Philip Bruce Henderson

Numerical and experimental investigation of the stability of radio-frequency (RF) discharges at atmospheric pressure

The stability and uniformity of a radio-frequency (RF) discharge is limited by a critical power density. Beyond this critical power density, instability occurs in the form of physical changes in the plasma (such as contraction due to arcing). The RF discharge used in this study is the non-equilibrium Atmospheric Pressure Plasma Jet (APPJ ® ) developed by Apjet, Inc. This discharge is known to operate uniformly in helium gas. However, for some proposed applications such as surface modification, there is a need to operate with reactive gases such as O2. Our experimental studies show that addition of molecular gas to a discharge operating in helium increases its power density (W cm −2 ), until it reaches the critical unstable arcing limit. Moreover, an increase in the frequency of operation (from 13 to 27 MHz) allows the plasma to sustain higher molecular gas concentrations and power densities before instability occurs. Further, it is observed that this critical power density is dependent on the type of molecular gas added. These results provide a motivation for the development of a mathematical model that can provide insight into the causes of instability and potential methods of suppression. The two commonly studied modes of instability are (1) thermal instability (TI) and (2) α–γ –arc mode transition. For the APPJ ® discharge conditions, the development time scales of TI are much longer (∼1 ms) as compared with discharge oscillation period (∼100 ns). Hence, if the instability was indeed thermal, discharge frequency increase would have no consequence, contrary to experimental findings. A 1D fluid model based on the local field approximation is developed to study instability in APPJ ® discharge. The analysis of modeling results confirmed our hypothesis that the instability development actually takes place via breakdown of sheath i.e. α–γ –arc mode transition and not by TI. (Some figures in this article are in colour only in the electronic version)

Weak Chemical Complexation of PH3 with Ionic Liquids

We present a combined theoretical and experimental study on weak chemical complexation between PH(3) and a few selected Cu(I)- and Al-based ionic liquids (ILs). PH(3) molecules were found to covalently bind with the cationic sites of the ILs. Effects of cations, anions, ion pairing, and solvents on the binding strength were systematically examined. The weak coordination of PH(3) on the ILs allows the PH(3) gas to be stored at near ambient conditions with a high capacity.

Novel epoxidation reaction of perfluoroalkenes with trimethylamine N-oxide and iodosylbenzene

Trimethylamine N-oxide and iodosylbenzene were found to be useful reagents for the epoxidation of tri- and tetra-substituted perfluoroalkenes in good to excellent yields. A catalytic epoxidation method was also developed by coupling this reaction with the N-oxidation of trimethylamine by hydrogen peroxide or perbenzoic acid.

PERSISTENT PERFLUOROALKYL RADICAL INVESTIGATIONS UNDER REDUCTIVE ENVIRONMENT : REACTION WITH ELECTRON-DONATING REAGENTS

Abstract The reactivity of persistent perfluoroalkyl radical, perfluoro-3-ethyl-2,4-dimethyl-3-pentyl ( 1 ), with various electron-donating reagents was investigated. It is revealed that 1 which is robust under oxidative conditions is rather vulnerable under reductive conditions. Thus, Lewis bases such as triethylamine and triphenylpnictogens (Ph 3 Pn, Pn=N, P, As, Sb, Bi) and some soft anions such as iodide or tetraphenyl borate reacted with 1 to give perfluoro-3-isopropyl-4-methylpent-2-ene ( 2 ) quantitatively. Even very weak Lewis bases such as diethyl ether and diethylsulfide also reacted with 1 to give 2 and additionally a hydrido product, perfluoro-3-ethyl-3- H -2,4-dimethylpentane ( 4 ). Hydrogen gas did not react with 1 at all without a catalyst, but in the presence of metal Pd adsorbed on charcoal, smoothly reacted to give 2 in quantitative yield. Metal hydrides such as LiAlH 4 , NaBH 4 , NaH, BH 3 (THF complex), Bu 3 SnH, Me 2 PhSiH reacted with 1 to give 2 and 4 . That an electron transfer mechanism is operating in the formation of 2 is obvious, but not conclusive in the formation of 4 .

Experimental and Modeling Study of the Stability of the Atmospheric Pressure Plasma Jet

Summary form only given. The stability and uniformity of a radio-frequency (RF) discharge is limited by a critical power density. Beyond this critical power density, instability occurs in the form of with physical changes in the plasma (such as contraction due to arcing). Levitsky identified and studied the two glow modes, alpha and gamma, of operation of an RF discharge. A detailed description of each mode can be found in the literature. The RF discharge under consideration in the current study is the non-equilibrium Atmospheric Pressure Plasma Jet (APPJ) developed by Apjet, Inc. This plasma operates uniformly in helium gas. Flowever, for some proposed applications, such as surface modification, there is a need to operate with reactive gases such as O2. Experimentally, an increase in molecular gas concentration in helium increases the power density (W.cm-3) until it reaches the unstable arcing limit. Moreover, an increase in the frequency of operation (from 13 to 27 MHz) allows the plasma to sustain higher molecular gas concentrations and power densities before instability occurs. The critical power densitv is dependent on the type of molecular gas added. Addition of O2 makes the discharge more stable, while the addition of CO2 decreases stability. These results provide a motivation for the development of a model that can provide insight into the causes of instability and potential methods of suppression. The two commonly studied modes of instability are 1. Thermal instability (TI), and 2. Alpha-gamma-arc mode transition (AGAT). For our discharge conditions, the development time scales of TI are much longer (~1 ms) as compared to discharge oscillation period (~100 ns). Hence, if the instability was indeed thermal, discharge frequency increase would have no consequence, contrary to experimental findings. A 1D fluid model was developed with a local field approximation (LFA) assumption. The analysis of modeling results confirmed our hypothesis that the instability development actually takes place via breakdown of sheath i.e. AGAT and not the TI mode.

Novel epoxidation of perrfluoroalkenes with N,N-diethylhydroxylamine

N,N-Diethylhydroxylamine epoxidizes various kinds of perfluoroalkenes in good to excellent yields.