Abstract:
A particle generator is able to generate pure particles for solid or liquid materials with melting points over several hundred degrees Celsius. The material is heated to generate the vapor in a small chamber. Heated nitrogen or some inert gas is used as the carry gas to bring the mixture into a dilution system. As the super saturation ratio of the material is large enough and over a critical value, particles are formed in the dilution system by homogenous nucleation, and grown in the same dilution system as well. The different size distributions and concentrations of the particles can be obtained by varying dilution parameters, such as residence time and dilution ratio.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to particle generators. 
     2. Background Art 
     To generate particles, the traditional aerosol atomizer needs first to mix material in some liquid. Then, using compressed air flow through a nozzle, the system generates liquid drops including the selected material. Those drops move with air into some devices, for example, a diffusion dryer. Liquid is removed by the dryer. Finally, particles are left in the air stream. 
     However, if there is no suitable liquid available, or some residual included in the liquid, or the device is not able to remove the liquid completely, pure particles for the material cannot be generated. Water is the most popular solvent with the current technology for particle generation with the traditional atomizer, but many residual particles are contained in the pure water, even for HPLC grade water, or ultra high purity water. Therefore, there are many limitations on the ability of the traditional atomizer to generate pure particles. 
     Background information may be found in U.S. Pat. Nos. 4,264,641; 4,410,139; 4,746,466; 4,795,330; 6,331,290; and 6,764,720. 
     For the foregoing reasons, there is a need for an improved pure particle generator. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved pure particle generator. 
     This particle generator is able to generate pure particles for solid or liquid materials with melting points over several hundred degrees Celsius. The material is heated to generate the vapor in a small chamber. Heated nitrogen or some inert gas is used as the carry gas to bring the mixture into a dilution system. As the super saturation ratio of the material is large enough and over a critical value, particles are formed in the dilution system by homogenous nucleation, and grown in the same dilution system as well. The different size distributions and concentrations of the particles can be obtained by varying dilution parameters, such as residence time and dilution ratio, etc. 
     In more detail, the pure particle generator generates pure particles for the selected material, and can generate solid and liquid pure particles. When the mixture including the selected material vapor is cooled down and diluted in the dilution system, and if super saturation ratio for the selected material is obtained and over a critical value, pure particles are formed by homogenous nucleation. The homogenous nucleation is defined as the nucleation of vapor on embryos essentially composed of vapor molecules only, in the absence of foreign substances. 
     Different size distributions and concentrations of the particles are obtained by adjusting the chamber temperature, and the dilution parameters for the dilution system, where dilution parameters are defined as dilution ratio, dilution air temperature, residence time, etc. In a preferred embodiment, a two-stage dilution system is used. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a pure particle generator made in accordance with a preferred embodiment of the invention; and 
         FIG. 2  is a block flow diagram illustrating the working principle of the pure particle generator. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to  FIG. 1 , the pure particle generator is generally indicated at  10 . Pure particle generator  10  consists of orifices  12 ,  14 , high efficiency particulate (HEPA) filter  16 , heating cartridge  18 , carry gas  20 , temperature controllers  22 ,  24 , stainless steel block  26  and cover  28 , thermocouples  40 ,  42 , ejectors  50 ,  52 , stainless steel tunnel  54 , etc. 
     Nitrogen or some inert gas is used as the carry gas  20 . The carry gas  20  should be chosen so as to avoid chemical reaction between the material and the carry gas under the high temperature. The flow rate of the carry gas  20  is restricted by orifice  12 , and flow rate of carry gas  20  is controlled by vacuum generated by the first ejector  50  in the two-state dilution tunnel  70 . Under a certain carry gas pressure, when vacuum generated by ejector  50  is over a critical value, the critical flow is obtained over orifice  12 . As a result, higher vacuum will not change the flow of carry gas. 
     A high efficiency particulate (HEPA) filter  16  is installed upstream of orifice  12  to remove the particles from the carry gas  20 . In this way, there is no contamination on the generated particles. 
     The carry gas  20  is heated by a heating tube  72  before entering the heated stainless steel block  26 . There is a heating tape wrapped on the heating tube  72 , and heating tube  72  is insulated by high temperature insulation. The material of heating tube  72  could be either metal or non-metal. The tube material should be selected so as to avoid chemical reaction between the carry gas  20  and the tube material. The temperature of the carry gas  20  is measured by a thermocouple  40 , which is installed in the heated tube  72  and able to measure the gas temperature directly. The temperature for carry gas  20  is set according to the physical property of the material, and controlled by temperature controller  22 . The purpose for heating the carry gas is to avoid particle formation in the stainless steel chamber when the vapor of the selected material is cooled down. At most of conditions, temperature is set slightly lower than the stainless steel chamber temperature. 
     The outlet of heating tube  72  is connected to stainless steel block  26  to allow the carry gas  20  to flow into the chamber in the stainless steel block  26 . There are one or multiple heating cartridges  18  immersed in the block  26 . The thermocouple  42  extrudes into the chamber, and measures the gas temperature in thechamber. The gas temperature in the chamber is controlled by temperature controller  24 , which controls on/off for the heating cartridges  18  in the block  26 . As a result, the expected gas temperature in the chamber can be obtained. 
     The selected material  74  in a container  76  sits in stainless steel block  26 . Container  76  is made of high temperature material, and does not react to the selected substance (for particles) under high temperatures. The material  74  in the heated block  26  can be installed and removed by opening the stainless steel cover  28 . The stainless steel cover  28  is secured on the block  26  by bolts. To avoid leaks at the contact surface between the cover  28  and the block  26 , a metal gasket is installed between cover  28  and block  26 . An outlet tube  80  is welded on the cover  28 . The mixture of the carry gas  20  and the substance vapor can flow out the chamber through outlet tube  80 . To minimize the heat transfer from heated block  26  to the ambient surroundings, heated block  26  is insulated with high temperature insulation  84 . As well, outlet tube  80  is insulated with insulation  82 . 
     The temperature of the mixture of the carry gas  20  and the vapor is set higher than the melt point of the material  74  and able to provide the high enough super saturation ratio for homogenous nucleation. The saturation ratio is defined as the ratio of the partial pressure of material at temperature T and the saturation vapor pressure of the same material in equilibrium with its liquid phase at the same temperature. When the saturation ratio is larger than 1.0, this is called the super saturation. 
     The carry gas  20  flows through the chamber of the heated block  26 , and brings the mixture into a dilution system. Within the dilution system, the residence time and dilution ratio can be controlled. In the illustrated exemplary implementation, an ejector-type two-stage dilution system is employed. 
     The inlet of the first ejector  50  of the two-stage dilution tunnel  70  is connected to the outlet of the chamber on the stainless steel cover  28 . The length of the connection tubing  80  should be as short as possible. The tubing  80  is wrapped with insulation  82  to minimize the heat transfer with the ambient air. 
     The two-stage dilution tunnel  70  has first and second ejectors  50 ,  52 . An ejector is operated by the particle free compressed air. When compressed air flows through an annular orifice or a nozzle in the ejector, vacuum is generated at the inlet of the ejector. The vacuum sucks in the mixture of the carry gas  20  and vapor into the ejector. Inside of the ejector, compressed air mixes with the mixture mentioned above. As a result, the mixture (or sample) is cooled down and diluted. Both ejectors  50 ,  52  work under the same work principles, and such ejectors are commercially available. 
     The outlet of the first ejector  50  is connected to the cone  90  of a stainless steel tunnel  54 . There are two cones on tunnel  54 . The first cone  90  on the tunnel  54  and connected to ejector  50  distributes the flow from ejector  50  to reduce the flow velocity difference in the tunnel  54  at the cross-section of the tunnel. The second cone  92  is used to vent the extra flow from the ejector  50 . Both cones  90 ,  92  are welded on the tunnel  54 . There are several sampling ports  94  on the tunnel  54 . Under the same flow rate from ejector  50 , different residence times can be obtained by using different sampling ports  94 . By changing the sample position on the stainless steel tunnel  54 , the residence time of the dilution tunnel is changed. When the sample port location is far from the sample inlet at the tunnel, the residence time is increased. When one sample port is chosen, the others will be blocked. 
     The second ejector  52  is connected to one of sample ports  94 , which is chosen per the expected residence time at the certain flow rate. The functions of the second ejector  52  are to freeze the change of particle concentration and size distribution in the sample flow by diluting. 
     One small orifice  14  is in the front of the second ejector  52 . Orifice  14  restricts sample flow from the tunnel  54 . As a result, the expected dilution ratio on the second stage can be obtained by adjusting compressed air pressure on the second stage ejector  52 . The outlet of ejector  52  is connected to a tee  98 , having one port connected to the instrument, and the other port being for venting the extra flow from the ejector  52 . The flow including pure particles moves into the instrument. 
     As mentioned above, the dilution ratio on an ejector is controlled by adjusting the compressed air pressure. Normally, when compressed air pressure is higher, more dilution air is provided to the ejector. Due to small orifice  14  upstream from ejector  52  to restrict the flow, the sample flow into ejector  52  is changed slightly or non-changed if the critical flow has been obtained. By this approach, the dilution ratio is increased while higher compressed air pressure is provided. In the opposite, a lower dilution ratio is obtained. 
     In operation, the pure particles are formed in the first stage of the dilution tunnel  70  by homogenous nucleation while the mixture of the vapor and carry gas from the heated chamber is diluted and cooled down. To have the homogenous nucleation, gas temperature in the chamber of the stainless steel block  26  and the dilution ratio in the first stage can be adjusted to provide high enough super saturation ratio. 
     It is appreciated that different concentrations and size distributions of the pure particle can be obtained by making appropriate adjustments. 
     One possible adjustment is to adjust the temperature set point in the chamber. With the higher temperature, more vapor of material  74  will be generated. When the vapor is cooled down in the dilution system, higher super saturation ratio is obtained. As a result, more particles could be formed in the tunnel for the first ejector. In the opposite case, less particles may be formed. When the super saturation ratio cannot be obtained or be over a critical value due to the lower gas temperature of the chamber, no particles may be formed. 
     Another way to make an adjustment is to adjust the residence time on the two-stage dilution tunnel  70 . With the longer residence time in the dilution tunnel, more particles are formed and the size distributions of the particles have potential to move toward larger size ranges. There are mainly two approaches to changing the residence time. The first approach is to change the flow in ejector  50 . With higher compressed air pressure in ejector  50 , more dilution air flows through the ejector. At the same sample location, the residence time is shorted. In the opposite case, the longer residence time will be obtained. In the second approach to changing the residence time, the sampling location is moved on the tunnel. By moving the sampling port  94  far from ejector  50  without changing the flow rate from ejector  50 , the longer residence time can be obtained. In the opposite case, the shorter residence time is obtained. 
     In another possible adjustment, the dilution ratio may be increased in the second stage. By increasing the dilution ratio in the second stage, the concentration of the particles in the sample flow is reduced. In the opposite case, the concentration is increased. 
     With reference to  FIG. 2 , a block diagram illustrates the working principle of the pure particle generator. According to the working principle, vapors are generated under high temperature (block  110 ). Vapor mixes with carry gas and flows into the dilution system (block  112 ). Vapor is cooled down and diluted in the dilution system (block  114 ). Particles are formed by homogenous nucleation if super saturation ratio is over a critical value (block  116 ). By adjusting the chamber temperature, dilution ratio, and residence time, different size distributions and concentrations of pure particles are obtained. 
     While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.