Microwave-Assisted Graphite-Support Synthesis of Imidazolones

5(4H)-Oxazolones react with ammonium acetate under microwave irradiation and using graphite as support in an eco-friendly process. The reaction was carried out under solvent-free conditions and the imidazolones were obtained quantitatively. Moreover the reaction time was reduced too.     

The transverse acoustic impedance of dilute solutions of3He in superfluid4He

The transverse acoustic impedanceZ=R–iX of dilute solutions of³He in superfluid⁴He has been measured at a frequency (/2) of 20.5 MHz at temperaturesT from 30 mK to the transition at T. The³He concentrations studied werec=0.014, 0.031, 0.053, 0.060, and 0.092 below 1 K, thoughc decreased slightly near the point. The impedance was found from the temperature dependence of the quality factor and the resonant frequency of anAT-cut quartz crystal resonator immersed in the liquid. Below 1 K,Z is due to the Fermi gas of³He quasiparticles, and in the collisionless limit, 1 ( is a relaxation time),R remains constant whileX goes to zero. Measurements ofR(c, T) andX(c, T) were analyzed to determine the momentum accommodation coefficient (c, T) and (c, T). The relaxation times were in good agreement with previous work, while (c, T) was independent ofc, but increased from 0.29±0.03 below 0.1 K to 1.0±0.1 above 0.8 K. Various mechanisms are suggested to explain this. Between 1.0 and 1.5 K the³He quasiparticles and the thermally excited rotons are in the hydrodynamic region, 1. Values of the total viscosity (c, T) were obtained and analyzed to give the³He gas viscosity and the³He-³He and roton-³He scattering rates, both of which were energy-dependent. The superfluid healing length a was also measured. Near the point we founda=(0.1±0.03)^–2/3 nm, where =1–T/T, proportional to the phase coherence length . Our data are consistent with the hypothesis that _s/T is a universal constant for superfluid dilute solutions, where _s is the superfluid density. Between 1.0 and 1.8 K we found thata(c, T) was comparable to measurements in³He-⁴He films.     

The transverse acoustic impedance of He II

The complex shear acoustic impedance of liquid He II has been measured at frequenciesf(=/2) of 20.5, 34.1, and 47.8 MHz from 30 mK to the -point T (2.176 K). The impedanceZ was found from the temperature dependence of the quality factor and the resonant frequency of a thickness shear mode quartz crystal resonator immersed in the liquid. The relationship for a hydrodynamic viscous liquidZ(T)=(1–i)(f _n )^1/2 was used to measure the temperature dependence of the viscosity (T) using tabulated values of the normal fluid density _n (T). Deviations from hydrodynamic behavior occurred when the viscous penetration depth was less than the superfluid healing length, the phonon mean free path, and the roton mean free path. Near the -point,Z(T)/Z(T) was frequency dependent and a value for the superfluid healing lengtha=(0.10±0.01)^–2/3 nm was found, where =(T–T)/T. The effects of van der Waals forces near the crystal surface were also observed and a layer model was used to interpret the measurements. Below 1.8 K only rotons contribute significantly toZ and we determined the roton relaxation time as _r =8.5×10^–14 T ^–1/3 exp (8.65/T) sec. Below 1.2 K, _r >1 and we investigated the breakdown of hydrodynamics in this region. ForT<0.6 K the resonant frequency of the crystals decreased by f/f=2×10^–7, but the origin of this effect is not yet known.Financial support provided by the SERC, Bedford College, and the Central Research Fund, University of London.     

Alumina coated with oxazolone derivative for extraction of trace amounts of cadmium and copper from water and plant samples

A solid phase extraction procedure is proposed for simultaneous separation and preconcentration trace amounts of Cu(II) and Cd(II) using alumina coated with N'-{4-[4-{1-[4-(dimethylamino)phenyl]methylidene}-5-(4-H)oxazolone]phenyle}acetamide and determination by flame atomic absorption spectrometry. Using 0.1g of the sorbent, the metal ions were sorbed at pH 7 and recovered with 5.0 mL of 0.5 mol L(-1) HNO(3). It was found that extraction can be performed from the sample volumes of 2000 and 800 mL for Cu and Cd, respectively (preconcentration factors of 400 for Cu and 160 for Cd). Obtained sorption capacities for 1g sorbent were 8 mg Cu and 14 mg Cd. The linearity was maintained in the concentration range of 0.1 ng mL(-1) to 7.0 μg mL(-1) for Cu and 0.13 ng mL(-1) to 2.0 μg mL(-1) for Cd in the original solution. Eight replicate determinations of a mixture containing 1.0 μg mL(-1) each of the elements in the final solution gave relative standard deviation ±1.6 and ±1.3% for Cu and Cd, respectively. The detection limit was calculated as 0.06 and 0.05 ng mL(-1) for Cu and Cd, respectively. The proposed method was successfully applied to the determination trace amounts of Cu and Cd in the water and plant samples.     

The transverse acoustic impedance of He II under pressure

The transverse acoustic impedanceZ of bulk liquid He II has been measured at a frequency of 20.5 MHz at temperatures from 30 mK to 2.2 K, at pressures up to 25 bar. Results at SVP have been published previously. The impedance was found from the temperature dependence of the quality factor and resonant frequency of an AT-cut quartz crystal resonator immersed in the liquid. Below 0.5 K the crystal also acts as a microbalance and a change in frequency associated with a loss peak at 10 bar is interpreted as the localization of a second layer of⁴He atoms. At higher pressures no further growth of solid⁴He on the crystal surfaces was observed. Between 0.6 and 1.6 K we interpretZ as being primarily due to rotons and we have determined the roton relaxation time at all pressures as =(8.5±0.4)×10^–14 T ^–1/3 exp (/kT) sec, using the theory by Roberts and Donnelly, where is the roton energy gap. The transition from hydrodynamic viscous behavior to the nonhydrodynamic regime was investigated in detail. The data were consistent with completely diffuse scattering of the rotons at the gold-plated electrodes on the crystal. Measurements near the -point, forT/T >0.99, were analyzed to determine the superfluid healing length asa(T)=(0.10±0.02)^–2/3 nm, where =1–T/T , at all pressures. An analysis of data for 0.1 gave a slightly larger value, equivalent toa(T)=(0.13±0.02)^–2/3 nm. Below 1.6 K the excess normal fluid density associated with the healing layer is similar to that found in thin films.     

Epitaxial Si Sensors at Low Temperatures: Non-Linear Effects

Cryogenic bolometric sensors made from epitaxially grown Si:As have been tested down to 40 mK. The sensors were grown by chemical vapour deposition with a doped layer 8.4 μm thick. The dopant concentration was measured using SIMS and was constant, ±1%, with an excellent box profile. Arsenic concentrations up to 7.5×10¹⁸ cm⁻³ were achieved. Above 100 mK the low power resistanceR(T) followed the variable range hopping law, or Efros-Shklovskii law for a Coulomb gap,R(T)=R ₀ exp(T ₀/T)^1/2 withT ₀∼25 K, typically. A double sensor arrangement was used to measure the electronphonon coupling in the sensors and the phonon coupling to the heat sink. As the dc current bias through a sensor was increased, spontaneous voltage oscillations were observed across the sensor below 100 mK, which limited the sensitivity of the sensors in this region. These are circuit-limited oscillations between high and low resistance states. A phase diagram was established for the spatio-temporal coexistence of the two states, with a critical temperatureT _c=115 mK. We show that this is an intrinsic phase transition within a thermal model of the electron-phonon coupling. For a resistance-temperature characteristic given by the Efros-Shklovskii law we findT _c=0.00512T ₀, independent ofR ₀ and the coupling strength. This predictsT _c=115±4 mK in this case. The model gives excellent agreement for the critical voltage and current, by assuming that the breakdown occurred via the formation of a filamentary region of high current density and high electron temperature. At higher currents, the response was temperature independent and given byI(E)=I(0) exp{−(E ₀/E)^1/2} whereE is the average applied electric field andE ₀∼380 V/cm, in agreement with a thermal model which includes the phonon-phonon coupling to the heat sink.     

The ac response of a 2-D electron gas on liquid helium in a magnetic field

The low frequency ac response of a 2-D electron gas on liquid helium in a magnetic field is analysed in terms of _xx and _xy , the components of the magnetoresistivity tensor. The electrons are screened by parallel electrodes and the system forms a 2-D transmission line. The 2-D wave equation is solved numerically for a bounded electron sheet in a rectangular geometry which is excited by one of the electrodes. For 1, where is a relaxation time, heavily damped voltage waves propagate along the transmission line. In a field these waves propagate along the edges of the electron sheet with characteristic decay lengths or 2-D skin depths, parallel and perpendicular to the edges, which depend on _xx and _xy . The effects of these skin depths on measurements of the magnetoresistance and ac Hall effect are demonstrated. The relationship to the dc Hall effect and to edge magnetoplasmons is shown. The effects of incomplete screening, density inhomogeneities and edge capacitance are also discussed.     

The transverse acoustic impedance of He I under pressure

The complex shear acoustic impedance of liquid He I has been measured at a frequencyf(=/2) of 20.5 MHz at temperatures from the transition to 4.2 K and at pressures from SVP to 25 bar. The impedanceZ = R – iX was found from the changes in the quality factor and the resonant frequency of a thickness shear mode quartz crystal resonator immersed in the liquid. The viscosity (T, P) of He I was obtained from the expressionZ = (1-i)(/2)^1/2 for the impedance of a liquid with density . The results were in reasonable agreement with the calculation of Ryoo, Jhon, and Eyring using a significant structure theory which incorporates zero-point motion. The acoustic reactanceX was increased by the enhanced density and visocisity of the first few atomic layers of⁴He near the crystal surface produced by van der Waals forces. Calculations show that this effect is primarily due to the second atomic layer.