Abstract:
A piezoelectric micropump in which a pump chamber is isolated by a diaphragm. A piezoelectric element is disposed on a back surface of the diaphragm, and the diaphragm is deformed by bending deformation of the piezoelectric element to change the volume of the pump chamber and transport fluid in the pump chamber. A support member for supporting a back surface of the piezoelectric element is provided so that the support member inhibits bending of a peripheral portion of the diaphragm in a reverse direction. The support member thus prevents the piezoelectric element from being floated. Accordingly, the displacement of the piezoelectric element can be reliably transmitted as the change in volume of the pump chamber, thereby enhancing the fluid transportation performance.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of International Application No. PCT/JP2007/052323, filed Feb. 9, 2007, which claims priority to Japanese Patent Application No. JP2006-079424, filed Mar. 22, 2006, the entire contents of each of these applications being incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to piezoelectric micropumps, and more particularly to a micropump using a piezoelectric element which undergoes bending deformation. 
     BACKGROUND OF THE INVENTION 
     Hitherto, there has been known a micropump using a piezoelectric element which undergoes bending deformation in a bending mode by application of a voltage. Patent Document 1 discloses a micropump in which a pump chamber is formed in a pump body, and a piezoelectric element is attached onto a back surface of a diaphragm which defines a top wall of the pump chamber. 
       FIG. 9(   a ) schematically illustrates a pump structure described in Patent Document 1. A pump chamber  21  is provided in a case  20 . A piezoelectric element  23  is attached onto a diaphragm  22  which defines a top wall of the pump chamber  21 . The diaphragm  22  is formed of an organic material such as polyimide. However, referring to  FIG. 9(   b ), when the piezoelectric element  23  undergoes bending deformation, a change in volume of the pump chamber  21 , which is expected to be generated by bending of the piezoelectric element  23 , partly becomes inefficient as a result of a displacement of the diaphragm  22  at both end portions of the piezoelectric element  23 . In other words, the piezoelectric element  23  is merely moved in a floated manner via the diaphragm  22 . Hence, a displacement of the piezoelectric element  23  cannot be transmitted as a change in volume of the pump chamber  21 . This phenomenon occurs because, for example, when the piezoelectric element  23  is deformed to bulge toward the pump chamber  21  so as to pump out incompressible fluid (liquid) filled in the pump chamber  21 , a pressure of the liquid is applied to the diaphragm  22 , and a peripheral portion of the diaphragm  22  (portion where the piezoelectric element  23  is not attached) is displaced in a reverse direction away from the pump chamber  21  by the pressure of the liquid. In contrast, when the piezoelectric element  23  is deformed to bulge away from the pump chamber  21 , the peripheral portion of the diaphragm  22  is bent toward the pump chamber  21 . 
     When the diaphragm  22  is formed of a hard material such as a metal plate, bending of the peripheral portion of the diaphragm  22  can be inhibited, and hence, the phenomenon as shown in  FIG. 9(   b ) does not occur. However, if the diaphragm  22  is hard, the diaphragm  22  inhibits the bending deformation of the piezoelectric element  23 , thereby decreasing the amplitude of the bending deformation and the change in volume of the pump chamber  21 . Also, a drive frequency of the pump is decreased, and hence, fluid transportation performance is deteriorated. Further, in the known configuration, unless the piezoelectric element  23  is attached to the center of the diaphragm  22 , the left-right balance of a displacement is disrupted, and the change in volume of the pump chamber  21  cannot be correctly transmitted. Thus, it is necessary to increase a positional accuracy of attachment between the diaphragm  22  and the piezoelectric element  23 .
     Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-214349   

     SUMMARY OF THE INVENTION 
     Accordingly, an object of a preferred embodiment of the present invention is to provide a piezoelectric micropump capable of efficiently transmit a displacement of a piezoelectric element as a change in volume of a pump chamber even when a diaphragm is formed of a soft material, and having good fluid transportation performance. 
     To attain the above-mentioned object, the present invention provides a piezoelectric micropump, in which a pump chamber is isolated by a diaphragm, a piezoelectric element is disposed on a back surface of the diaphragm, the diaphragm is deformed by bending deformation of the piezoelectric element, and the volume of the pump chamber is changed, to transport fluid in the pump chamber. In the micropump, a support member is provided, the support member being in contact with a back surface of the piezoelectric element to support the piezoelectric element. 
     When an alternating voltage (rectangular wave voltage or alternating voltage) is applied to the piezoelectric element, the piezoelectric element undergoes bending deformation in a plate-thickness direction, and the diaphragm is deformed by the bending deformation. If the diaphragm is formed of a soft material, a peripheral portion of the diaphragm (portion where the piezoelectric element is not arranged) is bent in a reverse direction opposite to the piezoelectric element as a result of a change in pressure of the fluid filled in the pump chamber. Hence, as with the known micropump shown in  FIGS. 9(   a ) and  9 ( b ), the displacement of the piezoelectric element cannot be efficiently transmitted as the change in volume of the pump chamber. However, since in the present invention the back surface of the piezoelectric element is supported by the support member, the support member inhibits bending of the peripheral portion of the diaphragm in the reverse direction, and prevents the piezoelectric element from being floated. Accordingly, the displacement of the piezoelectric element can be reliably transmitted as the change in volume of the pump chamber, thereby enhancing the fluid transportation performance. 
     The back surface of the piezoelectric element is merely in contact with the support member, and restriction is not provided by the support member by bonding or the like. The support member does not inhibit the bending deformation of the piezoelectric element, and hence, the piezoelectric element can be efficiently driven. It is noted that the back surface of the diaphragm according to the present invention is a surface of the diaphragm opposite to the pump chamber, and the back surface of the piezoelectric element is a surface of the piezoelectric element opposite to the pump chamber. 
     It is preferable that the piezoelectric element be attached to a center portion of the diaphragm, however, in this embodiment, even if the diaphragm is shifted from the center portion, the support member inhibits a shift of the piezoelectric element toward the back surface of the piezoelectric element. Thus, the performance of the piezoelectric element is hardly deteriorated. In addition, the performance of the piezoelectric element is hardly deteriorated even when the diaphragm is markedly larger than the piezoelectric element. A soft diaphragm (with low Young&#39;s modulus) may be used, and a pumping action is likely to be obtained by a piezoelectric element driven with a low voltage. 
     The support member may be, for example, an inner wall of a case that supports the diaphragm, or may be an additional member disposed in the case. The support member may be formed of a relatively hard material similarly to the case, or may be formed of an elastic member such as rubber. The diaphragm may be formed of an organic material such as polyimide similarly to the known configuration. Alternatively, the diaphragm may be formed of any elastic material such as rubber or elastomer. Still alternatively, the diaphragm may be a metal plate. However, a soft elastic material having a Young&#39;s modulus of 20 MPa or smaller, and a thickness of 100 μm or smaller is desirable. 
     According to a preferable embodiment, the support member may be a flat member that supports an entire area of the back surface of the piezoelectric element in a non-drive state. In this case, the support member supports a back surface of an outer peripheral portion or back surfaces of both end portions of the piezoelectric element when the piezoelectric element is deformed to bulge toward the pump chamber, whereas the support member supports a back surface of a center portion of the piezoelectric element when the piezoelectric element is deformed to bulge away from the pump chamber. Accordingly, the diaphragm can be constantly displaced toward the pump chamber regardless of the direction the piezoelectric element is deformed, and hence, the volume of the pump chamber can be decreased. Accordingly, the fluid in the pump chamber can be reliably pumped out, and the fluid transportation performance can be enhanced. 
     According to a preferable embodiment, the piezoelectric element may be formed into a rectangular shape, the support member may support back surfaces of both end portions of the piezoelectric element in a longitudinal direction, and a space for the bending deformation of the piezoelectric element may be provided on a back-surface side of a center portion of the piezoelectric element. The shape of the piezoelectric element may be a circular shape or a rectangular shape. When a rectangular piezoelectric element undergoes bending displacement in a mode in which both end portions in the longitudinal direction (short two sides) of the piezoelectric element serve as supporting points, a larger volume displacement can be obtained, as compared with a case in which a circular piezoelectric element undergoes bending displacement in a mode in which an outer peripheral portion of the piezoelectric element serves as a supporting point. Hence, when the rectangular piezoelectric element is used as a diaphragm-drive actuator, a pumping efficiency can be enhanced. When the support member supports the entire area of the back surface of the piezoelectric element, the diaphragm can be constantly displaced toward the pump chamber regardless of the direction the piezoelectric element is deformed. However, the volume displacement of the pump chamber is smaller than a case in which the piezoelectric element is deformed to bulge away from the pump chamber. Hence, the support member supports the back surfaces of both end portions in the longitudinal direction of the piezoelectric element. Accordingly, the diaphragm is displaced such that the center portion thereof is pushed up when the piezoelectric element is deformed to bulge toward the pump chamber, whereas the diaphragm is displaced such that the center portion thereof is pulled down when the piezoelectric element is deformed to bulge away from the pump chamber. In either case, a large volume displacement can be obtained. Accordingly, the volume of the pump chamber can be periodically markedly varied, thereby enhancing the pumping efficiency. 
     According to a preferable embodiment, the piezoelectric element may be formed to be smaller than a displaceable region of the diaphragm, and the diaphragm may have a margin in a whole circumferential portion of the diaphragm located outside the piezoelectric element, the piezoelectric element being not arranged at the margin. When the piezoelectric element has a size equivalent to that of the displaceable region of the diaphragm, the diaphragm has almost no margin. Hence, when the piezoelectric element is displaced, an excessively large force is partly applied to the diaphragm; thereby the displacement of the piezoelectric element may be restricted. In contrast, when the piezoelectric element is smaller than the displaceable region of the diaphragm, and the diaphragm has the margin outside the piezoelectric element, the margin of the diaphragm can be freely expanded or contracted when the piezoelectric element is displaced. Thus, the displacement of the piezoelectric element is not restricted. Accordingly, the piezoelectric element may undergo bending displacement freely, and the pump efficiency can be enhanced. 
     According to a preferable embodiment, the piezoelectric element may be face-bonded onto the diaphragm. In this case, since the diaphragm is moved while the diaphragm is closely attached onto the piezoelectric element, the displacement of the piezoelectric element can be reliably transmitted to the diaphragm. In addition, the piezoelectric element can be prevented from freely moving in a left-right direction. An adhesive may be an elastic adhesive such as a silicone adhesive or a urethane adhesive. Even when the piezoelectric element is slightly shifted from the center portion of the diaphragm, the shift does not seriously affect the pumping efficiency. 
     According to a preferable embodiment, a gap between the diaphragm and the support member in a thickness direction may be smaller than a thickness of the piezoelectric element, and the piezoelectric element may be pressed to the support member by elasticity of the diaphragm. The piezoelectric element can be preliminarily pressed to the support member and held by the elasticity of the diaphragm. Since the piezoelectric element and the support member are in contact with each other securely, the volume of the pump chamber can be reliably changed by the bending deformation of the piezoelectric element. As described above, when the piezoelectric element is preliminarily pressed to the support member and held by the elasticity of the diaphragm, the piezoelectric element and the diaphragm do not have to be bonded to each other. When the piezoelectric element and the diaphragm are not bonded to each other, the piezoelectric element can be freely displaced without restriction by the diaphragm. Accordingly, the piezoelectric element can be efficiently driven with a low voltage. When the piezoelectric element and the diaphragm are not bonded to each other, the piezoelectric element may be shifted from the diaphragm in a plane direction. Thus, the support member may preferably have a peripheral wall portion that regulates the position of an outer peripheral surface of the piezoelectric element with a predetermined gap interposed therebetween. In this case, the piezoelectric element can be prevented from being shifted, and the peripheral wall portion does not restrict the displacement of the piezoelectric element. Thus, the piezoelectric element can be efficiently driven. 
     As described above, with the present invention, since the support member supports the back surface of the piezoelectric element, the support member inhibits a displacement of the peripheral portion of the diaphragm. The support member thus prevents the piezoelectric element from being floated. Accordingly, the displacement of the piezoelectric element can be reliably transmitted as the change in volume of the pump chamber, thereby enhancing the fluid transportation performance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing a piezoelectric micropump according to a first embodiment of the present invention. 
         FIG. 2  is an exploded perspective view showing the piezoelectric micropump in  FIG. 1 . 
         FIG. 3  is a longitudinal cross section showing the piezoelectric micropump in  FIG. 1 . 
         FIG. 4  is a cross section taken along line IV-IV in  FIG. 3 . 
         FIGS. 5(   a ),  5 ( b ) and  5 ( c ) are cross sections schematically showing an operation of the piezoelectric micropump in  FIG. 1 ,  FIG. 5(   a ) showing a non-drive state,  FIG. 5(   b ) showing an upwardly bulging state, and  FIG. 5(   c ) showing a downwardly bulging state. 
         FIG. 6(   a ) illustrates an alternating current to be applied to the piezoelectric element, and  FIG. 6(   b ) illustrates a change in discharge flow rate of the micropump. 
         FIG. 7  is a schematic cross section according to a second embodiment of the present invention. 
         FIGS. 8(   a ),  8 ( b ) and  8 ( c ) are cross sections schematically showing a third embodiment of the present invention,  FIG. 8(   a ) showing a non-drive state,  FIG. 8(   b ) showing an upwardly bulging state, and  FIG. 8(   c ) showing a downwardly bulging state. 
         FIGS. 9(   a ) and  9 ( b ) are cross sections of an example of a known micropump,  FIG. 9(   a ) showing a non-drive state, and  FIG. 9(   b ) showing a state where a piezoelectric element is deformed. 
     
    
    
     REFERENCE NUMERALS 
     
         
         
           
             P micropump 
               1  bottom plate 
               1   a  recess (vibration chamber) 
               1   a   1  bottom wall (support member) 
               1   d  block (support member) 
               2  piezoelectric element 
               3  diaphragm 
               3   a  margin 
               4  frame 
               5  top plate 
               6  pump chamber 
               7  intake passage 
               8  discharge passage 
               10 , 11  check valve 
           
         
       
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, best modes of the present invention are described below with reference to embodiments. 
     First Embodiment 
       FIGS. 1 to 4  illustrate a piezoelectric micropump according to a first embodiment of the present invention. A micropump P of this embodiment includes a bottom plate  1 , a piezoelectric element  2 , a diaphragm  3 , a frame  4 , and a top plate  5 . These components are mutually layered and bonded. 
     The bottom plate  1  is formed of, for example, a glass epoxy board or a resin material. A rectangular recess  1   a  serving as a vibration chamber is formed at a center portion of the bottom plate  1 . In this embodiment, though described later, a bottom wall  1   a   1  of the recess  1   a  serves as a support member. The bottom wall  1   a   1  is in contact with a back surface of the piezoelectric element  2  and supports the piezoelectric element  2 . Two ports  1   b  and a plurality of through holes  1   c  are formed at a bottom surface of the recess  1   a . Leads  2   a  of the piezoelectric element  2  are led from the ports  1   b . The through holes  1   c  cause the vibration chamber to be exposed to the air. The recess  1   a  has a depth equivalent to or slightly smaller than the thickness of the piezoelectric element  2 . 
     The piezoelectric element  2  has a rectangular shape, and is housed in the recess  1   a . The outside dimension of the piezoelectric element  2  is smaller than the inside dimension of the recess  1   a . When the piezoelectric element  2  is housed in the recess  1   a , predetermined gaps δ (see  FIG. 3 ) are provided between four sides of the piezoelectric element  2  and inner edges of the recess  1   a . The gaps δ correspond to widths of margins  3   a  of the diaphragm  3 . The diaphragm  3  can be sufficiently expanded at the margins  3   a  when the piezoelectric element  2  undergoes bending deformation. The piezoelectric element  2  of this embodiment is a known bimorph-type ceramic piezoelectric element. The piezoelectric element  2  has electrodes at a lower surface thereof. The two leads  2   a  are connected to the electrodes. In response to application of a rectangular wave signal or an alternating current signal to the leads  2   a , the piezoelectric element  2  is vibrated in a bending mode in which both end portions in a longitudinal direction (short two sides) of the piezoelectric element  2  serve as supporting points, and a center portion in the longitudinal direction thereof serves as a maximum displacement point. Alternatively, the piezoelectric element  2  may be a unimorph-type piezoelectric element. 
     The diaphragm  3  is formed of an elastic sheet material, such as rubber, elastomer, or soft resin. The diaphragm  3  has a shape equivalent to that of the bottom plate  1 . An adhesive is applied onto an entire surface of a back surface, or a surface near the vibration chamber, of the diaphragm  3 . When the diaphragm  3  is closely attached onto the bottom plate  1 , in which the piezoelectric element  2  is housed, the diaphragm  3  is face-bonded onto the piezoelectric element  2 , and is bonded onto an upper surface of the bottom plate  1  in an area not occupied by the recess  1   a.    
     The frame  4  is formed of, for example, a glass epoxy board or a resin material. The frame  4  has a rectangular frame shape to define a pump chamber  6 . A side wall portion  4   a  for forming an intake passage  7  is provided outside a surface of one of short sides of the frame  4 . A side wall portion  4   b  for forming a discharge passage  8  is provided outside a surface of one of long sides of the frame  4 . An intake port  4   c  is formed at a side wall between the inside of the frame  4  (pump chamber) and the intake passage  7 . A check valve  10  is attached to a pump-chamber side of the intake port  4   c . The check valve  10  only allows liquid to flow into the pump chamber  6 . A discharge port  4   d  is formed at a side wall between the inside of the frame  4  (pump chamber) and the discharge passage  8 . A check valve  11  is attached to a discharge-passage side of the discharge port  4   d . The check valve  11  only allows liquid to be discharged from the pump chamber  6 . In this embodiment, the check valves  10  and  11  are formed of an elastic sheet of, for example, rubber, however, it is not limited thereto. A lower surface of the frame  4  is bonded onto an upper surface of the diaphragm  3 . 
     The top plate  5  is formed of, for example, a glass epoxy board or a resin material. The top plate  5  is bonded onto an upper surface of the frame  4 . By bonding the top plate  5 , the pump chamber  6 , the intake passage  7 , and the discharge passage  8  are defined between the top plate  5  and the diaphragm  3 . Tubes  9   a  and  9   b  are respectively connected to the intake passage  7  and the discharge passage  8 . The intake passage  7  and the discharge passage  8  are respectively connected to a liquid supply portion and a liquid discharge portion (not shown) via the tubes  9   a  and  9   b . In this embodiment, the tubes  9   a  and  9   b  are silicon tubes. 
       FIGS. 5(   a ) through  5 ( c ) are schematic diagrams of an operation of the above-described micropump P.  FIG. 5(   a ) illustrates a non-drive state or a voltage-switching state,  FIG. 5(   b ) illustrates a state where the piezoelectric element  2  is deformed to bulge upwardly, and  FIG. 5(   c ) illustrates a state where the piezoelectric element  2  is deformed to bulge downwardly. 
       FIG. 6(   a ) illustrates an alternating voltage applied to the piezoelectric element  2 . When alternating voltages of +V and −V are alternately applied, for example, the piezoelectric element  2  is deformed to bulge upwardly in a half period of +V as shown in  FIG. 5(   b ) whereas the piezoelectric element  2  is deformed to bulge downwardly in a half period of −V as shown in  FIG. 5(   c ). When the voltage is switched, the piezoelectric element  2  is restored to a flat shape as shown in  FIG. 5(   a ), and hence, the diaphragm  3  is restored to a flat shape. It is noted that the direction of the voltage and the direction of the deformation of the piezoelectric element  2  depend on the polarization direction of the piezoelectric element  2 . Thus, the piezoelectric element  2  may be deformed to bulge downwardly in a half period of +V whereas the piezoelectric element  2  may be deformed to bulge upwardly in a half period of −V, in a reverse manner. 
     When the piezoelectric element  2  is deformed to bulge upwardly, a center portion of the diaphragm  3  is displaced toward the pump chamber  6 , and the diaphragm  3  pumps out the liquid in the pump chamber  6 . At this time, although the diaphragm  3  is pushed in a reverse direction by a pressure of the liquid in the pump chamber  6 , since both end portions in the longitudinal direction of the piezoelectric element  2  are in contact with the bottom wall  1   a , of the recess  1   a  of the bottom plate  1  and are supported by the bottom wall  1   a , the diaphragm  3  is not bent in the reverse direction away from the pump chamber  6 . Thus, the diaphragm  3  can efficiently pump out the liquid. Since the margins  3   a  having the widths δ are provided at the four sides of the diaphragm  3 , when the piezoelectric element  2  is deformed to bulge upwardly, the margins  3   a  corresponding to both end portions in a short-side direction (two long sides) of the piezoelectric element  2  are expanded. Accordingly, the piezoelectric element  2  may undergo large bending deformation without the displacement of the piezoelectric element  2  being restricted. In contrast, when the piezoelectric element  2  is deformed to bulge downwardly, the center portion in the longitudinal direction of the piezoelectric element  2  is in contact with the bottom wall  1   a   1  of the recess  1   a  of the bottom plate  1 . Hence, both end portions of the piezoelectric element  2  are raised, a peripheral portion of the diaphragm  3  is displaced toward the pump chamber  6 , and thus, the diaphragm  3  pumps out the liquid in the pump chamber  6 . At this time, the margins  3   a  corresponding to both end portions in the longitudinal direction of the piezoelectric element  2  (two short sides) and the margins  3   a  corresponding to both end portions in the short-side direction of the piezoelectric element  2  (two long sides) are expanded. Accordingly, the piezoelectric element  2  may undergo bending deformation without the displacement of the piezoelectric element  2  being restricted. 
       FIG. 6(   b ) illustrates a change in discharge flow rate of the micropump P. As described above, since the piezoelectric element  2  constantly causes the diaphragm  3  to be displaced toward the pump chamber  6  regardless of the direction the piezoelectric element  2  is deformed, the liquid is discharged from the pump chamber  6  at short intervals, and hence, the liquid can be substantially continuously discharged from the pump chamber  6 . The discharge flow rate when the piezoelectric element  2  is deformed to bulge upwardly is larger than the discharge flow rate when the piezoelectric element  2  is deformed to bulge downwardly. Accordingly, as shown in  FIG. 6(   b ), discharge with a large flow rate and discharge with a small flow rate alternately appear. 
     In the micropump having the above-described configuration, when the size of the pump chamber  6  was 25.5 mm×12.5 mm×1.6 mm, and a rectangular wave voltage with ±5V at 17 Hz was applied to the piezoelectric element  2  to drive the piezoelectric element  2 , a discharge flow rate of 6.4 μl/s and a pump pressure of 350 Pa were obtained. 
     Second Embodiment 
       FIG. 7  illustrates a preferable second embodiment of the present invention. This embodiment is an example in which a gap H between the diaphragm  3  and the bottom wall  1   a   1  of the recess of the bottom plate  1  according to the first embodiment is set smaller than a thickness T of the piezoelectric element  2 . In this case, the piezoelectric element  2  can be pressed to the bottom wall  1   a  and held by the elasticity of the diaphragm  3 . Hence, the piezoelectric element  2  and the diaphragm  3  do not have to be bonded to each other. However, the piezoelectric element  2  and the diaphragm  3  may be bonded to each other. 
     When the piezoelectric element  2  and the diaphragm  3  are not bonded to each other, the piezoelectric element  2  may undergo bending deformation more freely as compared with the case where both components are bonded to each other. Thus, a large displacement can be obtained. This can enhance a pumping efficiency. 
     Third Embodiment 
       FIGS. 8(   a ) through  8 ( c ) illustrate a preferable third embodiment of the present invention.  FIG. 8(   a ) illustrates a non-drive state or a voltage-switching state,  FIG. 8(   b ) illustrates a state where the piezoelectric element  2  is deformed to bulge upwardly, and  FIG. 8(   c ) illustrates a state where the piezoelectric element  2  is deformed to bulge downwardly. 
     In this embodiment, blocks (support members)  1   d  are provided at the recess  1   a  of the bottom plate  1 . The blocks  1   d  support both end portions in the longitudinal direction, namely, two short sides of the piezoelectric element  2 . The piezoelectric element  2  is merely placed on the blocks  1   d , and is not bonded to the blocks  1   d . The blocks  1   d  may be integrally formed with the bottom plate  1 , or may be fixed onto the bottom plate  1  as additional members. A vibration space  1   e  is provided between the blocks  1   d . The piezoelectric element  2  is freely deformable in the vibration space  1   e.    
     As described above, both end portions in the longitudinal direction of the piezoelectric element  2  are supported by the blocks  1   d , so that the piezoelectric element  2  is lifted in the vibration chamber. Accordingly, when the piezoelectric element  2  is deformed to bulge upwardly as shown in  FIG. 8(   b ), the piezoelectric element  2  pushes up the diaphragm  3  at an almost center portion thereof to decrease the volume of the pump chamber  6 . Thus, the liquid in the pump chamber  6  can be pumped out. In contrast, when the piezoelectric element  2  is deformed to bulge downwardly as shown in  FIG. 8(   c ), the piezoelectric element  2  is displaced such that the diaphragm  3  is pulled down. Since the vibration space  1   e  is provided between the blocks  1   d , a center portion of the piezoelectric element  2  can be markedly displaced downwardly. The diaphragm  3  is simultaneously displaced by the downward displacement of the piezoelectric element  2 , so that the volume of the pump chamber  6  can be increased. Thus, the liquid can be sucked into the pump chamber  6 . 
     In this embodiment, the liquid can be sucked into the pump chamber  6  when the piezoelectric element  2  is deformed to bulge downwardly, whereas the liquid in the pump chamber  6  can be discharged when the piezoelectric element  2  is deformed to bulge upwardly. When the piezoelectric element undergoes upward or downward bending displacement in a bending mode, the blocks  1   d  constantly support both end portions of the piezoelectric element  2 . Hence, the piezoelectric element  2  is not floated, and the displacement of the piezoelectric element  2  can be effectively transmitted as a change in volume of the pump chamber  6 . With such a micropump of this embodiment, unlike the first embodiment, the bending of the piezoelectric element  2  in the reverse direction away from the pump chamber  6  can be effectively utilized. Thus, the discharge flow rate of the pump can be increased, and the pumping efficiency can be enhanced. 
     In the above-described embodiments, the piezoelectric element  2  is a bimorph-type piezoelectric element. The piezoelectric element of this type undergoes bending displacement equivalently in both directions when an alternating voltage is applied. Alternatively, for example, a piezoelectric element capable of being markedly displaced only in a direction may be employed. In the first embodiment, the discharge rate depends on the deformation to bulge upwardly of the piezoelectric element  2 . Hence, if a piezoelectric element capable of being largely displaced only upwardly is employed, the pumping efficiency can be enhanced. The piezoelectric element capable of being largely displaced only in a direction is obtained by a layer structure in which upper and lower layers are asymmetric to an intermediate layer. Alternatively, even with a layer structure in which upper and lower layers are symmetric, a piezoelectric element may be markedly displaced only in a direction if a positive voltage to be applied and a negative voltage to be applied are asymmetric and a large voltage is applied only to one of the upper and lower layers. Still alternatively, if both structures are combined, a further large displacement can be obtained. 
     In the above-described embodiments, the rectangular piezoelectric element is used. However, a square or circular piezoelectric element may be employed. It is noted that the rectangular piezoelectric element achieves a larger volume displacement than the square or circular piezoelectric element does. Thus, the rectangular piezoelectric element can realize a small, high-efficient micropump. 
     In the above-described embodiments, the bottom plate defining the case serves as the support member for supporting the back surface of the piezoelectric element. However, the support member may be an additional member which is separated from the case. In this case, the material of the support member is not limited to a hard material, and may be a soft material such as elastic rubber. Further, the case is not limited to one including the bottom plate, the frame, and the top plate as shown in  FIG. 2 . The case may have any structure as long as the pump chamber is isolated by the diaphragm, and the support member for supporting the back surface of the piezoelectric element may be provided.