Patent Application
20230345611
The present invention relates to plasma torches and in particular inductively coupled plasma torches.
Inductive plasma torches have established themselves as the tool of choice for ceramic and metallic powder processing and may be used in a variety of applications, such as densification, purification, spheroidization and nanoparticle synthesis. A raw powder is processed by injecting it into the plasma plume, having temperatures exceeding 10,000 kelvin, where powder particles become molten, and surface tension forces cause them to assume dense, spherical shapes. Upon exiting the plasma, the particles re-solidify in-flight and are collected in an adjoining chamber. When these processed particles are used in applications such as thermal coatings or part production, they provide advantages that include increased processing speeds (due to improved powder flowability) and denser, more uniform coatings and parts. The industries which benefit from these processed powders include transportation, which use them to produce wear-resistant and corrosion-resistant coatings, and aerospace and medical, which use them in metal injection molding. Additionally, emerging 3D printing and additive manufacturing techniques such as directed laser metal sintering (DLMS), selective laser melting (SLM), selective electron beam melting (SEBM) and directed energy deposition (DES) depend on the processed powders. All these applications require high powder flowability and high packing efficiency to maximize printing speeds and minimize part porosity. The plasma torches have also been long used in thermal spray of metallic and ceramic coatings, and, more recently, in hybrid plasma spraying, physical vapor deposition (PVD) and chemical vapour deposition (CVD) processes.
The advantages of plasma torches compared to the popular direct current (D.C.) torches are that, due to the lack of electrodes, the plasma is clean and is not contaminated by the electrode material, the plasma has large volume with relatively small temperature gradients, and the plasma velocity and temperature fields are axisymmetric. Importantly, materials are centrally injected into the torch. This feature allows uniform processing of the injected powders or liquids.
Additionally, the plasma torch benefits from the absence of electrodes that wear over time and contaminate the processed powder. Conventional plasma torches consume far more gases and power than required for processing materials. Attempts have been made to address these issues since the inception of plasma torches in the 1960s. These efforts were led by Reed, who in 1961 introduced a double flux design that improved torch flow patterns by using a secondary outer gas flow to create a low-pressure zone inside the torch to stabilize the plasma. The tubular geometry used in this design is still used in modern-day plasma torches. The years which followed included several efforts with partial success that focused on varying torch geometry, implementing water cooling, using multiple coils, optimizing operating parameters and adjusting torch size.
Although these efforts have reduced gas and power consumption and improved particle flow patterns in plasma torches, there has been limited success in varying torch geometry to address these issues. The conventional designs do not greatly improve the yield of processed particles and have a high operating cost. Thus, there is a need for a radio-frequency inductively coupled plasma torch (RF-ICP) with unique geometry, which eliminates the existing shortcomings of the current state-of-the-art plasma torches.
A radio-frequency inductively coupled plasma (RF-ICP) torch is disclosed in an example embodiment of the present subject matter. The RF-ICP torch comprises a tubular component to process sample particles. The tubular component comprises a first cylindrical tube portion at the first end of the tubular component. The tubular component also comprises a conical tube portion at a second end of the tubular component and coupled to the first cylindrical tube portion. The conical tube portion is coupled to the first cylindrical tube portion such that a smaller diameter end of the conical tube portion is coupled to the first cylindrical tube portion and the conical tube portion is to output processed sample particles.
The RF-ICP torch further comprises an injector tube. The injector tube comprises a posterior end concentrically placed inside the tubular component. The posterior end of the injector tube also comprises an outlet to deliver a plasma gas and the sample particles into the conical tube portion of the tubular component. The posterior end is concentrically placed inside the conical tube portion such that the posterior end does not touch the inner walls of the tubular component. The injector tube also comprises an anterior end that lies outside the first end of the tubular component. The injector tube also comprises an inlet to input the plasma gas and sample particles to be delivered by the outlet into the conical tube portion. Furthermore, a load coil is wound on an outer surface of the tubular component to energize the plasma gas in the tubular component.
In operation, plasma gas and sample particle are delivered via the injector tube into the conical tube portion. The plasma gas is energized by passing an electric current through the load coil. Energized plasma gas creates a plasma plume which processes the sample particles and the sample particles are outputted from the conical tube portion.
The outputted sample particles may be used in various applications, such as powder processing, thermal spraying, nano-powder synthesis, and waste treatment, material processing, crystal growth, spectrochemical analysis, cladding in the fiber optics industry, X-ray target manufacturing, 3D printing, and additive manufacturing.
The RF-ICP torch of the present subject matter has narrowed upstream section due to the presence of conical tube portion. This improves plasma gas injection into the conical tube portion. This feature reduces plasma gas consumption and achieves higher temperatures with less amount of plasma gas. High temperature plasma leads to high velocities of processed sample particles which improves the output yield of processed sample particles. Thus, the RF-ICP torch of the present subject matter has a higher efficiency and increased processed sample particle yield as compared to state-of-the art plasma torches. The RF-ICP torch of the present subject matter provides increased productivity with lower plasma gas consumption and lower power consumption.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements. The figures are not necessarily to scale, and the size of some parts may be exaggerated to more clearly illustrate the example shown. Moreover, the drawings provide examples and/or implementations consistent with the description; however, the description is not limited to the examples and/or implementations provided in the drawings.
For simplicity and illustrative purposes, the present disclosure is described by referring mainly to examples thereof. The examples of the present disclosure described herein may be used together in different combinations. In the following description, details are set forth in order to provide an understanding of the present disclosure. However, it will be readily apparent that the present disclosure may be practiced without limitation to all these details. Also, throughout the present disclosure, the terms “a” and “an” are intended to denote at least one of a particular element. As used herein, the term “includes” means includes but not limited to, the term “including” means including but not limited to. The term “based on” means based at least in part on, the term “based upon” means based at least in part upon, and the term “such as” means such as but not limited to.
The conical torch of the present invention has an injector tube (2) for the injection of carrier gas into the torch. The inner diameter of the injection tube forms a carrier flow path (16). The distal end (21) of the injection tube (2) is conical and defined by a cone angle α (26). The conical end of the injector tube is configured to match the conical section of the torch tube to form an annular gap with a gap size, d (27). An outer flow is injected into the annular gap (17). The conical end of the injection tube and the conical section of the torch tube are configured to increase the outer gas velocity. Accelerated gas velocity in the flow path can be in the range of 10 to 100 m/s, and ideally between 20-60 m/s, and optimally around 40 m/s. High outer gas flow velocities provide better cooling of the torch even with low overall inlet gas flow rates. This prevents thermal damage to the torch components and saves gas, and therefore, reduces operation costs. The plasma gas and the sample particles are delivered from the inlet 16 of the injector tube 2. The injector tube 2 is concentrically placed inside the tubular component 1 such that posterior end of the injector tube 2 does not touch inner walls of the tubular component 1 and there is a gap between the injector tube 2 and the tubular component as shown in
Referring to
A load coil (3) is provided around the outer surface of the torch tube 1 to generate an inductively coupled plasma (ICP) (4), which has an induction zone (5). The energy induced by the load coil forms a plasma in the torch cavity (6). A distal injector fixture (7) and a proximal injector fixture (8) are used to hold the injector tube and position it concentrically within the torch tube and within the load coil. In one embodiment, in order to collect particles, a collection chamber (25) can be connected to the conical torch by a mounting plate (24) to collect processed particles. The present conical torch provides high plasma temperatures within an induction zone that is closer to a central axis of the conical torch in comparison with conventional torches.
In an example, the torch tube 1 and the injector tube 2 may be made up of dielectric materials, such as quartz, alumina, boron nitride, silicon nitride, silicon carbide, zirconia, yttria, ceria, beryllium oxide. In another example, the tubular component 1 and the injector tune 2 may be made up of stainless steel.
In operation, a plasma gas and sample particles are injected into the torch tube 1 via the injector tube 2. In an example, the sample particles may be in powder form, such as metal powders, ceramic powders etc. In another example, the sample particles may be small pieces of metals, particle suspended in a liquid etc. In an example, the plasma gas may be one of helium, argon, oxygen, hydrogen, nitrogen, air, or a mixture of these and other gases.
The plasma gas and the sample particles are injected into the inlet 16 of the injector tube 2. The plasma gas and the sample particles are subsequently delivered in the torch tube 1 by the outlet 18 of the injector tube 2. A load coil 3 is wound around the torch tube 1 to energize the plasma gas. In operation, the RF inductively coupled torch is held upstream using a torch holder so that the torch tube can be positioned within the load coil 3. Further, the plasma gas in the conical tube portion is energized by passing radio frequency alternating electric current through the load coil 3 which heats up the plasma gas in the conical tube portion to generate an Inductively coupled plasma, ICP (of the conical torch), 4, with an Induction zone (of the conical torch), 5, where all the energy is induced from the load coil into the plasma. The plasma is formed in the torch cavity, 6.
The plasma plume 4 heats up the sample particles and the samples particles are molten. The molten sample particles cool and solidify in flight just after existing the plasma plume. These solidified sample particles assume a smooth uniform spherical shape and are called as processed sample particles. Thus, the processed sample particles are outputted from the conical tube portion.
The sample particles are processed to produce smooth, dense, spherical powdered samples particles from rough, porous, irregularly shaped sample particles. Due to the high temperatures of the plasma plume 4 (typically more than 10,000 Kelvin), the sample particles are melted and processed. A wide range of material, such as metals, ceramics can be processed using the RF-ICP torch of the present subject matter. The processed sample particles have advantages, such as improved flowability, consistent texture, and consistent sizes of particles. Thus, the processed sample particles enable a faster and more consistent rate of sample particle delivery in applications such as the spraying of thermal barrier coatings and various other industrial uses. Further, the processed particles ensure higher packing efficiency which results in resulting in higher quality end products.
Further, due to high temperature achieved in the RF-ICP torch of the present subject matter, sample particles having higher melting points can be processed in the RF-ICP torch of the present subject matter. Further, due to smaller diameter at one end of the conical tube portion, the induction zone 5 is created closer to the sample particles resulting in efficient processing of the sample particles.
In an example, there may be two gas paths through which the plasma gas may be introduced into the tubular component 1. A first inner gas flow path 16 may be used to introduce the plasma gas through the inlet of the injector tube 2. It should be noted that the sample particles are also introduced through the first inner gas flow path 16. Further, an outer gas flow path 17 may also be created between the injector tube 2 and the tubular component 1. The plasma gas may also be introduced through the outer gas flow path 17. The injector tube 2 has a flared portion 40 that coincides with the conical portion of the torch tube. The flared portion 40 prevents the plasma gas from directly going into the plasma plume 4 which would otherwise extinguish the plasma plume 4.
The swirling flow of the plasma gas results in some radial spreading which results in a wider particle size distribution but improves the temperature of the plasma plume. The axial or swirling manner introduction of the plasma gas may be used based on the application and usage of the RF-ICP torch.
In operation of the present radio-frequency inductively coupled plasma (RF-ICP) torch as shown in
In another example, the processed sample particle may be deposited on a substrate placed in front of the RF-ICP torch to thermally coat the substrate with the processed sample particle.
The range of parameters used in the RF-ICP torch of the present subject matter: The power in the load coil 3 can be maintained between 5 kW to 250 kW, preferably between 20-50 kW, more precisely between 25-45 kW. Oscillation frequency (i.e., radio- or microwave frequency) of the alternating current in the load coil 3 can be between 400 kHz to 100 MHz, preferably between 3 MHz to 40 MHz. The pressure can be between atmospheric pressure (1 bar) to partial vacuum. The plasma gas flow rate through the outer gas flow path 17 can be between 20-150 litres/minutes (L/min), preferably between 50-100 L/min. For a RF inductively coupled torch operating at 45 kW, the optimum value is 85 L/min.
The RF-ICP torch 100 was optimized by following a single parameter optimization procedure for the parameters β, α, d, and Ri, in that order (see
Titanium carbide was chosen as the sample particle due to its high specific heat capacity and latent heat of melting relative to commonly processed materials, which would test the limits of the torch.
The optimization begins with parameter β, which was varied from 0-45° while parameters α, d, and IR; were held fixed at 10°, 2 mm, and 16.8 mm, respectively. It was found that a value of 45° caused significant radial spreading in the processed sample particles due to centrifugal forces generated by the plasma gas introduced through the outer gas flow path. This causes particles to prematurely exit the plasma and not experience complete melting, resulting in lower spheroidization ratios (see
Next, parameter d was varied from 1-2.5 mm. It was found that parameter d did not have a significant impact on spheroidization ratios, but more so on the temperature of walls of RF-ICP 100, since it determines the thickness and velocity of the plasma gas introduced through the outer gas flow path 17. It was found that lower values of d, while producing a higher velocity flow which provided greater convective cooling to the outer wall of the RF-ICP torch 100, also decreased the thickness of the layer of plasma gas introduced through the outer gas flow path. A thinner layer of plasma gas allows the plasma to get closer to the outer wall of RF-ICP torch 100 and increases the amount of heat transferred to it. Additionally, it was found that smaller values of d resulted in flow instabilities due to the higher velocity flow, so this was taken into consideration. A value of 1.5 mm provided the optimum balance between these trade-offs by minimizing torch wall temperature and flow instabilities as show in
Finally, parameter Ri was varied from 12.8-16.8 mm. It was found that a smaller Ri, which reduces the internal volume for the RF-ICPtorch, also causes an increase in velocity and temperature due to a more concentrated plasma. This increases the heat transfer to particles, boosting spheroidization ratios as show in
However, Ri cannot be decreased indefinitely, as doing so would increase the temperature of walls of RF-ICP torch beyond the set limit of 1000 K. Increasing the flow rate of plasma gas introduced through the outer gas flow path 17, to combat is not desired since a goal of this design is to reduce gas consumption. An Ri of 13.8 mm was found to be the minimum value that prevented the wall temperature from exceeding the temperature limit.
The optimization procedure yielded values of 22.5°, 10°, 1.5 mm and 13.8 mm for parameters β, α, d, and Ri, respectively.
The performance of the present conical RF-ICP torch was compared to a conventional industrial scale plasma torch of a similar power level, specifically the conventional plasma torch named, Tekna PL-50. This conventional torch typically operates at flow rates of 120 L/min for the outer gas and 30 L/min for the intermediate gas (45° swirl angle). For both the present conical RF-ICP torch and the conventional torch, the plasma gas flow rate was varied between 5-7 L/min while the sample particle feed rate and power were held at 20 g/min and 45 kW, respectively. The present torch outperformed the conventional torch at all flow rates, particularly at lower ones, as seen in
The overall length of the plasma in the conical torch 100 is made shorter (i.e., ˜540 mm for the conical torch 100 versus ˜750 mm for the conventional plasma torch) due to the more concentrated plasma and fewer coil turns. Additionally, as mentioned previously when optimizing parameter β, the swirling flow present in the conical torch produces some radial spreading of sample particle which causes them to enter the hotter parts of the plasma while also decreasing the thermal loading effect. However, the spreading of particles can also result in a wider particle size distribution of the end product collected in the collection chamber due to more variability in the heat transferred to particles. The spreading is proportional to the outer gas swirl velocity angle β, so if desired β can be tuned to eliminate this.
For both the conical torch and the conventional torch, the torch power was varied between 22.5-45 kW while the sample particle feed rate and plasma gas flow rate were held at 20 g/min and 7 L/min, respectively. The conical torch was found to outperform the conventional torch at all power levels as show in
For both the conical torch and the conventional torch, the sample particle feed rate was varied between 15-25 g/min while the power and carrier gas flow rate were held at 45 kW and 7 L/min, respectively. The conical torch was found to outperform the conventional torch at all feed rates as show in
Another point of comparison between RF-ICP torch 100 and the conventional torch is the gas consumption, specifically when argon is used as the plasma gas. The conical torch operates at a plasma gas flow rate of 85 L/min (from outer gas flow path) while the conventional torch operates at 120 L/min. Furthermore, the conical torch does not have an intermediate gas flow, which consumes 30 L/min for the conventional torch. The net result is 42% in gas savings for the conical torch.
In summary, the present conical torch can produce a higher quantity of processed particles (i.e. higher spheroidization ratios) at a faster rate while operating with lower gas and energy consumption.
What has been described and illustrated herein are examples of the present disclosure along with some of its variations. The terms, descriptions and figures used herein are set forth via illustration only and are not meant as limitations. Many variations are possible within the spirit and scope of the subject matter, which is intended to be defined by the following claims and their equivalents in which all terms are meant in their broadest reasonable sense unless otherwise indicated.
| Number | Date | Country | Kind |
|---|---|---|---|
| 63088170 | Oct 2020 | CA | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CA2021/000088 | 10/4/2021 | WO |
