature is lowered to 730–740°C. A relatively low optical waveguide loss of around 1 dB cm^?1 was obtained, presumably because of the decreased growth rate of the ZnO film.     

Nonepitaxial Gold‐Tipped ZnSe Hybrid Nanorods for Efficient Photocatalytic Hydrogen Production

For the first time, colloidal gold (Au)–ZnSe hybrid nanorods (NRs) with controlled size and location of Au domains are synthesized and used for hydrogen production by photocatalytic water splitting. Au tips are found to grow on the apices of ZnSe NRs nonepitaxially to form an interface with no preference of orientation between Au(111) and ZnSe(001). Density functional theory calculations reveal that the Au tips on ZnSe hybrid NRs gain enhanced adsorption of H compared to pristine Au, which favors the hydrogen evolution reaction. Photocatalytic tests reveal that the Au tips on ZnSe NRs effectively enhance the photocatalytic performance in hydrogen generation, in which the single Au‐tipped ZnSe hybrid NRs show the highest photocatalytic hydrogen production rate of 437.8 µmol h^?1 g^?1 in comparison with a rate of 51.5 µmol h^?1 g^?1 for pristine ZnSe NRs. An apparent quantum efficiency of 1.3% for hydrogen evolution reaction for single Au‐tipped ZnSe hybrid NRs is obtained, showing the potential application of this type of cadmium (Cd)‐free metal–semiconductor hybrid nanoparticles (NPs) in solar hydrogen production. This work opens an avenue toward Cd‐free hybrid NP‐based photocatalysis for clean fuel production.