Langmuir probe measurements of electron temperature in an inductively coupled plasma

The feasibility of using double Langmuir probes to measure electron temperature (T_e) in an Ar inductively coupled plasma (ICP) was evaluated. Experimental methods for probing the plasma and for reducing rf interference were devised. Despite these measures, the probe signal was noisy and erratic if the ICP had the normal analytical configuration with a hole through its center, so measurements were restricted to an ICP without an axial channel. Theoretical criteria indicated that Langmuir probe measurements in an atmospheric pressure ICP were in a borderline regime in which the measured T_e values may have been depressed somewhat (relative to the actual T_e values in the ICP) due to cooling of electrons as they approached the probe. The T_e values obtained from the center of the ICP were 7500 K at a forward power of 1.0 kW and 10 000 K at 1.25 kW for a measurement position 8 mm above the load coil. Electron density (n_e) measurements by the Langmuir probe method were comparable to or higher than n_e values calculated from the Saha equation at the measured T_es. The T_e and n_e values were high enough to indicate that, if electron cooling and ion-electron recombination occurred near the probes, these effects were not extreme and/or the use of two probes compensated for them in some fashion. The probe measurements also indicated that T_e increased with the potential difference between the probes. This latter observation provided tentative evidence that the electron kinetic energy distribution was non-Maxwellian with an excess of higher energy electrons relative to lower energy electrons.     

Post-irradiation grafting of tetrafluoroethylene at low temperatures

Low-temperature postirradiation grafting of tetrafluoroethylene (TFE) to polyolefines and silicone elastomers was the subject of study. After preliminary dissolution of TFE in the polymer at 273°K the system was slowly cooled to 77°K. In this process a certain part of TFE in polyolefines (ethylene-propylene copolymer and polypropylene) is retained by the vitreous polymer matrix. When slowly heated after radiolysis at 77°K this system shows graft polymerization of dissolved TFE after T_g. The graft copolymer is soluble and its IR spectra contain absorption bands characteristic of polytetrafluoroethylene. In polydimethylsiloxane rubber (SKT) the dissolved TFE when frozen to 77°K remains sorbed between the SKT crystallites rather than in a separate phase. When these radiolyzed samples are heated the graft-polymerization occurs primarily over the temperature range between the TFE and SKT melting points. The technique provedes for 100–150% grafting of TFE. This method also permits grafting to silicone rubbers and to several other polymers and elastomers.     

Some aspects of plasma polymerization of fluorine-containing organic compounds. II. Comparison of ethylene and tetrafluoroethylene

Plasma polymerizations of ethylene and tetrafluoroethylene are compared. In the plasma polymerization of ethylene and of tetrafluoroethylene, glow characteristics play an important role. Glow characteristic is dependent on a combined factor of W/F_m, where W is discharge power and F_m is monomer flow rate. At higher flow rates, higher wattages are required to maintain “full glow.” In the plasma polymerization of tetrafluoroethylene, simultaneous decomposition of the monomer competes with plasma polymerization. Above a certain value of W/F_m, decomposition becomes the predominant reaction, and the polymer deposition rate decreases with increasing discharge power. ESCA results indicate that the plasma polymer of tetrafluoroethylene that is formed in an incomplete glow region (low W/F_m) is a hybrid of polymers of plasma polymerization and of plasma-induced polymerization of the monomer. Polymers formed under conditions of high W/F_m to produce “full glow” are similar, regardless of the extent of decomposition of the monomer. They contain carbons with different numbers of F(CF₃, ? CF₂? , >CF? , >C<) and carbons bonded to other more electronegative substituents.     

Electron production and loss processes in a spectrochemical inductively coupled argon plasma

The behavior of inductively coupled plasmas for spectroscopic purposes has been studied extensively in the past. However, many questions about production and loss of electrons, which have a major effect on this behavior, are unanswered. Power interruption is a powerful diagnostic method to study such processes. This paper presents time resolved Thomson scattering measurements of the electron density n_e and temperature T_e in an inductively coupled argon plasma during and after power interruption. In the center of the plasma the measured temporal development of n_e and T_e can be attributed to ambipolar diffusion, three-particle recombination and ionization. However, at the edge of the plasma an additional electron loss process must be involved. In addition, the high electron temperature during power interruption indicates the presence of an electron heating mechanism. The energy gain by recombination processes is shown to be insufficient to explain this electron heating. These discrepancies may be explained by the formation and destruction of molecular argon ions, which can be present in significant quantities.     

Emulsifier-free emulsion polymerization of tetrafluoroethylene by radiation. II. Effects of reaction conditions on polymer particle size and number

The size, distribution, and number of PTFE particles formed by radiation-induced emulsifier-free polymerization were measured by electron microscope and automatic particle analyzer (centrifugation method). From the electron micrographs we found that the particles are formed within 5 min. The change in the number of polymer particles (n_p) with reaction time (t) depends on the relative concentration of growing polymer chains to stabilizing species produced by the radiolysis of water and monomer; that is, it was governed by TFE pressure/dose rate ratio and classified into three cases: case I, dn_p/dt = 0 (e.g., at 3 × 10⁴ rad/hr and 20 kg/cm²); case II, dn_p/dt < 0 (e.g., at dose rate below 1.9 × 10⁴ rad/hr and 20 kg/cm²); case III, dn_p/dt > 0 (e.g., at 3 × 10⁴ rad/hr and 2 kg/cm²). The polymer molecular weight above 10⁶ is almost independent of the particle size. The polymerization loci are mainly on the surface of polymer particles dispersed in the aqueous phase in cases I and II except in the initial stage. In case III new particles are formed successively during polymerization. Therefore the polymerization loci are mainly in the aqueous phase. Especially in case I, we concluded that after the generation of particles the propagation proceeds mainly on the surface of polymer particles like the core shell model proposed by Granico and Williams.     

Emulsifier-free emulsion polymerization of tetrafluoroethylene by radiation. III. Simultaneous formation of hydrofluoric acid

Simultaneous formation of hydrofluoric acid (HF) in the radiation-induced polymerization of tetrafluoroethylene (TFE) was investigated. HF concentration in PTFE latex was determined mainly by conductometric titration with 0.01 and 0.001N NaOH. The amount of HF formed is almost independent of agitation speed and the amount of n-hexadecane added and is maximal at ca. 70°C corresponding to the rate of polymerization. The rate of HF formation increases with the initial pressure of TFE monomer and dose rate and decreases with polymerization or TFE consumption. This fact suggests that HF is formed mainly by TFE reactions and not by the degradation of PTFE. The mechanism of HF formation in this reaction system in the absence of oxygen is shown in the following two schemes: scheme I is the reaction of TFE with primary radicals (OH·, H·, e) from the radiolysis of water; scheme II is the reaction of water with the species from the radiolysis of TFE. On the assumption that HF is formed only according to scheme I, the G value of HF formation G(HF)_calc can be calculated as 11.25. All observed G values G(HF)_obs are larger than G(HF)_calc. When the polymerization is carried out at 20 kg/cm² under various dose rates, G(HF)_obs increases with the dose rate. When the polymerization is carried out at 3.0 × 10⁴ rad/hr under various pressures, G(HF)_obs decreases with the decrease in pressure from 20 to 2 kg/cm² and is fairly close to G(HG)_calc at 2 kg/cm². This indicates that HF formation is due mainly to scheme II at high pressure (in the presence of enough TFE) and to scheme I as the pressure is lowered.     

Molecular mass distribution of oligomers in products of tetrafluoroethylene radical telomerization

The molecular mass distributions (MMD) of perfluorinated oligomers in products of tetra-fluoroethylene (TFE) radical polymerization in various solvents (telogens) were determined from an analysis of differential thermogravimetric curves and data of gel permeation chromatography and mass spectrometry. Radiolysis of the telogens generates radicals initiating polymer chain growth. The choice of the solvent and TFE concentration makes it possible to obtain oligomers with the controlled average chain length from 4 for 40 CF₂ fragments and specified terminal groups. The polymerization of TFE in THF and propylene oxide affords oligomers with cyclic terminal groups capable of further polymerization due to ring opening. The appearance of two MMD maxima (low-molecular-weight at n ₁ ～6–8 and high-molecular-weight at n ₂ > 10 shifting towards high n with an increase in the TFE concentration) is caused by the formation of colloidal solutions of oligomers.     

Emulsion copolymerization of tetrafluoroethylene and propylene with redox system containing tert-butylperbenzoate. II. Polymerization mechanism

Emulsion copolymerization of tetrafluoroethylene (TFE) and propylene (P) initiated by trilon-rongalite catalytic system containing tert-C₄H₉OH, initial monomer mixture, emulsifier (C₇F₁₅COONH₄) concentration, and monomer mixture/water ratio on the polymerization rate (R) and molecular weight (M?_n ) was investigated. Both R and M?_n increased considerably with TFE content in monomer mixture up to 75 mol %. Alternating rubber-like copolymers in a wide range of initial monomer mixture (from 55–85 mol %) were obtained. The reactivity ratio was found to be r_TFE = 0.005 ± 0.04 and r_p = 0.17 ± 0.07. Above the critical miscelle concentration, the effects of the initiating system Is and emulsifier Cs on R and M?_n were found to obey the following relations: according to which emulsion copolymerization proceeds by the I case of Smith-Ewart theory. Polymerization mechanism of the reaction studied was suggested. The copolymerization is mainly terminated by degradative chain transfer of the propagating radicals to propylene. © 1994 John Wiley & Sons, Inc.     

Effect of biaxial orientation on dielectric and breakdown properties of poly(ethylene terephthalate)/poly(vinylidene fluoride‐co‐tetrafluoroethylene) multilayer films

Polymer films with enhanced dielectric and breakdown properties are essential for the production of high energy density polymer film capacitors. By capitalizing on the synergistic effects of forced assembly nanolayer coextrusion and biaxial orientation, polymer multilayer films using poly(ethylene terephthalate) (PET) and a poly(vinylidene fluoride‐co‐tetrafluoroethylene) [P(VDF‐TFE)] copolymer were produced. These films exhibited breakdown fields, under a divergent field using needle/plane electrodes, as high as 1000 kV mm^?1. The energy densities of these same materials, under a uniform electric field measured using plane/plane electrodes, were as high as 16 J cm^?3. The confined morphologies of both PET and P(VDF‐TFE) were correlated to the observed breakdown properties and damage zones. On‐edge P(VDF‐TFE) crystals induced from solid‐state biaxial stretching enhanced the effective P(VDF‐TFE) layer dielectric constant and therefore increased the dielectric contrast between the PET and P(VDF‐TFE) layers. This resulted in additional charge buildup at the layer interface producing larger tree diameters and branches and ultimately increasing the breakdown and energy storage properties. In addition to energy storage and breakdown properties, the hysteresis behavior of these materials was also evaluated. By varying the morphology of the P(VDF‐TFE) layer, the low‐field dielectric loss (or ion migration behavior) could be manipulated, which in turn also changed the observed hysteresis behavior. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 882–896     

Fundamental studies on a planar-cathode direct current glow discharge. Part I: characterization via laser scattering techniques

A laser-scattering-based instrument was used to study an argon d.c. planar-diode glow discharge. The gas-kinetic temperature (T_g) was determined via Rayleigh scattering and the electron number density (n_e), electron temperature (T_e), and shape of the electron energy-distribution function were determined by Thomson scattering. Axial profiles of these parameters were obtained as the discharge current, voltage, and pressure were varied. Trends in the profiles of T_g and in the other parameters show the interdependence of these plasma species and properties. The results will be compared with current theoretical computer models in order to improve our understanding of the fundamental processes in glow discharges sustained under conditions appropriate for spectrochemical analysis.     

Plasma homo- and copolymerizations of tetrafluoroethylene and chlorotrifluoroethylene

The plasma homo- and copolymerizations of tetrafluoroethylene (TFE) and chlorotrifluoroethylene (CTFE) in a capacitively coupled tubular reactor (TR) with external electrodes were studied by means of microgravimetry and FT-IR and XPS analyses. The deposition rates for CTFE/TFE plasma copolymers, as well as the ratios of IR absorbances at 1180 and 1225 cm⁻¹, and the XPS-derived Cl/C and F/C ratios, varied regularly with mol % CTFE in the feed, all of which results were dependent upon the rf power at which the plasma copolymerizations were conducted. The deposition rates for the plasma homopolymers of TFE (PPTFE) and CTFE (PPTCFE) depended markedly on rf power (W) and monomer molar flow rate (F). The F/C ratio for PPTFE was nearly independent of the composite parameter,W/FM (whereM is the monomer molecular weight), while for PPCTFE, the F/C ratio decreased significantly and the Cl/C ratio increased slightly with increase inW/FM. The percentage of carbon as CF₃ was 20–24% in PPTFE and 7–14% in PPCTFE. Plots of deposition rate versusW/FM for PPTFE and PPCTFE obtained in a TR differed considerably from corresponding plots in the literature for the same homopolymers prepared in a glass-cross or bell-jar reactor.     

Langmuir probe study of the charged particle characteristics in an analytical radio frequency-glow discharge. Roles of discharge conditions and sample conductivity

The application of a tuned Langmuir probe to the measurement of the charged particle characteristics of electron number density, ion number density, electron energy distribution function, average electron energy and electron temperature, in an analytical radio frequency (r.f.)-glow discharge is described. Studies focus on the roles of discharge operating conditions and plasma sampling position for conductive (copper) and nonconductive (Macor) samples. Based on the data obtained here, apparent differences in plasma characteristics between conductive and nonconductive samples can be reasonably explained. For example, the sputtering of conductive samples results in plasmas with obviously higher electron and ion number densities than the sputtering of nonconductive samples (e.g. n_i = 1.8 × 10¹⁰ cm⁻³ and n_e = 1.5 × 10⁹ cm⁻³ for copper, and n_i = 8 × 10⁹ cm⁻³ and n_e = 5 × 10⁸ cm⁻³ for Macor under the conditions of argon pressure = 4 Torr, r.f. power = 30 W and sampling distance = 4.5 mm). Conversely, nonconductive samples yield electrons with higher energies (average electron energies of 15 and 7.5 eV and temperatures of 6.5 and 3.5 eV respectively for the Macor and copper samples). Lower d.c. bias potentials for the case of sputtering nonconductive samples yield reduced sputtering rates and charged particle densities, though the electrons in the latter case have higher energies and thus improved excitation capabilities. The differences between r.f.- and d.c.-glow discharge optical emission spectra are also discussed relative to reported electron energy characteristics. Studies such as these will lay the ground-work for extensive evaluation of inter-matrix type standardization for r.f.-glow discharge atomic emission spectrometry.     

Chain dimensions and entanglement spacings in dense macromolecular systems

In this article, we reexamine and extend a relationship proposed earlier between entanglement density and chain dimensions in polymer melts. The power-law equation presented in the earlier work, relating the entanglement molecular weight M_e, melt chain density ρ, and the packing length p is tested with additional polymer species. Now included are additional polydienes and their hydrogenated derivatives, the isotactic forms of polypropylene and polystyrene, the essentially syndiotactic form of poly(methyl methacrylate), along with poly(tetrafluoroethylene), poly(vinylmethyl ether), various poly(methacrylates), and polymeric sulfur. We find that within experimental uncertainties, M_e/ρ and p are related through an equation (M_e/ρ = 218p³) that is insensitive to temperature (25°C ≤ T ≤ 380°C) and which seems to be universal for flexible Gaussian chains in the melt state. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1023–1033, 1999     

A theoretical calculation of dissociation rates of molecular hydrogen in electrical discharges

Dissociation rates of molecular hydrogen in electrical discharges have been calculated at different electron (T_e) and gas (T_g) temperatures (10000 T_e 23000 K, 500 T_g 4000 K), at different pressures p (5 p 50 torr) and electron number densities n_e (0 n_e 10¹² cm⁻³).The results have been obtained by solving a system of master equations, including V---T (vibration-translation), V---V (vibration---vibration) and e---V (electron---vibration) microscopic processes.The results obtained at n_e ≠ O show a “laser-type mechanism” in the dissociation of molecular hydrogen in electrical discharges. In particular one notices a strong increase of dissociation rates with decreasing gas temperature and pressure.The results show that this mechanism is as important as the mechanism of direct dissociation by electron impact.     

Order parameter studies from effective order geometry in 4O.Om and 7O.Om liquid crystalline compounds

The effective geometry parameter, α_g = n _o /n _e, is used to evaluate the orientational order parameter, S, in the case of N-(p-n-butyloxybenzylidene)-p-n-alkoxy anilines, 4O.Om and N-(p-n-heptyloxybenzylidene)-p-n-alkoxy anilines, 7O.Om compounds with m?=?3–7 and 9 in the former case and m?=?3, 5–7 and 9 in the later materials. The results obtained are compared with those calculated using the standard techniques of molecular polarisability and birefringence. The effective geometry parameter's influence on the deflection of light by the liquid crystal compounds is also studied. The variation of temperature gradient of the ordinary refractive index, dn _o /dT, and extraordinary refractive index, dn _e /dT, of the liquid crystals is also studied.     

Measurement of Spatial Distributions of Electron Density and Electron Temperature in Direct Current Glow Discharge by Double Langmuir Probes

The spatial distributions of electron temperature and density in a dc glow discharge that is created by a pair of planar electrodes were obtained by using double Langmuir probes. The contribution of double Langmuir probes measurement is to provide a relatively quantitative tool to identify the electron distribution behavior. Electrons gain energy from the imposed electric field, and electron temperature (T_e) rises very sharply from the cathode to the leading edge of the negative glow where T_e reaches the maximum. In this region, the number of electrons (N_e) is relatively small and does not increase much. The accelerated electrons lose energy by ionizing gas atoms, and T_e decreases rapidly from the trailing edge of the negative glow and extends to the anode. N_e was observed to increase from the cathode to the anode, which is due to the electron impact ionization and electron movement. The electron density was observed to increase with increasing discharge voltage while the electron temperature remained approximately. At 800 V and 50 mTorr argon glow discharge, Te ranged from 15 to 52 eV and Ne ranged from 6.3×10⁶/cm³ to 3.1×10⁸/cm³ in the DC glow discharge, and T_e and N_e were dependent on the axial position.     

A New Modified Pseudoequilibrium Calculation to Determine the Composition of Hydrogen and Nitrogen Plasmas at Atmospheric Pressure

This paper proposes a modified pseudoequilibrium calculation, which gives almost the same results as those of kinetic calculations to determine the composition of hydrogen and nitrogen plasmas at atmospheric pressure. The computing time is two to three orders of magnitude faster than that of the kinetic calculations. First, according to experimental results, a relationship between the electron temperature T_e and the heavy species one T_h has been proposed. The ratio T_e/T_h varies as a function of the logarithm of the ratio n_e/n _e ^max , _e ^max being the electron density in the plasma core for which equilibrium is achieved _e ^max ~ 10 ²³ ). The kinetic calculations have been performed assuming the microreversibility where the backward kinetic rate coefficient k_b is calculated by k_d/k_b=K_x, where k_d is the direct kinetic coefficient and K_x the molar fraction equilibrium constant. When electrons are involved in both direct and backward reactions, k_d and K_x are expressed as functions of T_e . However, when the direct reaction involves electrons while the backward one is due to collisions between heavy species (or the reverse), a temperature T* between T_e and T_h is introduced. T* is determined as a function of the ratio of the electron flux to that of neutral species in such a way that T*=T^e for n_e > 10²³ and T*=T_h for low values of n_e(n_e < 10¹⁵ m^–3). Compared to hydrogen, the nitrogen composition exhibits a very abrupt variation between 6000 and 6500 K, corresponding to a shift from the dissociation-dominated regime to that of ionization. It occurs because dissociation of nitrogen starts almost simultaneously with its ionization, which is not the case of H₂, for which dissociation is terminated long before ionization starts. If the charge transfer reaction, whose activation energy is low for both gases, is neglected, in both cases the electron density increases drastically below 9000 K. These results are quite similar to those obtained when calculating the composition with the multitemperature mass action law. The kinetic calculations are dominated by the reactions with a low activation energy: dissociation, dissociative recombination and charge transfer. Thus, a modified pseudoequilibrium calculation has been introduced, the plasma composition being calculated with the equilibrium constants corresponding to low activation energies[X₂ 2X, e+X ₂ ⁺ 2X, X ₂ ⁺ +X X⁺ + X₂ both for hydrogen (X=H) and nitrogen (X=N)] at the temperature T* between T_e and T_h. The results are in very good agreement with those of the kinetic calculations.     

Synthesis and characterization of original functional polymers of tetrafluoroethylene and 4,5,5‐trifluoro‐4‐ene pentyl acetate

The bulk radical copolymerization of tetrafluoroethylene (TFE) with 4,5,5‐trifluoro‐4‐ene pentyl acetate (FAc), initiated by tert‐butyl peroxypivalate to synthesize original, functionalized fluorinated poly(TFE‐co‐FAc), was investigated. FAc monomer was prepared from a five‐step process. The copolymerization was carried out in batch at different initial monomer molar ratios ([TFE]_o/[FAc]_o ranging from 95/5 to 10/90 mol %) and at different initiator concentrations (ranging between 0.075 and 1.100 mol % about the monomers) at 70 °C. All the experiments revealed the production of fluorooligomers as evidenced by an allylic‐transfer reaction from FAc. The microstructure of these copolymers (i.e., the molar percentage of both monomers in the copolymers) was assessed by ¹⁹F NMR spectroscopy. From the kinetics of copolymerization, two key characteristics were determined. First, the reaction order to the initiator (being 1.07) and that of FAc monomer (0.85) showed a heterogeneous character of the copolymerization and monomolecular chain‐transfer reaction to FAc. Second, from the Tidwell and Mortimer method, the reactivity ratios of both comonomers were determined, showing a tendency to alternance in a wide range of initial monomeric ratios (30/70–70/30): r_FAc = 0.20 ± 0.26 and r_TFE = 0.18 ± 0.15. Alfrey and Price's Q and e values of FAc were calculated by Greenley's technique [Q_FAc = 0.098 (from Q_TFE = 0.032) and e_FAc = 1.23 (vs e_TFE = 1.63)], indicating that FAc is a strong electron‐withdrawing monomer as TFE. The normalized monomer‐diad and triad fractions as a function of the polymer composition were obtained from the comonomer sequence‐distribution procedure. The average molecular weights and molecular weight distributions as well as the thermal properties (glass‐transition temperature and decomposition temperature) of the fluorocopolymers were assessed and are discussed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1693–1706, 2004     

An approximate quantitative analysis of non-equilibrium plasma transport for high density plasmas

For the evaluation of transport in high density plasmas numerical models have been developed in which simultaneously the conservation laws for mass, momentum and energy are solved. For high density plasmas, which are not too far from equilibrium the commonly used thermodynamic quantities are, electron temperature T_e, electron density n_e, heavy particle temperature and neutral density (or pressure). In this contribution an alternative formulation is described in which the plasma state is described by electron density ne and total pressure p and two non-equilibrium parameters: the deviation from Saha equilibrium of the neutral ground state (δb₁ = n₁/n₁ ^saha−1) and the deviation from thermal equilibrium between electrons and heavy particles δΘ = 1−T_h/T_e. The latter two parameters are zero in local thermodynamic equilibrium.

The advantage of this formulation is, that the transport coefficients and radiative properties can be reformulated as function of mainly n_e (at constant pressure), as the influences of non zero δb₁ and δΘ are small or can be explicitly given. As a result a simpler approximate formulation of the transport problem can be obtained. As an example the procedure is illustrated for atmospheric argon plasmas and for one aspect a comparison is made with work from e.g. E. Pfender.