Abstract:
A transducer for generating electrical energy from an expected force includes a single crystal ferroelectric material having a phase transition stress level. Mechanical stress is provided to this crystal at a level approaching the phase transition stress level, such that the expected external force will cause the phase transition. At least two electrodes are joined to the single crystal for receiving electrical energy created by the phase transition. The electrodes can be joined to conditioning and storage circuitry. In further embodiments, the phase transition is induced by an expected temperature change.

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 
     This application claims the benefit of U.S. Provisional Application No. 61/756,542, filed on Jan. 25, 2013, which is a continuation-in-part of U.S. patent application Ser. No. 13/053,577, filed on Mar. 22, 2011, and entitled “Crystalline Relaxor-Ferroelectric Phase Transition Transducer.” 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention is directed generally towards energy harvesting utilizing ferroelectric piezocrystal material transducers, and in particular, to a ferroelectric generator that utilizes a crystalline phase transformation to achieve higher energy density. 
     (2) Description of the Prior Art 
     While piezoelectric materials have been successfully utilized in sensors and actuators, their use as practical power sources for generating a useful amount of electricity in portable generators has been limited by the small amounts of power available and the efficiency of generating this power. Most practically applied piezoelectric energy harvesting is performed by piezoelectric composite materials. These materials incorporate a plurality of piezoelectric crystals in a matrix material. Efficiency is limited by the matrix material, coupling inefficiencies and crystal orientations. 
     Single crystal ferroelectric materials are also known in the art for energy harvesting. While more efficient than composite materials, these materials operate in the linear region of the ferroelectric response curve because the linear region is the operating region without pre-stress or bias electric field. 
     The current most promising class of materials for energy harvesting are relaxor-ferroelectric single crystals. These materials are single crystals of ferroelectric materials (for example, lead zinc niobate-lead titanate, known hereinafter as PZN-PT). These materials have been shown to deliver a high voltage at greater efficiency when the crystal is subjected to stress. In some special compositions (for example certain compositions of 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 or single crystal 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 of relaxor ferroelectric single crystals. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an efficient energy harvesting apparatus for a known force change. 
     It is a further object of the present invention to provide an efficient energy harvesting apparatus for a known temperature change. 
     Accordingly, there is provided a transducer for generating electrical energy from an expected external force that includes a single crystal ferroelectric material having a phase transition stress level. Mechanical stress is provided to this crystal at a level approaching the phase transition stress level, such that the expected external force will cause the phase transition. At least two electrodes are joined to the single crystal for receiving electrical energy created by the phase transition. The electrodes can be joined to conditioning and storage circuitry. In further embodiments, the phase transition is induced by an expected external temperature change. 
    
    
     
       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. 2A  is a perspective view of a first crystal orientation of a phase change piezoelectric crystal operating in the “32” mode; 
         FIG. 2B  is a perspective view of a second crystal orientation of a phase change piezoelectric crystal operating in the “33” mode; 
         FIG. 3  is a diagram of an apparatus utilizing a phase change crystal for energy harvesting with a controller and an electronically adjustable pre-stress; 
         FIG. 4  is a cross-sectional view of an embodiment of a phase change transducer utilizing a mechanical pre-stress; 
         FIG. 5  is a diagram of a phase change transducer configured as a flextensional transducer; 
         FIG. 6  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. 7  is a diagram of a phase change transducer particularly configure to capture energy from temperature changes. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention utilizes single crystal ferroelectric 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. 
     Binary materials showing this transition include Pb(B′ 1/3 Nb 2/3 )O 3 —PbTiO 3  where B′ is one metal selected from the set of metals including Mg, Mn, Zn, and Sc. 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 about 0.24-0.33 and y is in the range of about 0.28-0.33. Other ranges and similar compositions of binary and ternary crystals may have beneficial properties. 
       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. In one known material, this compressive pre-stress occurs in the region between about 24 MPa and 21 MPa as indicated in the FIG. The resulting microstrain (με) is between −2,500 and −5,400 as shown in the FIG. The critical stress is a characteristic of the particular material composition, and it can be determined experimentally for a given temperature and operating condition. 
     Ferroelectric relaxor single crystals can be used to provide an electrical response when subjected to stress in the “32” mode or in the “33” mode.  FIG. 2A  shows crystal orientation for stress in the “32” mode, and  FIG. 2B  shows crystal orientation for stress in the “33” mode. In  FIG. 2A , 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. 2B  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. 3  provides a schematic for an energy harvesting device  30  according to one embodiment. A relaxor-ferroelectric single crystal  32  is positioned having a pre-stress indicated by arrows  34 . Crystal  32  harvests energy from an additional fluctuating stress in the same direction  34 . Crystal  32  can also harvest energy from a slight environmental temperature change that results in a phase transition. Crystal  32  has electrodes disposed thereon and joined to a conditioning and storage device  36 . Conditioning and storage device  36  can be any electronic circuitry capable of transforming transient voltage provided by crystal  32  into a useful form. In other embodiments conditioning and storage device  36  can be an electrical load directly joined to crystal  32 . The electrodes can be connected in either a “33” mode wherein the electric field is produced in the same direction as the strain or in a “32” mode wherein the electric field is produced orthogonal to the strain. 
     Device  30  can further include a controller  38  and a device  40  giving an adjustable pre-stress. Pre-stress device  40  is in series with crystal  32  and other pre-stress means. Device  40  can be a mechanical, piezoelectric, magnetostrictive or other device known in the art that is capable of contracting or expanding in response to a control signal. Insulating material can be positioned between pre-stress device  40  and crystal  32 . Controller  38  can also be joined to a pressure sensor  42  and a temperature sensor  44 . Pressure sensor  42  and temperature sensor  44  are positioned to detect environmental conditions affecting crystal  32 . Controller  38  can provide a signal to adjustable pre-environmental conditions. This allows controller  38  to create a pre-stress in crystal  32  that is calculated based on the environmental conditions to it 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. 4  shows an embodiment of the mechanical arrangement of an energy harvesting device  48  capable of creating a mechanical pre-stress in a transducer crystal  50 . This embodiment shows a “32” mode transducer. Electrodes  52  are positioned on opposing sides of crystal  50 . Electrodes  52  can be joined to a load (not shown). Crystal  50  is positioned between stress plates  54 . A stress bolt  56  having an axis  56 A is positioned to compress stress plates  54 . Bolt  56  has a head  56 B at a first end and threads  56 C at a second end. Compression can be adjusted by a nut  58  positioned on threaded end  56 C of stress bolt  56 . Tension in bolt  56  results in an adjustable pre-stress in crystal  50 . Crystal  50  and electrodes  52  should be insulated from plates  54  and bolt  56 , if these items are conductive. 
     In “32” mode operation, an increase or decrease in the tension in bolt  56  causes a phase change in crystal  50  and generates an electric field between electrodes  52 . 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 . This results in positive or negative strain along axis  48 A of bolt  48 . 
       FIG. 5  shows another embodiment of the current invention. This embodiment shows the apparatus formed as a flextensional type of transducer  60  for harvesting energy. 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 generate energy, tension in the shell  64  along axis  62 B provides a compressive stress along axis  62 A 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. The crystals  66  can be brought closer to phase transition point by pre-stress component  68 . Pre-stress component  68  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. 6  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 receive an electric field generated by single crystal  78  when subjected to strain in the electric field direction. This provides a “33” mode piezoelectric transducer energy harvesting device. 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 an electric field. 
     Another embodiment  100  given in  FIG. 7  features a device wherein an increase in temperature increases stress in a crystal  102  while the increased temperature is also promoting a phase transition. Electrical contacts (not shown) are positioned on crystal  102  in either a “32” or “33” mode of operation. Crystal  102  is mounted between stress members  104  in a housing  106 . Stress members have a thermal coefficient of expansion that is chosen to assist with providing stress to crystal  102  when subjected to a temperature change. As mounted, crystal  102  is subjected to a base level of pre-stress. Housing  106  has lower thermal coefficient of expansion than stress members  104  allowing housing  106  to maintain pre-stress. An increase in environmental temperature results in an expansion of stress members  104 , increasing the compression of crystal  102 . Increase in environmental temperature also brings the crystal  102  closer to its phase transition temperature. These temperatures can also be controlled by providing a heat source at stress members  104  and crystal  102 . Electrical energy released by crystal on phase transition is received at the contacts. Thus, there could be a hybrid device for harvesting energy from both temperature changes and stress changes or for magnifying stress and temperature changes. 
     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.