Increase In Surface Conductivity Research Articles

Grafting of polypropylene fibers. II. Electrokinetic properties of grafted polypropylene fibers

Electrokinetic properties of methacrylic acid- and acrylonitrile-grafted polypropylene fibers measured in the presence of cationic dyes are reported. The zeta potential of polypropylene fibers decreases, and the surface charge density along with surface conductivity increases as the concentration of the dyes in the streaming solution increases. The zeta potential at pH 7 decreases as the amount of graft increases in case of both acrylonitrile- and methacrylic acid grafted fibers. Both surface charge density and surface conductivity increase with the increase in dye concentration for both acrylonitrile- and methacrylic acid-grafted fibers. The results are explained on the basis of the cationic dye adsorption on the grafted fiber in the case of methacrylic acid graft. In the case of acrylonitrile-grafted fibers, this could be due to the strong attraction of cationic dye to the nitrile group of the grafted fibers.

Scanning tunneling microscopy on photoconductive semi-insulating GaAs

The increase in surface conductivity upon illumination of semi-insulating GaAs was used to enable surface-topography measurements with a scanning tunneling microscope (STM). Images were recorded in ultrahigh vacuum and in air using a HeNe laser and a halogen lamp for the generation of a photoconductive top layer. From the current-voltage characteristics of the tip-sample contact the maximum feedback-controlled tunnel current at a given voltage can be deduced. A calculation of the increase in carrier density and conductance upon illumination is given, which confirms the possibility of using STM on highly resistive photoconductive materials. Further implications and applications are discussed.

Kinetic studies of the decomposition of CH 3COOH and CH 3COOD on SnO 2 single crystals

The decomposition of CH 3COOH, CH 3COOD, and CH 4 on as grown SnO 2(101) faces, cleaned in UHV, was investigated by reactive scattering, thermal desorption spectroscopy (TDS), and four-probe surface conductivity measurements. After exposure to methane and acetic acid in TDS CO and CO 2 appear in a similar way. The spectra are different from the ones after exposures to CO. These findings are conceivable assuming a formate intermediate in equilibrium with CO and CO 2. This mechanism is known from the water gas shift reaction on ZnO. It implies a reaction of the methyl group of the acetic acid with lattice oxygen delivering surface hydrogen. On exposure to CH 3COOD also desorption of CH 3COOH, CH 4 and H 2O is observed. This exchange of H/D can only be caused by a decomposition of the methyl group. A complete reaction scheme is proposed explaining also a reversible and an irreversible increase of surface conductivity by hydrogen donors and oxygen vacancy donors, respectively.

Reduction of Surface Charge-Induced Electric Field Enhancement and Increase in AC Surface Flashover Voltage

Surfactant is painted partially on a cylindrical insulator having a back-side (Inner) electrode, to change the distribution of the accumulating surface charge (residual charge) generated by ac partial discharge. The surfactants used are Neogen T®, Catiogen L®, or Tween 20®; the insulating material is polyvinyl-chloride heat shrinkable tube. Ac flashover voltage in air (ACFOV) depends on the surfactant applied: ACFOV decreases in the order Neogen>Catiogen?non-painted? Tween; ACFOV of the Neogen-painted insulator is about 1.4 times higher than that of the Catiogen-painted one at an electrode distance of 140 mm, though the painted part of the two specimens has the same conductivity. The measured distribution of the accumulating charge differs between the Neogen-painted and the other specimens, though the maximum charge density on the surface does not differ much. These experimental results cannot be explained sufficiently by the decrease in the static electric field that is due to the increase in surface conductivity of the painted part. The calculation of the electric field enhancement due to the accumulating charge shows that the field near the electrode edge with the Neogen is lower than that for the other specimens. The higher ACFOV of the Neogen-painted insulator may be caused, not by increasing the surface conductivity, but by the decrease in the field enhancement on account of the difference in the distribution of the accumulating charge.

Decomposition of ethanol and acetaldehyde on clean ZnO prism and oxygen faces

The decomposition of ethanol and acetaldehyde on ZnO was investigated in UHV by thermal desorption spectroscopy (TDS) and measurement of surface conductivity. After adsorption of ethanol two different reactions are observed in TDS on both surfaces. One pair of reaction products consists of ethylene and water, the other pair consists of acetaldehyde and hydrogen. After heating above 400 K an increase of surface conductivity is observed. The reaction temperatures on the prism face are lower than on the oxygen face. Adsorption of acetaldehyde (10,000 L) and subsequent annealing up to 800 K does not change the surface conductivity on the oxygen face. In contrast, on the prism face the increase in surface conductivity can arise from a reaction of acetaldehyde with lattice oxygen to intermediate acetic acid. TDS from a sintered ZnO sample shows the desorption of methane and carbon dioxide, and of ketene and water. These decomposition products point again to intermediate acetic acid on the surface.

Decomposition of ethanol and acetaldehyde on clean ZnO prism and oxygen faces

Abstract The decomposition of ethanol and acetaldehyde on ZnO was investigated in UHV by thermal desorption spectroscopy (TDS) and measurement of surface conductivity. After adsorption of ethanol two different reactions are observed in TDS on both surfaces. One pair of reaction products consists of ethylene and water, the other pair consists of acetaldehyde and hydrogen. After heating above 400 K an increase of surface conductivity is observed. The reaction temperatures on the prism face are lower than on the oxygen face. Adsorption of acetaldehyde (10,000 L) and subsequent annealing up to 800 K does not change the surface conductivity on the oxygen face. In contrast, on the prism face the increase in surface conductivity can arise from a reaction of acetaldehyde with lattice oxygen to intermediate acetic acid. TDS from a sintered ZnO sample shows the desorption of methane and carbon dioxide, and of ketene and water. These decomposition products point again to intermediate acetic acid on the surface.

Variations of work function and surface conductivity on clean cleaved zinc oxide surfaces by annealing and by hydrogen adsorption

Abstract Crystals are cleaved in UHV normal and parallel to the c axis exposing polar and prism faces. Work function (Kelvin) and surface conductivity are measured, also after annealing at elevated temperatures in UHV and after adsorption of atomic hydrogen. The work function of all three faces decreases irreversibly by annealing in UHV, but only on the Zn face a measurable surface conductivity appears. Also hydrogen adsorption diminuishes the work function. Simultaneously an increasing surface conductivity can be observed on all three types of surfaces. Corresponding structures in the annealing curves demonstrate a correlation between work function and surface conductivity. By means of space charge calculations band bending values are derived and there-by the electron affinity, which is not accessible to direct measurement. It is concluded that the two clean cleaved polar faces exhibit a depletion layer, which remains on the O face also after annealing in UHV. On the prism face nearly flat band situation is found. The decrease of work function with increasing hydrogen coverage, i.e. surface electron density, can be interpreted by means of space charge calculations using the measured surface conductivity. For the two polar faces the band bending alone causes the decrease, whereas for the prism face an additional increase of electron affinity has to be assumed. The variation of electron affinity by annealing or by hydrogen adsorption might be due to a change of atomic distances in the uppermost layer, although no superstructure has been found in LEED studies.

Variations of work function and surface conductivity on clean cleaved zinc oxide surfaces by annealing and by hydrogen adsorption

Crystals are cleaved in UHV normal and parallel to the c axis exposing polar and prism faces. Work function (Kelvin) and surface conductivity are measured, also after annealing at elevated temperatures in UHV and after adsorption of atomic hydrogen. The work function of all three faces decreases irreversibly by annealing in UHV, but only on the Zn face a measurable surface conductivity appears. Also hydrogen adsorption diminuishes the work function. Simultaneously an increasing surface conductivity can be observed on all three types of surfaces. Corresponding structures in the annealing curves demonstrate a correlation between work function and surface conductivity. By means of space charge calculations band bending values are derived and there-by the electron affinity, which is not accessible to direct measurement. It is concluded that the two clean cleaved polar faces exhibit a depletion layer, which remains on the O face also after annealing in UHV. On the prism face nearly flat band situation is found. The decrease of work function with increasing hydrogen coverage, i.e. surface electron density, can be interpreted by means of space charge calculations using the measured surface conductivity. For the two polar faces the band bending alone causes the decrease, whereas for the prism face an additional increase of electron affinity has to be assumed. The variation of electron affinity by annealing or by hydrogen adsorption might be due to a change of atomic distances in the uppermost layer, although no superstructure has been found in LEED studies.

Binding states of zinc on the polar faces of ZnO

Abstract The desorption of zinc from cleaved polar faces was investigated in UHV. TDS (calibrated by evaporation of metallic Zn), surface conductivity, LEED and AES were used. The initial sticking coefficient is lower on the Zn face than on the O face, 0.2 and 1 respectively. On both faces two states of adsorption occur. The lower state on the Zn face has an activation energy of 35 kcal/mole(1.5 eV). The corresponding energy on the oxygen face is 16 kcal/mole (0.7 eV). The lower peak is connected with an increase in surface conductivity on both faces. On the Zn face this surface conductivity can be attributed to a surface donor 1.3 eV above the conduction band edge. Above one monolayer the formation of islands and an epitactical growing of a zinc lattice were found.

Binding states of zinc on the polar faces of Zno

Abstract The desorption of zinc from cleaved polar faces was investigated in UHV. TDS (calibrated by evaporation of metallic Zn), surface conductivity, LEED and AES were used. The initial sticking coefficient is lower on the Zn face than on the O face, 0.2 and 1 respectively. On both faces two states of adsorption occur. The lower state on the Zn face has an activation energy of 35 kcal/mole(1.5 eV). The corresponding energy on the oxygen face is 16 kcal/mole (0.7 eV). The lower peak is connected with an increase in surface conductivity on both faces. On the Zn face this surface conductivity can be attributed to a surface donor 1.3 eV above the conduction band edge. Above one monolayer the formation of islands and an epitactical growing of a zinc lattice were found.

Xps studies of surface charge on nickel oxide

Surface charge present on nickel oxide has been studied with samples prepared by heating the oxide at temperatures between 700 and 1450°C The effect of the charge on O(1s).C (1s) and Ni(2p) spectra has been examined in detail. The magnitude of the charge at 20°C is shown to be related to the defect concentrations and hence the bulk conductivity of the oxides The effect of increasing the sample temperature (up to 120°C) is to facilitate removal of the charge due to increased surface conductivity

The surface conductivity at the polystyrene-latex particle/electrolyte interface

The surface conductivity of polystyrene latex particles has been measured in a range of concentrations of potassium chloride, barium perchlorate, aluminium perchlorate, and hexol nitrate. There was an increase of surface conductivity with increasing concentration for the three simple electrolytes; the surface conductivity eventually attained the same constant value in each electrolyte. The measured surface conductivity was several orders of magnitude larger than that calculated for theGouy layer; the results have been interpreted as indicative of a large contribution to the total surface conductivity from the fixed layer and a negligible contribution from the diffuse layer.

Vapor Adsorption and Displacement on Porous Glass by Surface Conductivity

There is a marked increase in surface conductivity on adsorption of water vapor on porous glass. The relationship between the amount of adsorbed water vapor, the characteristic adsorption states of water molecules, and the surface conductivity for high surface area samples were obtained for nonmethylated and methylated conditions. Also the changes in conductivity with preadsorbed n-hexane on adsorption of water, methanol and acetone vapors were measured. Each of these replaces hexane as might be expected, but not all at equal rates. This effect is considered from the point of view of an interaction parameter for replacement.

Feldeffekt und photoleitung an ZnO-einkristallen

It has been shown that atomic hydrogen produces an n-type accumulation layer on the surface of ZnO crystals. The high surface-conductivity associated with this layer can be lowered by adsorption of oxygen or by heating in a vacuum. In this way the surface conductance, which is large compared to the bulk conductance at 90°K, is varied over a range of five orders of magnitude. Three different methods are used to change carrier density at the surface without changing the gas coverage: field-effect, photoexcitation with chopped light and photoexcitation with steady illumination. From field-effect (change of carrier density due to transverse electric fields) and from surface photoconduction, using chopped light in the intrinsic absorption region, a drift mobility is derived. The results of these two methods are in good agreement. The drift mobility increases with increasing surface conductivity for several orders of magnitude and reaches 30 cm 2/Vsec. Measurements of surface photoconduction, using continuous irradiation in the intrinsic absorption region, combined with the drift mobility, yield information on the density and lifetime of excited electrons at the surface. As the irradiation intensity is increased through four orders of magnitude, the lifetime decreases from 0.5 sec to 7 × 10 −4 sec. However, if the surface conductivity is changed by four ciders of magnitude by adsorption or desorption, the lifetime remains constant and is therefore independent of adsorbed gases. A consistent explanation of the results is possible with a surface model containing two sets of continuously distributed surface states, one just below the conduction band and one just above the valence band. An estimation for the density of these fast surface states is given.

Contact Charging between Nonconductors and Metal

An investigation has been made of the charging of fused quartz and borosilicate glass spheres rolling on a clean nickel surface. The results indicate that the process of acquiring charge depends only upon the nature and surface condition of the materials in contact and is independent of the pressure of an atmosphere of dry N2. The total charge, however, is limited by gaseous discharge to the metal and therefore does depend upon the pressure. Such gaseous discharge has been verified by the observation of individual discharge pulses. The rate of acquiring charge and maximum charge in vacuum has been found to depend upon rolling speed, both reaching a maximum at an intermediate speed, and to decrease with increasing surface conductivity. Since both fused quartz and borosilicate glass charge negatively in contact with clean nickel, it is inferred that both have higher effective work functions than nickel. The surface conductivity of borosilicate glass has been found to be three of four orders of magnitude higher than that of quartz. Therefore, since the charging of the two materials is of the same order of magnitude, the effective work function of the glass appears to be considerably higher than that of fused quartz.