AC Samples Research Articles

Kinetics and Mechanism of the Reversible Dissociation of Ammonium Carbamate: Involvement of Carbamic Acid

The kinetics of the reversible decomposition of ammonium carbamate, NH4CO2NH2 (AC), into NH3 (A) and CO2 (C) has been studied between 278 and 312 K by recording the time dependence of the total pressure above an AC sample that, after being previously evacuated, is exposed to the headspace of an isoteniscope. At early times (<1 s), the data show an inflection, which is enhanced in first derivative transformations. Three mechanisms are discussed, each containing six rate constants. Two assume the presence of gas phase carbamic acid (CA) as an intermediate. These mechanisms are explored further through ab initio computational studies of CA. These results indicate that gas phase CA is a very stable species. No evidence could be found for its presence from Fourier transform IR spectra of the equilibrium vapor above AC. The mechanism that we suggest involves the reversible decomposition of AC into surface-bound CA, A, and C molecules, with the latter two reversibly desorbing from the surface. The experimental k...

PC Based Simple ac Hall Measurement System

This paper describes the development of PC based ac-ac and dc-ac Hall measurement setup using the same circuit. In ac-ac system, both the magnetic field and the sample current are sinusoidal functions. Whereas in dc-ac system, dc magnetic field and ac sample current are used. This Hall measurement system is fully computer controlled and the user's interference is limited to only turning on and off the magnetic field and its polarity. The hardware and software enable the user to carry out functions such as magnet calibration, Van der Pauw, ac-ac and dc-ac Hall measurements.

Surface microstructure of CIS thin films produced by rapid thermal processing

The surfaces of polycrystalline CuInSe2 thin films produced by rapid thermal processing (RTP) have been analyzed by scanning tunnelling microscopy and spectroscopy in ambient air. Deviating from standard measurement techniques the tunnelling microscope is driven by an AC sample voltage for surface morphology mapping in the constant current mode. Additionally, a Fermi energy mapping of the semiconductor surface is performed by mapping significant features of the I–V tunnelling characteristic. The polarity of the tunnelling current proves to be a reliable measure of the conductivity type of the material (n- or p-type); the observation of leakage currents at small bias voltages allows the identification of gap states around the Fermi level or metallic phases. Current-voltage curves taken at positions of different conduction type verigy the spectroscopic information in the maps. Typical areas imaged are (1.5 μm)2. Intra- and inter-granular nonuniformities of the conduction type are observed. Although the bulk material of all samples investigated is p-conductive, abrupt changes of the conductivity type of the surfaces from p- to n-type are observed as a function of the overall copper-to-indium ratio. The dominant current flow direction in slightly Cu-rich thin film bulk material is associated with p-type conduction, whereas In-rich samples exhibit largely n-type conductivity at the surface. Surfaces of copper-rich bulk materials show Fermi level pinning. The spectroscopic results do not depend on material and geometry of the tunnelling tip.

Propane Dehydrogenation on Chromia/Silica and Chromia/Alumina Catalysts

The catalytic activity of CrOx/SiO2 (SC, 0.08 to 0.99% chromium by weight), CrOx/Al2O3 (AC, 0.16 to 1.55%), and α-Cr2O3 for the dehydrogenation of propane has been investigated at 723 to 823 K in a flow apparatus. Before catalytic experiments, SC and AC samples were activated with oxygen at 773 K (s.o. treatment) followed or not followed by reduction with CO or H2 at 623 K. Occasionally, SC samples were dehydrated under vacuum at 1023 K before the s.o. treatment. Assuming all chromium to be in the +6 state after s.o., the average oxidation number of chromium (n) is n ≍ 2 in SC samples reduced with CO and n ≍ 3 with H2. In AC catalysts, n values progressively increase in consecutive reduction experiments, from n = 2.2 to n = 4.2. All catalysts are highly selective to propene and undergo a deactivation process during time on stream. The s.o. treatment completely restores the activity. The reduction mode does not influence the catalytic activity. Surface species formed upon adsorption at room temperature (RT) of CO or NO were studied by IR spectroscopy on CrOx/SiO2 submitted to treatments analogous to those used before catalytic experiments. In SC catalysts reduced with CO, the formation of carbonyls and nitrosyls of three different CrII species was confirmed. Interestingly, all these species were absent when the sample was (i) reduced with H2 at 773 K, or (ii) heated with H2O after reduction with CO, or (iii) exposed to propane after reduction with either CO or H2. After treatments (i) through (iii), a carbonyl and a dinitrosyl of CrIII were the only species formed on CO or NO adsorption at RT. Relying on (i) the dependence of the activity on chromium content, (ii) IR results. and (iii) ESR data, it is suggested that mononuclear CrIII with two coordinative vacancies is the active site for the dehydrogenation of propane. The catalytic activity of CrOx/SiO2, CrOx/Al2O3, and α-Cr2O3 is compared with that of CrOx/ZrO2, measured previously. Possible reasons for the substantially higher activity of CrOx/ZrO2 are discussed.

Decorin and a large dermatan sulfate proteoglycan in bovine striated muscle

Proteoglycans were extracted and isolated from adult bovine muscle tissue by dissociative extraction followed by density gradient centrifugation, gel chromatography and ion-exchange chromatography. Two proteoglycans were characterized; one of large molecular size (PG-L) and one of small molecular size (PG-S). The recovery of PG-L and PG-S was 33% and 67% respectively. By cellulose acetate electrophoresis before and after treatment with chondroitinase AC and ABC both samples were shown to carry predominantly dermatan sulfate chains. The large proteoglycan was recognized with an antibody against a large dermatan sulfate proteoglycan from bovine sclera, whereas the small was recognized by an antibody against decorin from bovine sclera. Chondroitinase ABC treatment of PG-S followed by SDS-PAGe showed a core protein with a molecular weight of 45 kDa, which also reacted with the decorin antibody. Amino-acid analysis of both PG-L and PG-S revealed an amino-acid composition closely similar, although not identical, to the large dermatan sulfate proteoglycan from bovine sclera and decorin respectively. Immunohistochemical analyses of muscle tissue sections showed that decorin and the large dermatan sulfate proteoglycan are present in the perimysium layers of muscle tissue, although with a somewhat different pattern of distribution. Decorin was, in addition, found in the endomysium.

Critical current models and magnetic shielding of high temperature superconductors

The applicability of critical current theory to macroscopic magnetic phenomena related to the magnetic shielding ability of high temperature superconductors is discussed and some misuse of the Bean model is demonstrated. Critical current theory is used to analyze the relationships among magnetic shielding factors, breakthrough fields, the waveforms of penetrated fields, as well as their dependence on AC and DC bias fields and sample thickness. Good agreement between the waveforms of penetrated fields observed with an oscilloscope and those predicted theoretically was obtained. The axial gradient of the magnetic field in an axial direction inside a superconductor under a normal magnetic field has been analyzed both qualitatively and quantitatively. Finally, a novel experimental method to measure the distribution of the axial component of a magnetic field inside a high temperature superconductor is presented.

On the Determination of Intrinsic Properties of Ceramic High-Tc Superconductors by Ac Susceptibility Measurements

ABSTRACTVarious models have been proposed to interpret ac susceptibility curves measured on sintered high-To superconductors (HTSC) and to deduce parameters like critical current densities and critical fields. To get more clearness, systematic studies of position and height of the intergranular maximum in χ'' have been performed on several ceramic HTSC samples in dependence of both ac field amplitude and sample size. In the present paper, the data are used to discuss the various models proposed in the literature. It is shown that modifications of the critical state model using incorrect jc(Hi) functions as well as other interpretations of the χ'' peak yield not actually intrinsic but sample size and geometry dependent parameters. The dependence of both position and height of the intergranular maximum in χ '' on ac field amplitude and sample size can be explained in the framework of a modified critical state model using a power law for the jc(Hi) dependence.

Yellow-Poplar Site Quality Related to Slope Type in Mountainous Terrain

Abstract Site index and topographic variables were measured on 112, ¼-ac sample plots with yellow-poplar overstory in the Blue Ridge Province of the Southern Appalachian Mountains. The plots were grouped by slope type—nose, side, or head—and multiple regression analysis was used to develop equations for predicting height at 50 years. Estimated site index was significantly less on side slopes than on head slopes. Stand age was the most important variable in both equations, followed by aspect and land surface shape. The equations accounted for 79% of the variation of yellow-poplar height on head slopes and 84% on side slopes. North. J. Appl. For. 4:189-192, Dec. 1987.

A new direct method for the precision measurement of the temperature dependence of impurity resistivity and deviations from Matthiessen's rule

An AC circuit and sample apparatus have been devised which enable the temperature dependence of impurity resistivity of metals and deviations from Matthiessen's rule to be measured simply and directly, the latter with an accuracy hitherto unobtainable. By means of two transformers the difference of two resistivities is essentially obtained, and by interchanging the positions of the two samples in a specially designed sample holder, uncertain apparatus constants are eliminated from the expression for Delta rho / rho 0. The circuit consists essentially of an amplifier and phase-sensitive detector applied to the out-of-balance signal between an AC potentiometer and the potential to be measured in such a way that the entire measurement becomes independent of current. An example of the temperature dependence of the deviation from Matthiessen's rule in Cu+0.1 at.% Au is given.