Mid temperature plasma device

Details Mid temperature plasma device Mid temperature plasma device

2003-12-03

2004-11-09

Campbell, Thor (Department: 3742)

Electric resistance heating devices

Heating devices

Continuous flow type fluid heater

C219S121360, C219S121110

Reexamination Certificate

active

06816671

ABSTRACT:

A plasma is an electrically conductive gas containing charged particles. When atoms of a gas are excited to high energy levels, the atoms loose hold of some of their electrons and become ionized producing a plasma containing electrically charged particles - ions and electrons. It is well known as shown in the example below that this dissociation is key to processing powders.
The proposed invention is about a new device which produces an unique convective plasma in the 10
3
C temperature range (mid temperature plasma). The bulk of fundamental plasma research has mostly been concentrated on very high density, high temperature plasmas or cold room temperature plasmas. When such plasmas are used for materials processing the processing is also mostly carried out inside the plasma chamber whereas the proposed plasma device is a convective plasma generator capable of transfer along channels. Although very high temperature plasmas are now commonly available, almost all metallurgical processing work involves temperatures of the order of 10
3
C which falls in-between the two extremes where the bulk of plasma research has been carried out. Consequently, most of plasma devices have never been able to assist metal fabrication in an efficient manner except for coatings, and micro-bacterial cleaning/coatings where notable strides have been made. Some induction coupled plasmas (4000-12000 C) and transferred arc plasmas (~6000 C) which have been used in metallurgical industrial practices for gas modification or for the development of high temperature deposited coatings (e.g. plasma deposition of thermal barrier coatings on aircraft blades). These plasmas are difficult to direct or transfer in tubes to locations other than where they exist (typically between electrodes).
Several uses for very low temperature (cold) plasmas are also known for polymer systems processing. Cold plasmas are used for polymer surface cleaning or polymerization purposes. The effect of a plasma impingement on a given material is determined by the chemistry of the reactions between the surface and the reactive species present in the plasma. At the low-exposure energies typically present in glow-discharge plasma systems, the interactions occur only in the top few molecular layers. This layer is deeper for higher temperature plasmas. Plasma surfaces have unique reactions which are well known for low temperature plasmas and polymers but not as well know for metallic surfaces and medium temperature plasmas. In the case of polymers which are treated by cold plasmas, the gases, or mixtures of gases, used for the cold plasma treatment include air, nitrogen, argon, oxygen, nitrous oxide, helium, tetrafluoromethane, water vapor, carbon dioxide, methane and ammonia. Each gas produces a unique plasma composition and results in different polymer surface properties. For example, the high surface energies required for wettability and chemical reactivity may be increased very quickly and effectively by plasma induced oxidation, nitration, hydrolyzation, or amination (ammonia processing). Conversely, plasma induced fluorination depresses the surface energy, producing an inert and non-wettable surface. Such affects are often utilized for powder coating.
The two extreme plasmas (very hot and room temperature) mentioned above are mostly unsuitable for metallurgical work because of the extreme temperatures and very poor efficiencies. Although some plasma temperatures from conventional generators may be manipulated to have lower temperatures, there are other problems for economical use when such modifications are attempted. For example transferred arc induc ion plasmas are noisy and extremely costly for use in the 700 C range of temperatures where aluminum is melted and cast. Additionally, the conversion efficiency and power transfer efficiency of the transferred arc plasma is very low (single digits for these low temperatures) thus negating economical use. A new mid temperature range (700 C1300 C) convective plasma device is described herein. This new system is extremely quiet and seemingly offers the possibility of close to 100% power transfer efficiency. The use of this source with the novel heat transfer mechanism is expected to give rise to a host of new energy efficient technologies. It is to be noted here that until the availability of the this technology for medium temperature plasmas it was commonly recognized that thermal plasma processing faces a untenable economic prognosis in commercialization. Whenever conventional plasmas were considered in the past for metallurgical processing, invariably cost considerations prevented large scale applications, a fact highlighted often in the classic review by Pfender (E. Pfender, Plasma Chemistry and Plasma Processing, Vol.1, No.1, 1999, pg.1-28). The importance of this invention is made more obvious by the manageable cost of systems which can now become available. It is important therefore to develop processes with medium temperature plasmas. In this light, several plasma processes contemplated in (Plasma and laser Processing of Materials, eds. K. Upadhaya, TMS, 1991, and Carbide, Nitride and Boride Materials Synthesis and Processing, eds. Alan W. Weimer, Chapman Hall, 1997, could also benefit with the new source (this invention).
The plasma of this invention also may be used to vastly enhance heat transfer to a solid in order to improve productivity and save in the power lost to the surroundings by (i) concentrating the heat on the solid on account of the charge separation in the plasma and (ii) saving energy by processing faster such that the time in which it takes to melt a solid is so low that the surrounding device has little time to loose heat.
In an example below we find that heating the solid by the plasma of this invention uniquely allows the heat transfer coefficient to increase by orders of magnitude when compared to heating only by convection without plasma. Although, plasmas have been used to heat solids in the past, most heating configurations with the solids involve holding the solid (often a powder) inside the plasma. In the designs now possible, described below, the plasma is directed at the solid inside a chamber containing the surfaces to be heated which is kept far away from the plasma generating source which also provides for forced convection. Such solids can for example be aluminum ingots or scrap aluminum parts which require melting or iron surfaces requiring nitriding. The theoretical reason to expect such a benefit in heat transfer coefficient is discussed below.
Theoretical determination of the heat transfer coefficient (H) is an extremely difficult theoretical problem therefore numerous empirical and semi-empirical correlations are used to describe heat transfer to a spherical particle in a laminar flow. One of the popular methods is the Ranz-Marshall formula which describes the Nusselt number (Nu) for heat transfer to a spherical particle.
Nu=A+B Re
n
Pr
m
Here the constants A, B, n, m, are typically, 2, 0.6, 0.5; 0.5 respectively for forced convective flow over small particles.
Here Nu and Re are Nusselt and Reynolds numbers, respectively. These numbers are defined by the following equations:
Nu=Hd
p
/{overscore (&lgr;)}
p
  (6)
Re=[d
p
/v
f
−v
p
/{overscore (&rgr;)}
f
]/{overscore (&mgr;)}
f
(see below for definition of symbols)
And where Pr is the Prandtl number defined as
Pr
={overscore (
c
)}
f
{overscore (&mgr;)}
f
{overscore (&lgr;)}
p
−1
where p
p
is the density, c
p
is the specific heat capacity at constant pressure, &lgr;
p
is the thermal conductivity, T
p
is the absolute temperature, r
p
and d
p
are the radius and diameter of the particle, the dynamic viscosity is &mgr;
f
, the velocity is v. f and p signify fluid and particle respectively. The bars signify averaged temperature values. The particle temperature is the function of time &tgr; and its radial coordinate r.
There is a problem with fitting data if fluid plasma conditions exist and the classic paper of Young and Pf

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