Abstract
Some basic parameters functioning in oxidation rate of organic substances using low-temperature plasma asher with maximum high-frequency power of 30 W have been investigated. Approximately 50% of the high-frequency energy was admitted into the oxygen stream and a supposed electron temperature of around 15 eV in the plasma gas was calculated from the given physical dimensions of the apparatus. Linear relationship was found between the oxidation rate and (H. F. power)/(pressure) under certain limited range using sucrose as a test substance, while the latter was proportional to the electron temperature of the plasma. The oxidation rate was accelerated with increased flow rate until the latter reached to 20ml/min, and the oxidation rate was equilibrated by surface reaction rate at higher flow rate.
The plasma concentration in the oxygen stream was monitored by a thermistor probe consisting of a glass-covered thermistor element and a glass handle, a stainless steel net having been mounted on the thermistor probe for catalytic recombination of the dissociated species of the oxygen plasma. The stainless steel net provided a higher efficiency of the recombination than platinum net.
A distribution pattern of the oxidation rate along the plasma tube exhibited nearly a symmetric curve from the central position of the high-frequency coil, so that the quenching speed of the dissociated species produced in the discharge region was supposed to be considerably fast. Oxygen was then replaced with other gases and the same experiment was carried out. Air plasma oxidized sucrose having its distribution pattern of the oxidation rate which was not understood from the oxygen content in air. Complicated reactions between the nitrogen and the oxygen plasmas were therefore supposed. Carbon dioxide which dissociates into CO and O resulted a moderate oxidation with somewhat lower oxidation rate than air, probably because molecular oxygen which partly incorporated with plasma oxidation when oxygen or air was used was absolutely excluded from the carbon dioxide plasma. When pure argon was used as the plasma gas, a fragmentation of the organic substance caused by the electron impact was observed.
A comparison was finally made between the chemical structure and the oxidation rate by using various organic compounds the most of which were aliphatic and aromatic carboxylic acids. Acid anhydrides of low molecular weight exhibited extremely high rate of weight losses because of their volatilities stimulated by the surface oxidation of the materials. No significant difference of the oxidation rate due to the numbers of carboxylic groups and the lengths of alkyl chains was observed, while higher oxidation rate of unsaturated compounds than that of saturated ones was found in general. Low melting point samples gave some increase of weight due to diffusion of organic oxides produced at the surface into the samples.
A mass spectrometric study of the reactant gases found no fraction greater than m/e = 50, that meant practically complete combustion was taken place in the plasma apparatus.
