1. Field of the Invention
The present invention relates to aerosol sampling technology, and more particularly to a nozzle plate containing multiple micro-orifices for use in a cascade impactor and a method for manufacturing the same.
2. Description of the Related Art
The Micro-Orifice Uniform Deposit Impactor (MOUDI) invented by MSP Corporation has been widely used for size-classified aerosol sampling. Each stage of the MOUDI consists of a nozzle plate with a plurality of nozzles and an impaction plates to collect particles of a specific size range. By decreasing the nozzle diameter and increasing the air jet speed in the nozzle from the top to the bottom stages, the MOUDI is able to collect particles of subsequently smaller size ranges. In a 10 stage MOUDI, the cutoff aerodynamic diameter of the stage 0 to 10 is 18, 10.0, 5.6, 3.2, 1.8, 1.0, 0.56, 0.32, 0.18, 0.1, 0.056 μm, respectively, and there is a final after filter to collect particles smaller than 0.056 μm. To classify very small particles, the nozzle plates of the last 4 impaction stages, or stage 7 to 10, use 900-2000 micro-orifices with the diameter ranging from 140 to 52 μm to collect particles ranging from 0.32 to 0.056 μm in diameter.
U.S. Pat. No. 6,431,014 disclosed an improved MOUDI design with a series of differential pressure sensors for measuring the pressure drop across the nozzle plates. Additionally, the influence of particle accumulation and blockage in the micro-orifices on the performance of the MOUDI is also briefly discussed. The clogged orifices may cause the cut-point of the impactor to change which leads to measurement errors. The dust accumulation problem in the nozzle can be eliminated by periodic cleaning. However, an improper cleaning method, such as high intensity ultrasonic cleaning, may damage the nozzle plates whose wall thickness to define the nozzle diameter is very thin.
Ji et al. (2006) observed the 6th to 8th stage nozzle plate of a 8-stage MOUDI by using an electron microscope, and the results were published in a journal paper (Ji, J. H., Bae, G. N., Hwang, J., 2006. Observation and evaluation of nozzle clogging in a micro-orifice impactor used for atmospheric aerosol sampling, Particulate Sci. Technol. 24: 85-96). In the study, nozzle clogging caused by particle deposition in the nozzle was observed. The collection efficiency curves were shifted to that corresponding to smaller orifice sizes, and the 50% cutoff sizes were much smaller than those specified by the manufacturer for the three stages with nozzles less than 400 μm in diameter. The pressure drops across the clogged nozzles were also higher than the nominal values given by the manufacturer.
The inventor of the present invention used an optical microscope to observe the micro-orifices of the nozzle plate of the last several stages of the MOUDI. An uneven inner surface of the micro-orifices was observed (see
It is the main object of the present invention to provide a nozzle plate with multiple micro-orifices for a cascade impactor and a method for manufacturing the same, wherein the micro-orifices of the nozzle plate have a smooth inner surface, avoiding clogging of particles in the nozzle plate.
It is another object of the present invention to provide a nozzle plate for a cascade impactor and a method for manufacturing the same, wherein the uniform wall thickness and sturdy structure of the micro-orifices facilitate cleaning by an ultrasonic cleaner.
To achieve these and other objects of the present invention, a nozzle plate for a multi-stage cascade impactor comprises a plate body, and a plurality of micro-orifices formed on the plate body and cutting through top and bottom sides of the plate body. Each micro-orifice has a smooth inner surface and a diameter expanding gradually in direction from the bottom side of the plate body toward the top side thereof. Further, the number of the micro-orifices is preferably within 50-10000, and the diameter of each micro-orifice at the bottom side of the plate body is within 45-410 μm. The nozzle plate further comprises a plurality of annular protrusions protruded from the bottom side of the plate body around each micro-orifice.
To achieve these and other objects of the present invention, a method for making a nozzle plate containing multiple micro-orifices comprises the steps of:
(1) depositing a seed layer on a substrate; (2) coating the seed layer with a layer of first photoresist, radiating UV light through a first mask onto the first photoresist, and then developing the first photoresist; (3) etching the seed layer and removing the first photoresist, so as to form a plurality of through holes on the seed layer that cut through top and bottom sides of the seed layer; (4) coating a sacrificial layer on the substrate and the seed layer; (5) depositing a metal mask film on the sacrificial layer; (6) coating a layer of second photoresist on the metal mask film, radiating UV light through a second mask onto the second photoresist, and then developing the second photoresist; (7) etching the metal mask film and removing the second photoresist, so as to form a plurality of protrusions on the sacrificial layer; (8) etching the sacrificial layer until the substrate and the seed layer are exposed to the outside; (9) electroplating a metal material onto the seed layer; and (10) removing the substrate, the seed layer and the sacrificial layer. Further, the metal material used during step (9) is a mix of nickel and cobalt.
Referring to
(1) depositing a seed layer 22 on a glass substrate 20, as shown in
(2) coating the seed layer 22 with a layer of first photoresist 24, radiating UV light through a first mask 26 onto the first photoresist 24, and then developing the first photoresist 24, as shown in
(3) etching the seed layer 22 and removing the first photoresist 24, as shown in
(4) coating a sacrificial layer 28 on the glass substrate 20 and the seed layer 22, as shown in
(5) using copper or chromium to deposit a metal mask film 30 on the sacrificial layer 28 by a sputtering process, an evaporation process or a chemical vapor deposition process, as shown in
(6) coating a layer of second photoresist 32 on the metal mask film 30, radiating UV light through a second mask 34 onto the second photoresist 32, and then developing the second photoresist 32, as shown in
(7) etching the metal mask film 30 and removing the second photoresist 32, as shown in
(8) etching the sacrificial layer 28 until the glass substrate 20 and the seed layer 22 are exposed to the outside, as shown in
(9) electroplating a metal material 36 onto the seed layer 22 to a desired thickness D3, as shown in
(10) removing the substrate 20, the seed layer 22 and the sacrificial layer 28, thereby obtaining a nozzle plate 10, as shown in
Referring to
Further, the smooth inner surfaces of the micro-orifices 14 allow particles to pass therethrough smoothly without clogging the micro-orifices. Further, the uniform wall thickness and sturdy structure of the micro-orifices 14 facilitate cleaning by an ultrasonic cleaner and improve the convenience of use and the sampling quality. Further, subject to different desired cut-off aerodynamic diameters, the number of the micro-orifices 14 of the nozzle plate 10 and their final orifice diameter can be 900/140 μm, 900/90 μm, 2000/55 μm, 2000/52 μm, 980/49 μm, 1650/450 μm or 2000/55 μm. Preferably, the number of the micro-orifices 14 is within 50-10000, and the diameter is within 45-410 μm.
Although particular embodiments of the invention have been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.
Number | Date | Country | Kind |
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99105869 | Mar 2010 | TW | national |