Shigeyoshi Kittaka

A method for measuring the charging tendency of powder in pneumatic conveyance through metal pipes

Abstract The charging tendencies of powders of some synthetic high polymers are measured with a newly developed charging apparatus. The sample is continuously transported through a bent metal pipe using an air flow and is electrified by multiple frictions and collisions against the inner wall of the metal pipe. The transport speed V and the solid load d of the sample are changed from 1.4 to 2.8 m/s and 1.9 to 5.2 kg/m3, respectively. The charge build-up of each examined is given by the empirical equation: Q = Qs [1 − k exp (−t/τ)]. The saturation charge Qs and the initial rate of charge accumulation qo [ = (dQ/dt)t=0] are found to depend on the transport speed and the material of the pipe. The initial rate of charge accumulation qo also depends on the solid load and in the case of a spherical PE sample, it can be described by an experimental equation obtained with aluminium pipe: qo = −2.47 V0.4 d−1.07 × 10−6 C/kg s. The saturation charge Qs of each sample used in this measurement at the same transport speed corresponds to the position of each sample in the familiar previously published triboelectric series [1–3].

A new system for measuring the charging tendency of solid particles

Abstract The main part of the system is a cylundrical metal chamber 70 mm in diameter and 20 mm in height in which particles, supported by a circular air flow, rotates, and are charged electrostatically by friction against the inner surface of the chamber. Pellets of some synthetic high polymers are used to test the system. The build-up to the charge on a sample has been commonly observed almost to follow an exponential growth with good reproducibility. The time constant of the curve corresponds with the position in the triboelectric series of each sample. The saturation value of the charge is considered to be almost solely a function of the size and shape of the sample. In the case of a nearly spherical pellet of polyethylene 2.6 mm in mean diameter, the average saturation charge on a sample was 5.25 × 10−10C.

Growth of a Large Number of Iron Whiskers by the Reduction of Halides

A great number of iron whiskers have been grown by the reduction of iron halides with addition of a small amount of carbon black. The addition of carbon black to the halides was effective for growing a large number of iron whiskers in the atmosphere of hydrogen and a charcoal-air system. Some of the whiskers had a foreign particle at the tip, and their mechanical property and morphology were similar to that of the normal whisker. The whiskers of another type were fine and often grew outward from a carbon black particle in a bundle form. Most of the halide in the original charge was reduced to iron whiskers. Some properties of the whiskers and the growth mechanism were discussed. Experimental results indicate that a chemical reaction at a growth step is required and carbon black added provides the growth site as foreign particles.

Photoelectric Emission and Contact Charging of Vacuum-UV Irradiated Polymers

Photoelectric emission of high-density polyethylene, polypropylene and nylon 6 is found to increase considerably through pre-irradiation with vacuum UV of energy hν<10.74 eV. From the change of yield spectrum by pre-irradiation, we made a rough estimate of the surface-trap distribution of each polymer. Polyethylene has the largest density of surface trap of the three, and nylon 6 has the smallest. Irradiated polymers have a tendency to generate a more positive charge than non-irradiated ones when they come in contact with metals or polymers. This effect of pre-irradiation on the charging tendency is due to the change of the surface-state distribution caused by UV-irradiation. Polyethylene has the largest change of charging tendency because its density of surface trap is the largest of the three.

Contact Charging and Photoemission of Anthracene Single Crystal

The sign and magnitude of the charge generated on an anthracene single crystal by contact with metals depend on the metal work-function. Equilibrium of change exchange is achieved during a contact period shorter than the order of seconds. The charge generation by contact is described by electron-transfer from/to surface states. The results on contact charging show that the depth of the filled surface-states of anthracene is 4.58 eV from the vacuum level, and that of anthracene pre-uv irradiated in O2 gas is 4.93 eV. These values almost agree with the threshold energies of photoemission. The pre-uv irradiation in O2 increases the negative charge-generation. This is due to the increase of electron traps. The density of traps of pre-uv irradiated anthracene is of the order of 1012/cm2/eV.

Evidence for Electron Transfer as the Mechanism of Contact Charging of Polyethylene with Metals

The photoemission characteristic of high-density polyethylene is found to change after the polymer is electrified by contact with metals. In contact with Cr (work function=4.35 eV), the emission yield after contact (ya) is smaller than that before contact (yb) at hν 4.9 eV, ya is larger than yb. In contact with Al (work function=3.7 eV), ya is larger than yb over the whole range of photon energy examined. The field applied to the interface between the surfaces of polyethylene and metal changes both the charging tendency and the photoemission characteristic. These results give evidence that the mechanism of contact charging is electron transfer.

Effect of added carbon black on iron whisker growth

Abstract As previously reported, mass-growth of iron whiskers can be achieved by reduction of halide admixed with carbon black. In order to investigate the role of added carbon black on iron whisker growth, the growth rates and morphology have been investigated. It was found that the effect is related to an adsorption process at the tip growth sites which are limiting the reaction. The heat of adsorption of the halide increases and markedly multi-nucleous growth and large growth rates appear on many whiskers. There is no effect of carbon black below 500°C since the vapor pressure of the adsorbed halide is small.

Measurement of Charging Tendency of Pellets by Impact Charging

The charging tendency of pellets of some synthetic high polymers are measured with a newly developed charging chamber in which the pellets are charged repeatedly by impacts against a metal plate by the use of a compressed air flow. The charge build-up of the pellets follows an almost exponential growth. It is shown that the saturation charge is limited by air breakdown. Furthermore, the time constant of the build-up curve shows how easily the sample acquires the charge. In the case of nearly spherical pellets of polyethylene, 0.16 cm in radius, the saturation charge density is -2.10 x 10-9 C/cm2. This value is 36.3% of the maximum theoretical value. The charging tendencies of nylon-6, polyethylene, and polystyrene shown by both the value of the time constant and the polarity, of the charge, agree with the triboelectric series.

Growth of Copper Whiskers from Cupric Oxide

A great number of copper whiskers of the order of 109 have been grown in a single run when CuO is reduced in CO-CO2-N2 atmosphere with the addition of small amounts of CuCl and graphite. The whiskers grow on the surface of the graphite particles mostly in length of 0.1~1.5 mm, in some cases as long as 15 mm, their diameter being between 0.5 and 3µ and fairly uniform for a single run. Some interesting forms of whiskers are observed. The fundamental properties of the whiskers thus obtained are nearly the same as those formed by the known method of hydrogen reduction of copper halides.

Growth of iron-nickel whiskers

Abstract Iron-nickel whiskers have been grown by the hydrogen reduction of ferrous chloride and nickel oxide which were mixed with a small amount of carbon black or activated alumina powder. In nickel-rich mixtures, the whisker-crowd has been found to be mostly composed of iron-nickel alloy along with a pure nickel phase in an amount of about a tenth of the alloy phase for growth temperatures above 750°C. In iron-rich mixtures, an α-phase and two γ-phases have been obtained simultaneously in the growth temperature range between 650 and 750°C. Almost all the individual whiskers are single crystals and homogeneous in structure as characterized by 1000 kV electron microscopy.