Patent Publication Number: US-2011070110-A1

Title: Piezoelectric micro blower

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a piezoelectric micro blower that is capable of transporting a compressible fluid, such as air. 
     2. Description of the Related Art 
     A piezoelectric micro pump is used, for example, for a pump for transporting water for cooling a small-sized electronic device, such as a notebook computer, and a pump for transporting fuel in a fuel cell. On the other hand, a piezoelectric micro blower can be used as a blower that replaces a fan for cooling a CPU or other heat generating electronic component or as a blower to supply oxygen that is required for electric power generation in a fuel cell. A piezoelectric micro pump/piezoelectric micro blower is a device that includes a diaphragm that is deformed due to flexion when a voltage is applied to a piezoelectric element. The piezoelectric micro pump/piezoelectric micro blower has the advantages of a simple structure, a low-profile body, and low power consumption. 
     When an incompressible fluid, such as a liquid, is transported, it is common to provide check valves, each of which is made of a soft material, such as rubber or resin, at an inlet port and an outlet port. In addition, it is common to drive a piezoelectric element at a low frequency, such as tens of hertz. Although the maximum displacement is obtained when a piezoelectric element is driven approximately at the resonance frequency of a diaphragm (a first-order resonance frequency or a third-order resonance frequency), a check valve fails to operate in response thereto because the resonance frequency has a high frequency in the kHz range. Therefore, it is preferable to use a piezoelectric micro blower that does not include any check valve to transport a compressible fluid. 
     A flow generation device is disclosed in Japanese Examined Patent Application Publication No. S64-2793. The flow generation device disclosed therein includes a substrate that includes a pressurizing chamber that is filled with a fluid, a nozzle plate that includes nozzles that are directed to the pressurizing chamber, an opening, and an electric vibrator that is attached to the nozzle plate such that the nozzles are located approximately at the center of the opening. The nozzle plate and the electric vibrator are attached to the substrate. An alternating current that has a frequency that is close to the resonance frequency of the electric vibrator is supplied to the electric vibrator. When such a structure is used, a check valve can be omitted. In addition, it is possible to increase the flow rate by driving the vibrator at a high frequency. In a structure illustrated in FIG. 5 of Japanese Examined Patent Application Publication No. S64-2793, an air chamber into which air flows is provided in front of the nozzle plate. A fluid that is ejected through the nozzles sucks ambient air into the air chamber. The fluid is discharged through an exit port together with the ambient air. However, since the open area of the air chamber is relatively large, the pressure energy of the fluid that is ejected through the nozzles is dissipated into the periphery of the air chamber. For this reason, the structure disclosed in Japanese Examined Patent Application Publication No. S64-2793 has a disadvantage in that it is difficult to increase the flow rate through the exit port. 
     A micro blower that includes an ejection unit that sucks air from the outside and ejects the air, a cover that includes an exit port to discharge the air ejected from the ejection unit, and a base unit that is connected to the ejection unit is disclosed in a Japanese Unexamined Patent Application Publication No. 2005-113918. The micro blower includes an ejection plate that includes a suction hole and an ejection hole. A vibration plate that is provided with a magnetic sheet is attached to the back of the ejection plate. A pressurizing chamber is provided between the ejection plate and the vibration plate. A coil is used to vibrate the magnetic sheet. Ejection airflow is generated from a cavity as a result of the vibration of the magnetic sheet. The airflow sucks ambient air into a cover cavity that is located in front of the ejection plate. The fluid ejected is discharged through the exit port together with the ambient air. Since the open area of the cover cavity is greater than that of the pressurizing chamber, the pressure energy of the fluid that is ejected through the ejection hole is dissipated into the cover cavity. For this reason, the structure disclosed therein has the same disadvantage as that of the structure disclosed in the Japanese Examined Patent Application Publication No. S64-2793, that is, it is difficult to increase the flow rate through the exit port. 
     SUMMARY OF THE INVENTION 
     To overcome the problems described above, preferred embodiments of the present invention provide a piezoelectric micro blower which is capable of transporting a compressible fluid efficiently without using a check valve and achieving an increased flow rate. 
     A preferred embodiment of the present invention provides a piezoelectric micro blower including a blower body, a driver including a peripheral portion that is fixed to the blower body and a piezoelectric element, and a blower chamber provided between the blower body and the driver, wherein a voltage is applied to the piezoelectric element to cause the driver to vibrate due to flexion to transport a compressible fluid. The piezoelectric micro blower preferably includes a first wall of the blower body, the blower chamber being provided between the driver and the first wall, a first opening that extends through the first wall, the inside of the blower chamber being in communication with the outside of the blower chamber through the first opening, a second wall that is provided at a side opposite the blower chamber with the first wall being provided between the second wall and the blower chamber, the second wall being spaced at a distance from the first wall, a second opening that extends through the second wall, a central space that is provided between the first wall and the second wall, has an open area that is greater than the first opening and the second opening but less than the blower chamber, and is in communication with the first opening and the second opening, and an inlet passage that includes an outer end that is in communication with the outside of the piezoelectric micro blower and an inner end that is connected to the central space, wherein a bottleneck portion that has a passage area that is less than that of the inlet passage is provided in the inlet passage. 
     When a voltage is applied to the piezoelectric element to cause the driver to vibrate due to flexion, a fluid flows at high speed from the first opening to the second opening as the driver becomes displaced. With this arrangement, it is possible to suck a fluid from the inlet passage into the central space. That is, it is possible to suck a fluid from the inlet passage into the central space not only when the driver is displaced into a convex shape that is oriented downward but also when the driver is displaced into a convex shape that is oriented upward. The fluid sucked from the inlet passage and the fluid pressed out of the blower chamber flow into each other. The confluent fluids are discharged together through the second opening. Therefore, it is possible to obtain a discharging flow, the quantity of which is greater than the displacement volume of the driver. The inlet passage is connected to the central space between the first opening and the second opening. Since the inlet passage is not directly connected to the blower chamber, the inlet passage is less susceptible to a pressure change in the blower chamber. Even though a check valve is not provided, a fluid that flows at high speed from the first opening to the second opening does not flow backward into the inlet passage. Thus, it is possible to effectively increase the flow rate. The open area of the central space is preferably greater than the first opening and the second opening but less than the blower chamber. The fluid entering from the inlet passage temporarily accumulates in the central space. Entrained by the flow of a fluid that is blown out through the first opening, the fluid that has accumulated in the central space is discharged through the second opening together with the fluid blown out through the first opening. 
     When the driver vibrates up and down due to bending as described above, a fluid is sucked from the inlet passage into the central space by utilizing the pressure energy of a fluid pressed out of the blower chamber. The fluid sucked from the inlet passage and the fluid pressed out of the blower chamber flow into each other to be discharged through the second opening. If the inlet passage was directly connected to the central space, the pressure energy of the fluid in the central space would be dissipated into the inlet passage. To avoid such pressure energy dissipation, in a preferred embodiment of the present invention, a bottleneck portion having a passage area that is less than that of the inlet passage is preferably provided in the inlet passage. Since the bottleneck portion is provided in the inlet passage, the pressure energy that is present inside the central space is much less likely to be dissipated into the inlet passage. Accordingly, it is possible to efficiently direct the pressure energy toward the second opening, thereby achieving an increase in the flow rate of a fluid discharged through the second opening. 
     Preferably, the first opening is provided at a portion of the first wall that faces an area of the approximate center of the driver. With this preferred structure, since the first opening is arranged such that it faces an area of approximate the center of the driver, which is the area at which the displacement of the driver is the greatest, it is possible to obtain the maximum flow rate. In addition, preferably, the second opening provided at a portion of the second wall that faces the first opening. With this preferred structure, it is possible for a fluid ejected at high speed from the first opening to pass through the central space and be discharged through the second opening with reduced resistance. 
     Preferably, the bottleneck portion is provided at a region in which the inlet passage and the central space are connected. The bottleneck portion may be provided anywhere in the inlet passage. However, it is preferable to provide the bottleneck portion at a position close to the central space because the pressure energy that is present inside the central space is less likely to be dissipated into the inlet passage. Thus, it is possible to efficiently direct the pressure energy toward the second opening. 
     Preferably, the passage area of the bottleneck portion should gradually decrease in a direction of flow from the inlet passage to the central space. Since the average pressure in the central space is less than that in the inlet passage, a pressure gradient exists therebetween. Although pressure loss occurs in a flow channel because of friction between a fluid and wall surfaces, if no bottleneck portion is provided therein, vena contracta would be produced in the vicinity of the entrances to the central space because the pressure of the central space is less than the pressure decrease caused by the pressure loss. A vortex would develop in the vicinity of the vena contracta, which would result in loss. For this reason, the flow rate would decrease. To reduce such a flow loss, the bottleneck portion is preferably provided in the inlet passage. The passage area of the bottleneck portion preferably gradually decreases in a direction of a flow from the inlet passage to the central space. Thus, it is possible to suppress a vortex of a fluid that flows from the inlet passage into the central space from developing, which would thereby further reduce the average pressure in the central space. Therefore, the quantity of a fluid sucked into the central space is increased, which makes it possible to further increase the flow rate of a fluid discharged through the second opening. 
     Preferably, the inlet passage should include a plurality of passages extending in radial directions from the central space. In addition, preferably, the outer ends of the plural passages of the inlet passage should be respectively in communication with inlet ports. With this preferred structure, it is possible to ensure that the inlet passage has a sufficiently large passage area, which advantageously reduces the flow channel resistance. Therefore, it is possible to further increase the flow rate. 
     Preferably, the open area of the central space should be arranged such that the portion of the first wall that faces the central space vibrates when the driver vibrates. The vibration of the first wall acts to increase the quantity of a fluid that is caused to flow by the driver. Since the quantity thereof increases because of the displacement of the first wall, it is possible to achieve a further increase in the flow rate. It is more preferable that the portion of the first wall that faces the central space should vibrate in a sympathetic manner as the driver vibrates. That is, it is possible to cause the first wall to vibrate in a sympathetic manner in response to or by following the displacement of the driver by making the natural vibration frequency of the portion of the first wall that faces the central space close to the vibration frequency of the driver. When the first wall vibrates in a sympathetic manner, it is not always necessary that the first wall and the driver must vibrate in the same resonance mode. For example, both the first wall and the driver may vibrate in the first-order mode or a higher-order mode (e.g., third-order mode). Alternatively, one of the first wall and the driver only may vibrate in the first-order mode, while the other may vibrate in a higher-order mode. 
     The driver according to a preferred embodiment of the present invention may preferably be, for example, a unimorph-type driver that includes a piezoelectric element that expands and contracts in a planar direction and that is attached to one surface of a diaphragm, e.g., a resin plate or a metal plate, for example, a bimorph-type driver that includes piezoelectric elements that expand and contract in opposite directions and that are attached respectively to both surfaces of a diaphragm, or a bimorph-type driver that includes a multilayer piezoelectric element that is deformed due to flexion by itself and that is attached to one surface of a diaphragm. Alternatively, for example, the entire body of the driver may preferably be defined as a multilayer piezoelectric element. The shape of the piezoelectric element may be a disc, a rectangle, or a ring, for example. An intermediate plate may preferably be sandwiched between the piezoelectric element and the diaphragm. The structure may be modified in various ways as long as the driver vibrates in the direction of thickness due to flexion when an alternating voltage, which is either a sinusoidal-wave voltage or a rectangular-wave voltage, is applied to the piezoelectric element. 
     It is preferable to drive the driver including the piezoelectric element in a first-order resonance mode because the greatest amount of displacement is obtained in the first-order resonance mode. However, since the first-order resonance frequency is within an audible range, a problem of excessive noise might arise. In contrast, if a third-order resonance mode is used, it is possible to obtain an amount of displacement that is greater than that obtained when a resonance mode is not used at all, although the amount of displacement is less than that obtained in the first-order resonance mode. In addition, it is possible to drive the driver at a frequency that is not within the audible range. Therefore, noise will not be generated. The term “first-order resonance mode” means a mode in which the center portion of the driver and the peripheral portion thereof are displaced in the same direction. The term “third-order resonance mode” means a mode in which the center portion of the driver and the peripheral portion thereof are displaced in opposite directions. 
     As described above, a piezoelectric micro blower according to various preferred embodiments of the present invention produce the following advantageous effects. A driver is caused to vibrate due to bending so as to suck a fluid from a central space into a blower chamber through a first opening. The fluid is pressed out of the blower chamber at high speed to be discharged through a second opening. In the course of flowing, the fluid pressed out thereof entrains a fluid that is present inside the central space to be discharged together. Therefore, it is possible to obtain a discharging flow, the quantity of which is greater than the displacement volume of the driver, without providing a check valve. Thus, it is possible to provide a blower that is capable of blowing a large quantity of a fluid. In addition, since a bottleneck portion is provided in an inlet passage, the bottleneck portion reduces the likelihood that the energy of a pressure fluctuation in the central space will be dissipated into the inlet passage. Therefore, it is possible to efficiently direct the pressure energy toward the second opening. Consequently, it is possible to achieve a further increase in the flow rate. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view that illustrates an overall configuration of a piezoelectric micro blower according to a first preferred embodiment of the present invention. 
         FIG. 2  is an exploded perspective view of the piezoelectric micro blower illustrated in  FIG. 1 . 
         FIG. 3  is a sectional view taken along the line III-III of  FIG. 1 . 
         FIG. 4  is a sectional view taken along the line IV-IV of  FIG. 3 . 
         FIGS. 5A to 5E  are diagrams that illustrate the operation of the piezoelectric micro blower illustrated in  FIG. 3 . 
         FIG. 6  is a sectional view of a piezoelectric micro blower according to another example of a preferred embodiment of the present invention. 
         FIGS. 7A and 7B  are diagrams that illustrate flow rate characteristics relative to an applied voltage and flow rate characteristics relative to power consumption that were obtained from samples in which the material and thickness of separators are different from each other. 
         FIG. 8  is a sectional view of a micro blower having bottleneck portions according to another example of a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the accompanying drawings, preferred embodiments of the present invention will now be explained. 
     First Preferred Embodiment 
     A piezoelectric micro blower according to a first preferred embodiment of the present invention is shown in  FIGS. 1 to 4 . A piezoelectric micro blower A according to the first preferred embodiment of the present invention is an air blower that is preferably used to cool electronic equipment, for example. The piezoelectric micro blower A includes a top plate (second wall)  10 , a flow channel formation plate  20 , a separator (first wall)  30 , a blower frame member  40 , a driver  50 , and a bottom plate  60 , which are attached to define a layered structure in this order as viewed from the top. The peripheral portion of the driver  50  is fixed by bonding between the blower frame member  40  and the bottom plate  60 . The members described above, except the driver  50 , that is, members  10 ,  20 ,  30 ,  40 , and  60 , are components that define a blower body  1 . Each of the members  10 ,  20 ,  30 ,  40 , and  60  is preferably made of a flat rigid flat material, such as a metal plate or a hard resin plate, for example. 
     The top plate  10  is preferably a quadrangular flat plate, for example. A discharging port (second opening)  11  is provided as a through hole at the approximate center of the top plate  10 . 
     The flow channel formation plate  20  is preferably also a flat plate that has substantially the same outer shape as that of the top plate  10 . As illustrated in  FIG. 4 , a center hole (central space)  21  that preferably has a diameter greater than that of the discharging port  11  is provided at the approximate center of the flow channel formation plate  20 . A plurality of inlet passages (in the illustrated example, preferably four inlet passages)  22  extending in radial directions toward four corners are provided in the flow channel formation plate  20 . The outer ends of the inlet passages  22  are respectively in communication with inlet ports  8 , which will be explained later. In the first preferred embodiment of the present invention, the inlet passages  22  are preferably in communication with the center hole  21  from four different directions. Because of such a structure, a fluid can be sucked into the center hole  21  without substantial fluid resistance when the driver  50  performs a pumping operation. Therefore, it is possible to increase the flow rate. Each of the inlet passages  22  preferably includes a bottleneck portion  23 . The bottleneck portion  23  is defined by the tapered portion of the inlet passage  22  at which the width thereof is reduced toward the center hole  21 . In the first preferred embodiment of the present invention, the bottleneck portions  23  are preferably provided at regions at which the inlet passages  22  are connected to the center hole  21 . However, the regions at which the bottleneck portions  23  are provided are not limited thereto. The bottleneck portions  23  may be provided anywhere in the inlet passages  22 . 
     The separator  30  is preferably also a flat plate that has substantially the same outer shape as that of the top plate  10 . A communication hole (first opening)  31  is provided at the approximate center of the separator  30 , specifically, at a location corresponding to the position of the discharging port  11 . The communication hole  31  preferably has a diameter that is substantially the same as that of the discharging port  11 , for example. The diameter of the communication hole  31  may preferably be exactly the same as that of the discharging port  11 . The diameter of the communication hole  31  may preferably be different from that of the discharging port  11 . However, it is required that the diameter is less than that of the center hole  21 . Inlet holes  32  are provided near four corners of the separator  30 , specifically, at locations corresponding to the outer ends of the inlet passages  22 , respectively. In an assembled state in which the top plate  10  and the separator  30  are attached to the flow channel formation plate  20 , the center of the discharging port  11 , the center of the center hole  21 , and the center of the communication hole  31  are preferably aligned on the same or substantially the same axis. The center of the aligned openings  11 ,  21 , and  31  corresponds to the center of the driver  50 , which will be explained later. As will be explained later, preferably, the separator  30  is made of a thin metal plate so that a portion of the separator  30  that is located at an area corresponding to the center hole  21  can be vibrated in a sympathetic manner. 
     The blower frame member  40  is also preferably a flat plate that has substantially the same outer shape as that of the top plate  10 . A circular cavity  41  having a large diameter is preferably provided at the approximate center of the blower frame member  40 . Inlet holes  42  are preferably provided near four corners of the blower frame member  40 , specifically, at locations corresponding to the positions of the inlet holes  32 , respectively. Since the blower frame member  40  is fixed by bonding between the separator  30  and the driver  50 , the cavity  41  of the blower frame member  40  defines a blower chamber. The blower chamber  41  is not limited to an enclosed space. The blower chamber  41  may be a partially open space. 
     The bottom plate  60  is also preferably a flat plate that has substantially the same outer shape as that of the top plate  10 . A cavity  61  is preferably provided at the approximate center of the bottom plate  60 . The two-dimensional shape of the cavity  61  is substantially the same as that of the blower chamber  41 . The bottom plate  60  preferably has a thickness that is greater than the sum of the thickness of a piezoelectric element  52  and the amount of displacement of a diaphragm  51 . With this structure, when the piezoelectric micro blower A is mounted on, for example, a substrate, the piezoelectric element  52  is prevented from contacting the substrate. The wall of the cavity  61  surrounds the piezoelectric element  52  of the driver  50 , which will be explained later. Inlet holes  62  are preferably provided near four corners of the bottom plate  60 , specifically, at locations corresponding to the positions of the inlet holes  32  and  42 , respectively. 
     The driver  50  preferably includes the diaphragm  51  and the piezoelectric element  52 . The piezoelectric element  52  preferably has a circular shape, for example, and is attached to the lower surface of the diaphragm  51  at the central area thereof. Besides various metal materials, such as stainless or brass, resin can be used as the material of the diaphragm  51 , for example. For example, a resin plate that is made of glass epoxy resin may preferably be used as the diaphragm  51 . The piezoelectric element  52  preferably has a disc shaped having a diameter less than that of the cavity  41  of the blower frame member  40 . In the first preferred embodiment of the present invention, a single-plate piezoelectric ceramic including electrodes on both surfaces is preferably used as the piezoelectric element  52 , for example. The piezoelectric element is attached to the back of the diaphragm  51  (i.e., one surface of the diaphragm  51  that is opposite to the other surface facing the blower chamber  41 ). That is, the driver  50  is preferably configured as a unimorph driver, for example. When an alternating voltage (either a sinusoidal wave or a rectangular wave) is applied to the piezoelectric element  52 , the piezoelectric element  52  expands and contracts in a planar direction. As a result, the displacement of the entire body of the driver  50  in the direction of its thickness occurs due to bending. The alternating voltage applied to the piezoelectric element  52  causes the driver  50  to become displaced either in a first-order resonance mode or in a third-order resonance mode due to bending. Since such a voltage is applied thereto, it is possible to significantly increase the displacement volume of the driver  50  as compared to a case in which a voltage having any frequency other than the above-described frequency is applied thereto. Consequently, it is possible to significantly increase the flow rate. 
     Inlet holes  51   a  are preferably provided near four corners of the diaphragm  51 , specifically, at locations corresponding to the locations of the inlet holes  32 ,  42 , and  62 , respectively. The inlet holes  32 ,  42 ,  51   a , and  62  are aligned such that they define each of the inlet ports  8 . One end of the inlet port  8  is defined as a downward open end. The other end of the inlet port  8  is in communication with the inlet passage  22 . 
     As illustrated in  FIG. 3 , the inlet ports  8  of the piezoelectric micro blower A are open at the bottom of the blower body  1 . In addition, the discharging port  11  is open at the top of the blower body  1 . A compressible fluid can be sucked into the inlet ports  8  through the open ends thereof, which are preferably provided at the reverse side of the piezoelectric micro blower A, and then can be discharged from the discharging port  11 , which is preferably provided at the front side of the piezoelectric micro blower A. Such a structure is suitable for use as a blower to supply air to a fuel cell or a blower to cool a CPU using air. It is not always necessary that the open ends of the inlet ports  8  be oriented downward, and instead, the open ends may be open at the sides. 
     Although the driver  50  illustrated in  FIG. 3  preferably includes the diaphragm  51  and the piezoelectric element  52 , the structure of the driver  50  is not limited to the illustrated example. For example, as illustrated in  FIG. 6 , the driver  50  may preferably further include an intermediate plate that is sandwiched between the diaphragm  51  and the piezoelectric element  52 . A plate that is preferably made of metal, such as SUS, for example, can be used as the intermediate plate  53 . Since the intermediate plate  53  is sandwiched between the diaphragm  51  and the piezoelectric element  52 , a neutral plane at the time of the displacement of the driver  50  due to bending is located inside the intermediate plate  53 , which results in greater displacement efficiency. Therefore, it is possible to provide a piezoelectric micro blower that can produce a high flow rate with a low voltage. 
       FIGS. 5A to 5E  are diagrams that schematically illustrate an example of the operation of the piezoelectric micro blower A. To facilitate understanding, displacement is shown in an exaggerated manner. An initial state (when no voltage is applied thereto) is shown in  FIG. 5A .  FIGS. 5B to 5E  illustrate the displacement of the driver  50  and the separator  30  at intervals of ¼ of a cycle of a voltage (e.g., sinusoidal wave) applied to the piezoelectric element  52 . As a result of the application of an alternating voltage to the piezoelectric element  52 , operations shown in  FIGS. 5B to 5E  are repeated in a cycle. As illustrated therein, the vibration of the driver  50  causes the sympathetic vibration of the separator  30 . The vibration of the separator  30  occurs with a delay of a predetermined phase (in this example, approximately 90°) with respect to the vibration of the driver  50 . Because of the sympathetic vibration of the separator  30 , a large pressure wave is generated from the first opening  31  in an upward direction. Air is forced out of the central space  21  through the second opening  11  to the outside due to the pressure wave. Therefore, it is possible to increase the flow rate as compared to a case in which the separator  30  is not vibrated in a sympathetic manner. Since the air is forced out of the central space  21 , new air is sucked from the inlet passages  22  into the central space  21 , thereby generating a continuous airflow exiting from the second opening  11 . 
     In  FIGS. 5A to 5E , an example of the displacement of the driver  50  in a first-order resonance mode is illustrated. The same principle holds true for displacement in a third-order resonance mode. In the illustrated example, it is assumed that the amount of displacement of the separator  30  is greater than that of the driver  50 . However, depending on the dimensions of the central space  21 , the Young&#39;s modulus of the separator  30 , the thickness of the separator  30 , and other factors and parameters, there is a possibility that the amount of displacement of the separator  30  is less than that of the driver  50 . The phase delay of the separator  30  with respect to the driver  50  is not limited to approximately 90°. The above-described structure may be modified as long as the vibration of the separator  30  occurs with a certain delay with respect to the vibration of the driver  50 , and, for this reason, an actual change in the distance between the driver  50  and the separator  30  is greater than a change that would occur if the separator  30  did not vibrate at all. 
     A flow-rate measurement was performed for a piezoelectric micro blower A having the following specifications. 
     Driver: A unimorph element that includes a diaphragm, a single-plate piezoelectric ceramic element, and an intermediate plate that is sandwiched between the diaphragm and the piezoelectric ceramic element was used as the driver. The diaphragm was made of a 42Ni plate having a thickness of about 0.08 mm. The intermediate plate was an SUS430 plate having a thickness of about 0.15 mm and a diameter of about 11 mm. The piezoelectric ceramic element had a thickness of about 0.2 mm and a diameter of about 11 mm. 
     Blower chamber: about 0.15 mm in height, about 16 mm in diameter 
     Blower body: about 20 mm in length, about 20 mm in width, about 2.4 mm in height 
     Separator: SUS430 having a thickness of about 0.05 mm 
     First opening: about 0.6 mm in diameter 
     Second opening: about 0.8 mm in diameter 
     Central space: about 6 mm in diameter, about 0.5 mm in height 
     Inlet passages: about 2.5 mm in width, about 0.5 mm in height, four passages 
     Bottleneck portions: about 1 mm in width 
     A voltage having a sinusoidal waveform of 24 kHz and 20 Vp-p was applied to the piezoelectric micro blower A having the above-described specifications. As a result of the measurement, the flow rate was about 0.9 L/min under a condition of about 100 Pa. The driver was energized in the third-order mode. Alternatively, the driver may be energized in the first-order mode. Another experiment was performed for comparison with the use of a piezoelectric micro blower having the same specifications as described above except that it does not include any bottleneck portion. As a result of the measurement, the flow rate was about 0.77 L/min under the condition of about 100 Pa. From the above-described results, it was discovered that the bottleneck portions contribute to an increase in the flow rate. 
     It can be inferred that for the following reasons, the quantity of flow is relatively large in the above-described structure in which the inlet passages include the bottleneck portions. 
     When the driver  50  vibrates, a pressure wave having high energy is generated from the first opening  31 . Because of the pressure wave, air is forced out of the central space  21  through the second opening  11 . As the driver  50  vibrates, a portion of the separator  30  that defines the bottom wall of the central space  21  also vibrates (refer to  FIG. 5 ). The vibration of the separator  30  generates a large pressure fluctuation in the central space  21 . The pressure energy acts to be dissipated not only through the second opening  11  but also into the inlet passages  22 . If the sectional area of the inlet passage  22  is configured to be large so as to reduce air resistance, pressure energy loss is large. In contrast, since the inlet passages  22  include the bottleneck portions  23 , the pressure energy that is present inside the central space  21  is less likely to be dissipated into the inlet passages  22 . Accordingly, it is possible to efficiently direct the pressure energy that is present inside the central space  21  toward the second opening  11 , thereby achieving an increase in the flow rate. 
     Since there is a high-speed flow inside the central space  21 , its average pressure is less than pressure in the inlet passages  22 . For this reason, a pressure gradient is produced in a flow channel. The pressure gradient generates a flow from the inlet passages  22  toward the central space  21 . Although pressure loss occurs in the inlet passages  22  because of friction between air and wall surfaces, if no bottleneck portion was provided in the flow channel, vena contracta would be produced in the vicinity of entrances to the central space  21  because the pressure of the central space  21  is less than the pressure decreased due to the pressure loss. Therefore, loss due to a vortex being developed would occur in the vicinity of the vena contracta. Consequently, the flow rate of the blower would decrease. In contrast, since the bottleneck portions  23  are preferably provided in the vicinity of entrances to the central space  21 , the conformity of the shape of the flow channel to the shape of the flow increases. With the increased conformity, it is possible to prevent a vortex from developing and thus to reduce flow loss. Consequently, the flow rate of the blower is increased. 
     The separator  30  is preferably attached to the flow channel formation plate  20 . The central area of the separator  30  that corresponds to the central space  21  is configured as a regional portion that can vibrate. As illustrated in  FIGS. 5A to 5E , the vibration of the central area of the separator  30  has a strong affect on the flow rate. In view of these considerations, the size of the central space  21  (open area) is designed to have an appropriate diameter that makes it easier for the central area of the separator  30  to vibrate. However, it is impossible to restrain the separator  30  at regions at which the inlet passages  22  are connected to the central space  21 . In the flow channel formation plate  20  including the bottleneck portions  23 , since the front end of each of the bottleneck portions  23  has a tapered shape, the area at which the sidewalls of the central space  21  exist is relatively large. That is, as compared to a case in which no bottleneck portions are provided, it is possible to increase the area in which the separator  30  is supported. The shape of the area in which the separator  30  is supported is preferably substantially circular. With the bottleneck portions  23 , the flow channel formation plate  20  can support the central area of separator  30  more securely, which contributes to an increase in the flow rate. 
     Table 1 shows the flow rate obtained when the drive frequency of the driver  50  and the diameter of the central space  21  are changed. The unit of the flow rate is L/min. A diaphragm (42Ni plate) having a thickness of about 0.08 mm was used for a drive frequency of about 24.4 kHz. A diaphragm (42Ni plate) having a thickness of about 0.1 mm was used for a drive frequency of about 25.5 kHz. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                   
                   
                 Diameter of Central space 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                 Φ 5 mm 
                 Φ 6 mm 
               
               
                   
                   
               
               
                   
                 Frequency 
                 24.4 kHz 
                  0.7 L/min. 
                  0.8 L/min. 
               
               
                   
                   
                 25.5 kHz 
                 0.78 L/min. 
                 0.71 L/min. 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 1, the flow rate increases as the frequency increases when the diameter of the central space  21  is about 5 mm, whereas the flow rate increases as the frequency decreases when the diameter of the central space  21  is about 6 mm. As described above, it can be understood that the vibration of the central area of the separator  30  that corresponds to the central space  21  has a strong affect on the flow rate. The following reason can be inferred. The natural vibration frequency of the driver  50  differs depending on the material of the diaphragm  51  and the thickness thereof. It is possible to make the natural vibration frequency of the central area of the separator  30  that corresponds to the central space  21  close to the natural vibration frequency of the driver  50  by adjusting the diameter of the central space  21 , thereby producing the sympathetic vibration of the separator  30 , which increases the flow rate. 
       FIGS. 7A and 7B  show the results of an experiment conducted on a piezoelectric micro blower B having a structure in which the driver  50  includes the diaphragm  51 , the piezoelectric element  52 , and the intermediate plate  53  that is sandwiched between the diaphragm  51  and the piezoelectric element  52 . As shown in Table 2, the experiment was performed to compare the flow rate obtained when the material of the separator  30  and the thickness thereof are changed. A plate that is made of phosphor bronze with a thickness of about 0.05 mm was used as a separator in a first sample. A plate that is made of SUS304 with a thickness of about 0.1 mm was used as a separator in a second sample. Except for the above component, the specifications of the piezoelectric micro blower B are the same or substantially the same as those of the piezoelectric micro blower A. The specifications of the first sample are the same or substantially the same as those of the second sample except for the separator. The drive frequency of about 24.4 kHz was used for both the first sample and the second sample. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 First Sample 
                 Second Sample 
               
               
                   
               
             
            
               
                 Material of Separator 
                 phosphor 
                 SUS304 
               
               
                   
                 bronze 
                   
               
               
                 Thickness of Separator (mm) 
                 0.05 
                 0.1 
               
               
                 Diameter of First Opening 
                 0.6 
                 0.6 
               
               
                 (mm) 
                   
                   
               
               
                 Material of Top Plate 
                 nickel silver 
                 nickel silver 
               
               
                 Diameter of Second Opening 
                 0.8 
                 0.8 
               
               
                 (mm) 
                   
                   
               
               
                 Material of Blower Chamber 
                 nickel silver 
                 nickel silver 
               
               
                 Height of Blower Chamber (mm) 
                 0.15 
                 0.15 
               
               
                 Diameter of Blower Chamber 
                 16 
                 16 
               
               
                 (mm) 
                   
                   
               
               
                 Material of Diaphragm 
                 42 Ni 
                 42 ni 
               
               
                 Thickness of Diaphragm (mm) 
                 0.08 
                 0.08 
               
               
                 Thickness of Intermediate 
                 0.15 
                 0.15 
               
               
                 Plate (mm) 
                   
                   
               
               
                 Diameter of Intermediate 
                 11 
                 11 
               
               
                 Plate (mm) 
                   
                   
               
               
                 Thickness of Piezoelectric 
                 0.20 
                 0.20 
               
               
                 Element (mm) 
                   
                   
               
               
                 Diameter of Piezoelectric 
                 11 
                 11 
               
               
                 Element (mm) 
                   
                   
               
               
                 Diameter of Central space 
                 6 
                 6 
               
               
                 (mm) 
                   
                   
               
               
                 Height of Central space (mm) 
                 0.5 
                 0.5 
               
               
                   
               
            
           
         
       
     
     Typically, a plate that is made of SUS304 and has a certain thickness is one and a half times as rigid as a plate that is made of phosphor bronze and has the same thickness. Since the separator of the second sample is twice as thick as the separator of the first sample, the rigidity of the separator of the second sample is far greater than the rigidity of the separator of the first sample. In other words, a portion of the separator of the first sample that faces the central space vibrates, whereas a portion of the separator of the second sample that faces the central space does not significantly vibrate. The purpose of the experiment is to measure the influence of the vibration of a portion of the separator that faces the central space on the flow rate. 
     As illustrated in  FIG. 7A , for example, the flow rate of the second sample obtained when a voltage of about 20 Vp-p was applied thereto was approximately about 0.42 L/min. The flow rate of the first sample obtained when the same voltage of about 20 Vp-p was applied thereto was approximately about 0.78 L/min. Therefore, the flow rate of the first sample is roughly twice that of the second sample. That is, the vibration of the portion of the separator significantly contributes to an increase in the flow rate.  FIG. 7B  shows the results of a comparison of the flow rate based on power consumption. Although power consumption changes due to a change in impedance, the first sample is advantageous when compared based on the same power consumption. 
     Second Preferred Embodiment 
       FIG. 8  illustrates the shape of bottleneck portions according to a second preferred embodiment of the present invention. Since the structure of the flow channel formation plate  20  according to the second preferred embodiment of the present invention is substantially the same as that of the first preferred embodiment of the invention (refer to  FIG. 4 ) except for the shape of bottleneck portions, the same reference numerals are used for the components described earlier. The components described earlier are not explained below. In the second preferred embodiment of the present invention, bottleneck portions  24  that are not tapered are provided at regions at which the inlet passages  22  are connected to the central space  21 . The bottleneck portions  24  prevent the energy of a pressure fluctuation in the central space  21  from being dissipated into the inlet passages  22 . Thus, it is possible to achieve an increase in the flow rate. 
     In the foregoing preferred embodiments of the present invention, a portion of a separator (first wall) that corresponds to a central space preferably vibrates in a sympathetic manner as a driver vibrates. However, it is not always necessary for the separator to vibrate in a sympathetic manner. Any alternative structure in which the vibration of a driver excites a separator, and, in addition, the separator vibrates in response to or by following the vibration of the driver makes it possible to achieve an increase in the flow rate. The shape of an inlet passage is not limited to a linear passage extending in a radial direction as shown in  FIG. 4 . That is, the shape of the inlet passage may be arbitrarily selected. The number of inlet passages may also be arbitrarily selected depending on the required flow rate and the required level of noise. 
     In the foregoing preferred embodiments of the present invention, a plurality of plate members are preferably attached in layers to define a blower body. However, the structure of the blower body is not limited thereto. For example, the top plate and the flow channel formation plate  20  may be integrally molded as a single component that is made of resin or metal. The same applies to, for example, the separator  30  and the blower frame member  40 , or the flow channel formation plate  20  and the separator  30 . 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.