Small Active Volume Research Articles

Small-scale convection under the back-arc occurring in the low viscosity wedge

Water released from subducting slabs through a dehydration reaction may lower the viscosity of the mantle significantly. Thus, we may expect a low viscosity wedge (LVW) above the subducting slabs. The LVW coupled with a large-scale flow induced by the subducting slabs may allow the existence of roll-like small-scale convection whose axis is normal to the strike of the plate boundary. Such a roll structure may explain the origin of along-arc variations of mantle temperature proposed recently in northeast Japan. We study this possibility using both 2D and 3D models with/without pressure- and temperature-dependent viscosity. 2D models without pressure and temperature dependence of viscosity show that, with a reasonable geometry of the LVW and subduction speed, small-scale convection is likely to occur when the viscosity of the LVW is less than 1019 Pa s. Corresponding 3D model studies reveal that the wavelength of rolls depends on the depth of the LVW. The inclusion of temperature-dependent viscosity requires the existence of further low viscosity in the LVW, since temperature dependence suppresses the instability of the cold thermal boundary layer. Pressure (i.e. depth) dependence coupled with temperature dependence of the viscosity promotes short wavelength instabilities. The model, which shows a relatively moderate viscosity decrease in the LVW (most of the LVW viscosity is 1018∼1019 Pa s) and a wavelength of roll ∼80 km, has a rather small activation energy and volume (∼130 kJ/mol and ∼4 cm3/mol) of the viscosity. This small activation energy and volume may be possible, if we regard them as an effective viscosity of non-linear rheology.

Measurements of Gamma-Knife helmet output factors using a radiophotoluminescent glass rod dosimeter and a diode detector.

A radiophotoluminescent (RPL) glass rod dosimeter (GRD) and a small active volume p-type silicon diode detector are used for the measurement of the output factors from Gamma-Knife fields. The GRD system consists of small rod-shaped glass chip detectors and an automatic readout device. The output factors measured with the GRD from the 14, 8 and 4 mm helmets relative to the 18 mm helmet are 0.981, 0.942 and 0.877, respectively. Similarly, the corresponding output factors measured with the p-type silicon diode detector are 0.980, 0.949 and 0.867, respectively. The output factors are corrected for the end effect for each helmet. The output factors obtained from both detectors are in good agreement with the values in a recent publication and the values recommended by Elekta, the manufacturer. The directional dependence of these detectors is also measured. For the Gamma-Knife angle ranging from 6 to 36 degrees in the y-z plane of the stereotactic space, the measured angular dependence of the GRD is approximately 1.0% at a 4 MV x-ray beam. The response of the silicon diode detector indicates approximately 3-4% directional dependence for the same angular range for a 6 MV x-ray beam. The Gamma-Knife helmet output factors measured with the silicon diode detector are corrected for angular dependence.

Pressure Effect on Molecular and Lattice Dynamics in Pyridinium Nitrate

Measurements of spin-lattice relaxation time T 1 , second moment M 2 and neutron scattering have been performed for a polycrystalline sample of pyridinium nitrate as a function of hydrostatic pressure and temperature. The structure of this compound has also been analysed by the HF/6-31 G method. The results of the measurements and calculations have confirmed that reorientation of the pyridinium cation takes place between the potential minima of different values, and the asymmetry parameter decreases with increasing temperature. The temperature dependence of the asymmetry parameter is modified by the pressure applied. The presence of the hydrogen bond in this compound implies a very small activation volume and is responsible that the crystal does not undergo a phase transition to the disordered phase.

Structural and magnetic properties of (001) oriented FePt/Ag composite film

(FePt/Ag)n multilayers were deposited on MgO (100) single crystal with laser ablation and were found to have the same crystallographic orientation as the substrate when FePt forms the bottom layer. Through annealing, composite films consisting of FePt L10 and Ag phases were formed and a large perpendicular magnetic anisotropy constant Ku of the order of 107erg cc−1 was developed in the FePt grains. A thick Ag layer is found to be favourable for decreasing the dispersion of the easy-axis for magnetization. The measurement of time decay of magnetization gave rise to a small activation volume of the order of 10−19 cm3.

Analysis of the influence of germanium dead layer on detector calibration simulation for environmental radioactive samples using the Monte Carlo method

Germanium crystals have a dead layer that causes a decrease in efficiency, since the layer is not useful for detection, but strongly attenuates photons. The thickness of this inactive layer is not well known due to the existence of a transition zone where photons are increasingly absorbed. Therefore, using data provided by manufacturers in the detector simulation model, some strong discrepancies appear between calculated and measured efficiencies. The Monte Carlo method is applied to simulate the calibration of a HP Ge detector in order to determine the total inactive germanium layer thickness and the active volume that are needed in order to obtain the minimum discrepancy between estimated and experimental efficiency. Calculations and measurements were performed for all of the radionuclides included in a standard calibration gamma cocktail solution. A Marinelli beaker was considered for this analysis, as it is one of the most commonly used sample container for environmental radioactivity measurements. Results indicated that a good agreement between calculated and measured efficiencies is obtained using a value for the inactive germanium layer thickness equal to approximately twice the value provided by the detector manufacturer. For all energy peaks included in the calibration, the best agreement with experimental efficiency was found using a combination of a small thickness of the inactive germanium layer and a small detection active volume.

Motor cortex activation is preserved in patients with chronic hemiplegic stroke.

Many central nervous system conditions that cause weakness, including many strokes, injure corticospinal tract but leave motor cortex intact. Little is known about the functional properties of surviving cortical regions in this setting, in part because many studies have used probes reliant on the corticospinal tract. We hypothesized that many features of motor cortex function would be preserved when assessed independent of the stroke-affected corticospinal tract. Functional MRI was used to study 11 patients with chronic hemiplegia after unilateral stroke that spared regions of motor cortex. Activation in stroke-affected hemisphere was evaluated using 3 probes independent of affected corticospinal tract: passive finger movement, a hand-related visuomotor stimulus, and tapping by the nonstroke index finger. The site and magnitude of cortical activation were similar when comparing the stroke hemisphere to findings in 19 control subjects. Patients activated each of 8 cortical regions with similar frequency as compared to controls, generally with a smaller activation volume. In some cases, clinical measures correlated with the size or the site of stroke hemisphere activation. The results suggest that, despite stroke producing contralateral hemiplegia, surviving regions of motor cortex actively participate in the same proprioceptive, visuomotor, and bilateral movement control processes seen in control subjects.

FePt–Ag nanocomposite film with perpendicular magnetic anisotropy

Magnetic properties of FePt–Ag nanocomposite films synthesized by annealing (FePt/Ag)n multilayers deposited on a MgO(100) substrate by laser ablation are discussed in conjunction with the structure. After annealing over a temperature range from 540 to 650 °C, a large perpendicular magnetic anisotropy constant Ku of the order of 107 erg/cc developed. The so-called δM curve measurements imply that grains are magnetically decoupled. Increasing the Ag layer thickness is found to be useful for the improvement of the (001) orientation of FePt grains. The remanence ratio and coercivity increase with a rise in the annealing temperature. Magnetic grains of less than 10 nm and a small activation volume of the order of 10−19 cm3 are observed.

Investigation of body-tie effects on ion beam induced charge collection in silicon-on-insulator FETs using the Sandia nuclear microprobe

Silicon-on-insulator (SOI) technology exhibits three main advantages over bulk silicon technology for use in radiation environments. (1) SOI devices are immune to latchup, (2) the volume of the sensitive region (body) and hence total charge collection per transient irradiation is much reduced in SOI devices and (3) the insulating layer blocks charge collection from the substrate (i.e., no funneling effect). This effectively raises the single event upset (SEU) threshold for the SOI device. However, despite their small active volume SOI devices are still vulnerable to single event effects (SEE). Inherent in the SOI transistor design is a parasitic npn bipolar junction transistor (BJT), where the source-body-drain acts as an emitter-base-collector BJT. An ion strike to a floating (not referenced to a specific potential) body creates a condition where the excess minority carriers in the drain-body cause the parasitic BJT to turn on and inject more charge into the drain than was deposited in the device by the ion. In extreme cases the floating body effect (FBE) can trigger a high-current state called single-event snapback (SES) where channel conduction is sustained indefinitely through regenerative electron-impact ionization near the drain junction. Tying the body to the source limits the emitter-base current and reduces the sensitivity of the device to single ion strikes. Unfortunately, the body-tie loses effectiveness with distance due to resistivity, and in regions far enough from the tie the BJT is still in effect. Using the Sandia nuclear microprobe we have created charge collection maps on Sandia CMOS6rs SOI FETs of varying channel widths. These devices have body ties at both ends of the channel region. Results clearly demonstrate that distance of the ion strike from the body tie has an inverse effect upon charge collection and SES sensitivity due to the resistivity of the channel. Experimental results compare well with DAVINCI simulations and electrically induced snapback thresholds. In addition, an interesting saturation effect of SES versus the amount of injected charge is observed.

Observation of unequal activation volumes of wall-motion and nucleation processes in Co/Pd multilayers.

We report the difference in activation volumes of wall-motion and nucleation processes in Co/Pd multilayers. Each activation volume was estimated from field dependence of wall-motion speed and nucleation rate, obtained by real-time domain imaging using a magneto-optical Kerr effect microscope. Delicate analysis shows that the two activation volumes are generally unequal and the ratio between the volumes varies noticeably from 0.9 to 1.1 with change of the multilayered structure. We found that the inequality in the two activation volumes has a crucial influence on magnetization reversal behavior: a process having a smaller activation volume dominates.

A theory of work softening

Observed features of the yield drop in work softening support the view that the structure in a slip band is heterogeneous in a metal such as aluminium or copper. The centre of the band is almost empty of dislocations. These are concentrated round its boundary, particularly as walls of edge dipoles at its ends. It is argued that work hardening is due to the prevention of the passage of these dislocations by a forest of obstacles with small activation volumes and energies. Thermal energy enables them to cut through their obstacles and so to penetrate further into their walls, thereby reducing their back stress and enabling the Frank-Read sources, within the bands, to become active at lower applied stresses. The yield drop is explained as due to an overshooting, resulting from this thermally activated cutting, of the stress to operate the sources.

Deformation mechanisms in the O phase

The intermetallic Ti2AlNb has a ternary ordered, orthorhombic structure (which is a slightly distorted form of the hexagonal D019, Ti3Al phase) and is the major constituent of the ‘orthorhombic’ alloys of the Ti–Al–Nb system. We explore in this paper the mechanical behaviour of this intermetallic over a wide range of strain rates and temperatures. Thermal activation analysis of the flow behaviour indicates that at low temperatures the flow behaviour is controlled by a strong thermally activated process with small activation volumes typical of a Peierls-type barrier. Dynamic recovery occurs at higher temperatures, but a specific rate-controlling mechanism for flow remains to be identified. The creep of the intermetallic has also been examined in a temperature–strain-rate regime well removed from the DSA regime. Stress exponents decrease slowly from 7 to 5 with increasing temperature and the dislocation structure consists of a three dimensional network linked by attractive junctions. A network model of creep, as proposed for example by McLean, may be appropriate to describe the creep of this intermetallic. The effect of Nb content and a variety of quaternary additions on the mechanical behaviour of the intermetallic has also been evaluated. Nb is shown to strengthen the intermetallic through the thermal component of the flow stress, while Si is found to have the strongest solid solution strengthening effect at low temperatures. Zr and oxygen additions enhance the dynamic strain ageing effect at intermediate temperatures. The Nb content does not affect the creep strength of the intermetallic.

Relaxation time and thermal stability in magneto-optical recording media

The time dependence of the magnetization process in amorphous TbFeCo thin films and (TbFeCo/Pt) multilayers is studied. By assuming a Gaussian distribution of relaxation times for magnetic aftereffect, the non-Arrhenius behavior of the magnetization decay is found in almost all samples. From a fitting of experimental data one can determine the mean value of relaxation time /spl tau//sub 0/ and also its width /spl Delta//spl tau//sub 0/. The multilayers of TbFeCo/Pt show a high stability compared to single layers of TbFeCo with also a small activation volume.

Dependence on pressure of conduction by hopping of small polarons in minerals of the Earth's lower mantle

Electrical conductivity of the lower mantle-like assemblage (Mg,Fe)SiO3 perovskite-(Mg,Fe)O magnesiowustite is usually analyzed using the quasi-chemical Arrhenian approach of diffusion. The conductivity of this assemblage has often been attributed to hopping of small polarons, because of the low value of the activation energy and the small negative activation volume. However, the solid-state physics approach can provide more arguments, for or against conduction by polarons. We have tried to bridge the gap between the two approaches and identify the physical quantities entering the phenomenological activation parameters. In particular, we have investigated the pressure dependence of the activation energy, and the physical meaning of the activation volume. Hopping is controlled by the binding energy of the polaron and by the value of the exchange integral, which increases with pressure causing the observed decrease of the activation energy. From the physical theory and the results of experiments at pressures up to 40 GPa and temperatures up to 400 ∘C, we have estimated the values of parameters characteristic of polarons: radius, mobility, time between jumps and adiabaticity. These values are compatible with conduction by small adiabatic polarons. The consequences for extrapolations to lower mantle conditions of the presence of a temperature dependent preexponential term in the expression for conductivity have been examined. It was found that the extrapolations are not significantly different from those using the Arrhenius equation.

Reaction between [Ru(H2O)6]2+ and DMF: A Complete Volume Profile of a Monocomplex Formation Reaction on a d6 Low Spin Aquaion

The reaction between [Ru(H 2 O) 6 ] 2+ and dimethylformamide (DMF) was studied as a function of the temperature and the pressure at pH 1. When a slight excess of Ru aquaion is used, [Ru(H 2 O) 5 DMF] 2+ is the only product of the reaction observed in the NMR spectra and the DMF is bound through the oxygen atom. From variable temperature experiments, the rate constants, activation enthalpies and entropies for the formation and the dissociation of the monocomplex were determined. These are as follows: 10 4 x k f 298 =(8.2 ± 0.4) kg mol -1 s -1 ΔΗ ☨ f = 83.6 ± 3 k J mol -1 Δ S ☨ f = -23.7 ± 10 J K -1 mol -1 for the formation reaction, 10 3 x k d 298 = (1.10 ± 0.04) s -1 ΔΗ ☨ f = 90.2 ± 2 kJ mol -1 ΔS ☨ f , = +1.0 ± 7 J Κ -1 mol -1 for the dissociation reaction. The thermodynamic parameters (equilibrium constant at 298.15 K, reaction enthalpy and entropy) were also determined:K 298 eq = 0.74 ± 0.02 kg mol -1 , ΔH 0 = -6.6 ± 2 k j mol -1 , ΔS 0 =-24.7 ± 6 JK -1 . The k f value is similar to the previously reported rate constants for monocomplex formation reactions with other ligands and confirms that the entering ligand has no effect on the rate. From variable pressure experiments, the activation volumes for the formation and the dissociation reactions and the reaction volume were determined: ΔV f ☨ = +2.7 ± 0.6 cm3 mol -1 , ΔV d ☨ = 0.9 ± 0.7 cm 3 mol -1 , ΔV° = +3.6 ± 0 . 9 cm3 mol -1 respectively. These results show that the volume of the transition state is intermediate between the volume of the reactants and the volume of the products. It demonstrates that the mechanism is the same as for the water exchange on [Ru(H 2 O) 6 ] 2+ , i.e. an Id mechanism. Contraction of the bonds between Ru and the spectator ligands at the transition state may explain the abnormally small activation volumes. The need of theoretical calculations on these systems is emphasized.

Mechanistic Insight from Energy and Volume Profiles for CO Binding to a Lacunar Iron(II) Cyclidene Complex

The binding of CO to a lacunar Fe(II) cyclidene complex is characterized by very negative values for the activation entropy and activation volume, whereas the reverse dissociation reaction is accompanied by small positive activation entropy and activation volume data. The constructed volume profile suggests that, in the transition state, CO disappears completely into the ligand pocket during partial Fe−CO bond formation, followed by a high-spin to low-spin transition on Fe(II) during which the metal center moves into the ligand plane and accounts for the subsequent volume decrease.

Pressure dependence of self- and solute diffusion in bcc zirconium

The pressure effect of self-diffusion in the high-temperature bcc phase of zirconium has been studied with high accuracy using the radiotracer technique. Activation volumes of $0.184\ifmmode\pm\else\textpm\fi{}0.016\ensuremath{\Omega}$ (\ensuremath{\Omega}: atomic volume) at 1423 K and $0.213\ifmmode\pm\else\textpm\fi{}0.014\ensuremath{\Omega}$ at 1273 K were obtained. Simultaneously, the activation volumes of the ${}^{95}\mathrm{Nb}$ solute diffusion were measured which amount to $0.161\ifmmode\pm\else\textpm\fi{}0.014\ensuremath{\Omega}$ at 1423 K and $0.193\ifmmode\pm\else\textpm\fi{}0.023\ensuremath{\Omega}$ at 1273 K. The small activation volumes indicate a strong relaxation of the vacancy. The large relaxation volume reflects the inherent weakness of the bcc lattice towards a shear in $〈111〉$ direction. The results provide an explanation for the absence of positron trapping in group-IV transition metals.

Mantle convection and stability of depleted and undepleted continental lithosphere

We address the question of how convective processes control the thicknesses of oceanic and continental lithospheres. The numerical convection model involves a Newtonian rheology which depends on temperature and pressure. A repeated plate tectonic cycle is modeled by imposing a time‐dependent surface velocity. One part of the surface, representing a continent, never subducts. The asymptotic equilibrium thickness of the lithosphere varies with the viscosity at the base of the lithosphere, but is not directly sensitive to the pressure dependence of the viscosity law and to the plate velocity. For small activation volumes, and average upper mantle viscosities deduced from postglacial rebound, the equilibrium plate thickness is more than 400 km (regimes 1 and 2). The equilibrium thickness of the oceanic lithosphere (around 100 km) implies that the viscosity in the asthenosphere is less than 7×1018 Pa s. Only models with strongly pressure‐dependent viscosity laws (activation volumes greater than 9×10−6 m3/mol) are able to reconcile this value with the average upper mantle viscosity (5×1020 Pa s). For these models, there are two lithospheric thicknesses such that the heat supplied by convection at the base of the lithosphere equals the surface conductive heat flow (regime 3). They could be that of an aged oceanic lithosphere and that of a shield lithospheric root. They indeed appear as points of prefered thickness in our numerical models. However, convection triggered by the lateral density jumps at the boundaries between the root and the thinner lithosphere slowly destabilizes the thick lithosphere. A plausible degree of chemical buoyancy in a depleted lithospheric root does not prevent convective erosion. In our simulations, long‐term stability of a cratonic lithospheric root is best achieved when its material is both buoyant and more viscous than the surrounding mantle. Extensive devolatilization of the refractory rocks forming the root is invoked to explain this viscosity increase.

Simultaneous emission of the HF and N/sub 2/ lines from a plasma cathode TEA laser

Simultaneous laser action from HF and N/sub 2/ is obtained, from a plasma cathode TEA laser, for the first time. The sliding discharge along the surface of a dielectric is used as a plasma cathode, for the main volumetric discharge. The laser operates at atmospheric pressure, with a gas mixture of SF/sub 6/:C/sub 3/H/sub 8/:N/sub 2/:He. For a typical flow rate ratio of 0.27:0.024:0.2:19.8 1 min/sup -1/, it produces simultaneously 160 mJ HF and 0.6 mJ N/sub 2/ laser outputs at 0.43% and 1.4/spl times/10/sup -3/% efficiencies respectively, at the moderate charging voltage of 28.5 kV. These output characteristics are obtained from a small active discharge volume and length of 10/sup 6/ cm/sup 3/ and 38 cm respectively. These values extend the performance, recently reported in the literature, of a sliding discharge HF/N/sub 2/ laser with corresponding simultaneous energy outputs of 12 mJ HF and 1.1 mJ N/sub 2/, to a higher energy output level, thus making the device suitable for a broader range of applications. This novel dual wavelength HF/N/sub 2/ laser system presented, can be particularly convenient for medical experiments, where the IR beam can be used for tissue ablation, while the UV beam can be used as the excitation source for fluorescence spectroscopic measurements, for the evaluation of the ablation process. Details are presented on the dependence of the laser performance parameters, such as output energy, discharge voltage and current and structure of the laser output pulses on the mixture composition and the circuit parameters.

Carbon film based ring‐disk and split‐disk dual electrodes as detectors for microbore liquid chromatography

AbstractThe electrochemical properties of carbon film based ring‐disk and split‐disk electrodes in small active volume (0.24–2μL) radial flow cells were studied in order to employ them in conjunction with microbore liquid chromatography (LC) for determining catecholamines. The high conversion of dopamine (DA) at the disk of a ring‐disk electrode makes the DA limiting current lower than the theoretical value at a low flow rate. The collection efficiency (CE) increases with decreasing flow rate and cell thickness, which is different from previously reported results. A high CE of 0.96 was achieved because of the narrow gap of 5 μm.On the other hand, a higher limiting current was observed with the split‐disk than with a commercially available dual‐disk electrode because of its efficient geometry for analyte collection. The cross talk of the DA after reacting at one of the electrode is only 2%. The ring‐disk and split‐disk electrodes were suitable as series and parallel electrochemical detectors for small bore LC. A detection limit of 103 fg was obtained for DA at the disk of the ring‐disk because of the high signal in the thin‐layer radial flow cell and the low noise level of the carbon film. In contrast, very slight interference was observed from neighboring electrodes when determining catecholamines at the split‐disk.

Thermal activation and switching in c-axis aligned barium ferrite thin film media

The thermal activation volume and the magnetic switching volume were measured in c-axis uniaxially in-plane oriented barium hexaferrite thin film magnetic recording media. Excellent agreement among the experimental data obtained independently from the magnetic viscosity measurements and the measurements of the time dependence of coercivity implies that the thermally driven process is the cause of both phenomena. The presence of incoherent switching during magnetization reversal is indicated by the in-plane angular dependence of coercivity and is believed to cause the much smaller activation volume and switching volume than the grain size as determined from a TEM micrograph.