Abstract:
A piezoelectric transducer includes a single crystal piezoelectric material having a phase transition from one crystalline phase to a second crystalline phase at a predetermined stress level. A pre-stress is applied to the single crystal piezoelectric material so that the material is maintained near its phase transition point. An electrical field source is joined to the material such that, in cooperation with the pre-stress, an increase or decrease in the electrical field causes a crystalline phase transition in the single crystal piezoelectric material. Crystalline phase transition induces strain larger by an order of magnitude than that caused by the non-phase transition piezoelectric effect.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     CROSS REFERENCE TO OTHER PATENT APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention is directed generally towards piezoelectric transducers, and in particular, to a piezoelectric transducer that uses a crystalline phase transformation to achieve improved elongation. 
     (2) Description of the Prior Art 
     Most transduction devices are based on strain produced in a piezoelectric or magnetostrictive material. These materials can produce relatively large strain in a linear region, but in order to develop high strain they must be driven by a very high electric or magnetic field. There are several other classes of materials utilizing high strain associated with martensitic phase transition (namely, shape memory alloys). However, high strain in shape memory alloys (SMA) is thermally activated, and transduction devices based on these materials have frequency band limitations. For this reason, piezoelectric and magnetostrictive materials are most often used in transducer applications. 
     The current most promising class of transducer materials are relaxor-ferroelectric piezoelectric single crystals. These materials are single crystals of piezoelectric materials (for example, lead zinc niobate-lead titanate, known hereinafter as PZN-PT). These materials have been shown to deliver extraordinarily high strain when an external electric field is applied as compared to conventional polycrystalline piezoceramic. 
     In some special compositions (for example ternary lead indium niobate-lead magnesium niobate-lead titanate (PIN-PMN-PT)), the material will undergo a phase transformation accompanied by a very sharp hysteretic strain and a dramatic change in stiffness when subjected to external stress. This phase transformation can be invoked repeatedly at variable rates to induce large strains in the single crystal element. Known compositions exhibiting this type of phase change behavior include (1−x)PZN-xPT where 0.04&lt;x&lt;0.11. The composition where x was 0.06 has been tested at multiple temperatures and applied direct current (DC) bias fields. Under these conditions, it has been shown to exhibit a phase transformation. 
     It is known to combine single crystal piezoelectric materials with mechanical stress inducing means. This is typically performed in order to avoid putting the piezoelectric material in tensile stress because of the fragility of ceramic materials in tension. In a prior art mechanically induced stress application, the stress is calculated to be that which is optimal for insuring piezoelectric material life in the operating conditions of the application. These operating conditions can include varying environmental temperatures and pressures. 
     It is also known to include an electrically controllable stress element in combination with a piezoelectric single crystal material. This element can be either a piezoelectric (voltage driven) element or a magnetostrictive effect (MS) element. These hybrid magnetostrictive-piezoelectric transducer systems are known to work effectively in a linear region. 
     In both mechanical and electrical stress generation means, applications avoid utilizing stress near the phase transition stress level. This is because the piezoelectric material shows a non-linear response at this level that may be triggered by environmental temperature or pressure. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide transducer that has a strain response to an electric field that is much greater than heretofore known. 
     It is a further object of the present invention to provide a transducer utilizing a crystalline phase transition as its mode of operation. 
     Accordingly, there is provided a piezoelectric transducer that includes a single crystal piezoelectric material having a phase transition from one crystalline phase to a second crystalline phase at a predetermined stress level. A pre-stress is applied to the single crystal piezoelectric material so that the material is maintained near its phase transition point. An electrical field source is joined to the material such that, in cooperation with the pre-stress, an increase or decrease in the electrical field causes a crystalline phase transition in the single crystal piezoelectric material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become apparent upon reference to the following description of the preferred embodiments and to the drawings, wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawings and wherein: 
         FIG. 1  is a graph showing strain versus stress for a representative phase change piezoelectric material; 
         FIG. 2  is a graph showing an applied electric field superimposed with the strain response for a single crystal piezoelectric material undergoing phase change; 
         FIG. 3A  is a perspective view of a first crystal orientation of a phase change piezoelectric crystal operating in the “32” mode; 
         FIG. 3B  is a perspective view of a second crystal orientation of a phase change piezoelectric crystal operating in the “33” mode; 
         FIG. 4  is a diagram of an apparatus for utilizing a phase change piezoelectric crystal; 
         FIG. 5  is a cross-sectional view of an embodiment of a phase change transducer utilizing a mechanical pre-stress; 
         FIG. 6  is a diagram of an apparatus utilizing a phase change crystal with an electronically adjustable pre-stress; 
         FIG. 7  is a diagram of a phase change transducer configured as a flextensional transducer; 
         FIG. 8  is a diagram of a phase change transducer configured as a Tonpilz transducer employing mechanical pre-stress, adjustable electronic pre-stress and a single piezoelectric crystal; and 
         FIG. 9  is a diagram of a phase change transducer configured as a bender bar transducer. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention utilizes single crystal piezoelectric materials, known more specifically as relaxor-ferroelectric single crystal compositions, which operate near the morphotropic phase boundary (MPB). In these types of crystals, the morphotropic phase boundary is the temperature/pressure region in which the crystalline structure changes from a rhombohedral structure to a tetragonal structure through intermediate states. In this process, energy is stored in the crystal before the phase change, and this energy is suddenly released during the phase change. Binary materials showing this transition include (1−x)Pb(B′ 1/3 Nb 2/3 )O 3 -xPbTiO 3  where B′ is Mg, Mn, Zn, or Sc. One known material in this range, that is useful for this purpose incorporates Zn as B′, and x is in the range of 0.04&lt;x&lt;0.11. Ternary single crystal compositions, such as xPb(In 1/2 Nb 1/2 )O 3 -(1-x-y)Pb(Mg 1/3 Nb 2/3 )O 3 -yPbTiO 3  (PIN-PMN-PT), also show these properties. Useful crystals in this group have been found where x is in the range of 0.24-0.33 and y is in the range of 0.28-0.33. Other ranges and similar compositions of binary and ternary crystals may have beneficial properties. A particularly desirable characteristic of these materials is a sharp phase transition. 
       FIG. 1  is a graph that generically depicts a hysteresis curve  10  of a single crystal material having a sharp elastic instability near the phase transition point. As compressive stress increases in the material, strain response increases linearly in region  12 A. A phase transition point occurs at  14 A. During phase transition the response is shown as indicated at  16 A. Strain response is rapid during phase transition. The phase transition completes as indicated at  18 A and a second linear region begins as indicated at  20 A. As compressive stress is reduced, strain declines linearly in region  20 B. Once stress declines sufficiently, a phase transition occurs at  18 B. The response during phase transition is shown at  16 B, and phase transition completes at  14 B. The response enters the linear region at  12 B as stress declines further. For a given material, the position of this curve and its inflection points depend on pressure, temperature and the electrical field to which the material is subjected. 
     A pronounced strain change such as that indicated at  14 A and  14 B is essential for the proposed transducer. This jump can be induced by any possible external driving element. For example, this driving element can be a magnetostrictive element driven by a magnetic field generated in a coil. Alternatively, this can be secondary driving piezoelectric element or the single crystal element can be preloaded to a certain critical stress. The critical stress is a characteristic of the particular material composition, and it can be determined experimentally for given temperature and electric field. 
       FIG. 2  provides a graph of an applied electrical field versus time superimposed with the strain response of the single crystal piezoelectric material. The applied electrical field is indicated with hollow, circular datums, and the strain response is shown with filled, circular datums. The applied electrical field is a sine wave. The strain response is similar to a square wave; however, there are some features worth noting. The linear response region, shown in  FIG. 1  as  12 A and  12 B, is the angled region shown at the top of the strain response curve in  FIG. 2 . The phase transition occurs in the vertical region of the strain response curve. Another linear response region is evident at the bottom of the curve. In this material at this phase, additional voltage doesn&#39;t result in a significant amount of additional strain. 
     In application, the single crystal piezoelectric material begins with an initial strain (˜−0.002) when the applied electric field is near zero and the pre-stress is about 10 MPa. When the electric field approaches about −1 KV/cm induced phase transition strain is about −0.006 with a total strain of ˜0.004. The strain curve levels out once the phase transition is complete. As shown by the remainder of the electric field and strain curves, this process can be repeated periodically with essentially the same strain response. 
     Strain caused by crystalline ferroelectric phase change can be distinguished from that caused by ordinary piezoelectric strain by the magnitude of the strain. PZT composite ceramics have a piezoelectric coefficient of around 200 pm/V and are capable of inducing a strain of ˜0.00002 (2×10 −5 ). Single crystal piezoelectric materials when operated in the linear mode have a piezoelectric coefficient of ˜2000 pm/V. For the drive field E (max) ˜100,000 V/m, the generated strain is ˜0.0002 (2×10 −4 ). In the phase change piezoelectric single crystals used in the current invention, generated strain can range from ˜0.001 (1×10 −3 ) to possibly as high as ˜0.008 (8×10 −3 ). Experimentation has shown strain values of 0.004 (4×10 −3 ) with an electric field of ˜0.1-0.2 MV/m when the material is subjected to a pre-stress of at least 10-20 MPa. 
     These crystals can be driven in the “32” mode or in the “33” mode.  FIG. 3A  shows crystal orientation for driving in the “32” mode, and  FIG. 3B  shows crystal orientation for driving in the “33” mode. In  FIG. 3A , stress σ 22  and strain ε 22  occur in the direction indicated by arrow  22 . The X 1  axis is in the direction given by the vector [0  1 1]. The X 2  axis is in the direction given by the vector [100], and the X 3  axis is in the direction given by the vector [011]. The arrows indicated by P s  and vector directions [  1 11] and [111] show possible polarization vectors for these materials driven in the “32” mode. Strain in direction  22  is responsive to an electric field in the direction indicated by arrow  24 . 
       FIG. 3B  shows a piezoelectric crystal driven in the “33” mode having stress σ 33  and strain ε 33  in the direction indicated by arrow  26 . This is in response to an electric field generated in the direction indicated by arrow  28 . The crystal has axes X 1  in the direction given by the vector [100], X 2  in the direction given by [010], and X 3  in the direction [001]. Possible polarizations are indicated at the vectors identified as P s  in directions [111], [1  1 1], [  1 11], and [  1   1 1]. 
       FIG. 4  provides a schematic for an apparatus joined to a phase change transducer  30 . The phase change transducer  30  has electrodes joined to a controller  32 . The electrodes can be connected in either a “33” mode wherein the electric field is provided in the same direction as the strain or in a “32” mode wherein the electric field is orthogonal to the strain. Controller  32  is joined to a voltage source  34 . Controller  32  is also joined to a pressure sensor  36  and a temperature sensor  38 . Pressure sensor  36  and temperature sensor  38  are positioned to detect environmental conditions affecting transducer  30 . Controller  32  provides a base level of voltage to transducer  30  dependent on the environmental conditions. This allows controller to create a pre-stress in transducer  30  that is calculated based on the environmental conditions to place transducer  30  near the phase transition point indicated at  16  or  18  of  FIG. 1 . The calculated pre-stress can be based on experimental data or theoretical information related to the single crystal or the crystalline composition. 
       FIG. 5  shows an embodiment of the mechanical arrangement of a phase change transducer  40  capable of creating a mechanical pre-stress in a transducer crystal  42 . This embodiment shows a “32” mode transducer. Electrodes  44  are positioned on opposing sides of crystal  42 . Electrodes  44  can be joined to an electrical field source (not shown). Crystal  42  is positioned between stress plates  46 . A stress bolt  48  having an axis  48 A is positioned to compress stress plates  46 . Bolt  48  has a head  48 B at a first end and threads  48 C at a second end. Compression can be adjusted by a nut  50  positioned on threaded end  48 C of stress bolt  48 . Tension in bolt  48  results in an adjustable pre-stress in crystal  42 . Crystal  42  and electrodes  44  should be insulated from plates  46  and bolt  48 , if these items are conductive. 
     In “32” mode operation, an increase or decrease in the electric field applied to electrodes  44  causes a phase change in crystal  42 . This results in positive or negative strain along axis  48 A of bolt  48 . Because piezoelectric materials are reciprocal, a change in stress applied along axis  48 A also results in an electrical field being generated between electrodes  44 . 
       FIG. 6  provides a diagram showing an embodiment using an electrically controllable, adjustable pre-stress  52 . In this embodiment, a piezoelectric crystal  54  is mechanically joined to the adjustable pre-stress  52 . The crystal  54  and adjustable pre-stress  52  assembly is subjected to a mechanical pre-stress or containment as indicated by arrows  56 . Adjustable pre-stress  52 , crystal  54  and mechanical pre-stress  56  are in series such that an increase in one component causes an increase in the stress of all of the components. Adjustable pre-stress  52  and crystal  54  are separately joined to a controller  58 . Insulating material  59  can be positioned between adjustable pre-stress  52  and crystal  54 . Adjustable pre-stress  52  can be a piezoelectric composite or single crystal material, a magnetostrictive material, or some other material capable of increasing its dimension in response to controller  58 . In the case of a piezoelectric material, controller  58  can provide an electrical field to the piezoelectric adjustable pre-stress  52 , increasing or decreasing its dimension and thereby changing the pre-stress in the assembly. In the case of a magnetostrictive material, controller  58  can increase current in a coil which subjects the magnetostrictive material to a magnetic field. This increases the relevant dimension in the magnetostrictive material resulting in increased or decreased pre-stress in the assembly. These are merely examples of an electrically controlled, adjustable pre-stress, and it is envisioned that other structures known in the art could provide this functionality. Optionally, controller  58  can be joined to sensors such as those shown in  FIG. 4 . 
       FIG. 7  shows another embodiment of the current invention. This embodiment shows the apparatus formed as a flextensional type of transducer  60 . Small displacements along the semi-major axis  62 A result in large displacements along the semi-minor axis  62 B providing an efficient low frequency transducer. In flextensional transducer  60 , a flexing shell  64  acts as the mechanical pre-stress. The resonance frequency of transducer is determined by the flexing shell  64  size and material. Phase change crystals  66  are positioned in series with an electrically controllable, pre-stress component  68 . Pre-stress component  68  can be a magnetostrictive transducer or a separately controlled piezoelectric transducer. 
     For high strain, phase change operation, flexing shell  64  applies a static stress to the single crystals  66  by way of the shell&#39;s  64  elasticity. This compressive stress brings the single crystals  66  close to the phase transition point. To excite the transducer  60 , pre-stress component  68  provides a dynamic stress that causes the single crystals  66  to change state. Releasing this stress causes crystals  66  to return to the original state. The required dynamic stress is related to how close the static stress can come to the phase transition point and remain stable. As provided previously the dynamic stress can be generated electrically by the piezoelectric effect through the crystals  66  or through an external magnetostrictive or electrostrictive actuator such as pre-stress component  68 . Pre-stress component can adjust static stress as needed, for example, to compensate for changes in pressure and temperature that would affect the shell&#39;s  64  compressive load. 
       FIG. 8  shows a cross-sectional view of another transducer  70  utilizing a Tonpilz configuration with a magnetostrictive pre-stress component  72 . Magnetostrictive pre-stress component  72  can be a magnetostrictive material  74  (i.e., TERFENOL or GALFENOL) positioned within a solenoid coil  76 . Using a variable current source, solenoid coil  76  can provide a magnetic field which acts on the magnetostrictive material  74  to induce a static stress. Magnetostrictive pre-stress component  72  is joined to a piezoelectric phase change single crystal  78 . Electrodes  80  are positioned to provide an electric field through single crystal  78  causing strain in the electric field direction. This provides a “33” mode piezoelectric transducer. Mechanical pre-stress is applied to magnetostrictive pre-stress component  72  and phase change single crystal  78  by a tension bolt  82  having a bolt head  84  at a first end. A second end  86  is threaded and fitted with a nut  88 . Bolt head  84  and nut  88  apply compression to compression plates  90  positioned next to the combined magnetostrictive pre-stress component  72  and phase change single crystal  78 . Tension bolt  82  provides a static pre-stress to component  72  and crystal  78 . The static pre-stress positions crystal  78  near its phase change point. 
     Current supplied to solenoid coil  76  increases the critical dimension of magnetostrictive material  74  and causes additional stress in crystal  78 . This stress can be controlled to optimize phase change in crystal  78  in view of varying environmental conditions. Crystal  78  can then operate in the passive mode in which compression of crystal  78  causes a phase change and results in the creation of a an electric field, or, in the active mode, in which an electromagnetic field causes a phase change resulting in strain in the transducer. 
     In  FIG. 9 , there is shown a diagram of a bender bar embodiment  94  of the phase change transducer. In this embodiment, a bending bar  96  made from a magnetostrictive material is bonded to a single crystal layer  98  such that the single crystal layer  98  is held in compression. Electrodes  100  are in contact with the outer surface and inner surface of the bent crystal layer  98 . Applying an electric field to the electrodes  100  causes strain in the crystal layer  98 . Magnetostrictive material  96  is joined to a coil  102 . Pre-stress in crystal layer  98  can be adjusted by providing an electrical current to coil  102  which causes a stress in bending bar  96 . As in other embodiments, external stress can induce an electric field in single crystal layer  98  allowing for passive operation of the embodiment  94 . 
     It will be understood that additional variations and alternatives in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made as understood by those skilled in the art within the principles and scope of the invention as expressed in the appended claims. The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description only. It is not intended to be exhaustive nor to limit the invention to the precise form disclosed; and obviously many modifications and variations are possible in light of the above teaching. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of this invention as defined by the accompanying claims.