Abstract:
A method of poling a ferroelectric sample suitable for use in a fuel injector of an internal combustion engine, the method comprising providing a ferroelectric sample having a stack of ferroelectric layers, wherein adjacent layers are separated by internal electrodes, forming a first group and a second group of electrodes; applying a multiaxial pressure to the ferroelectric sample; reducing the bonding strength between each pair of adjacent ferroelectric layers and internal electrodes; and generating a first electric field between the first and second group of electrodes to pole each ferroelectric layer in a first poling direction.

Description:
TECHNICAL FIELD  
       [0001]    The present invention relates to a method of poling a sample of ferroelectric material, comprising a plurality of ferroelectric layers arranged in a stack, so as to induce bulk piezoelectricity. In particular, but not exclusively, the invention relates to a method of poling multilayer ferroelectric samples of the type suitable for use in piezoelectric actuators for fuel injection systems for internal combustion engines. 
       BACKGROUND OF THE INVENTION  
       [0002]      FIG. 1  is a schematic view of a piezoelectric actuator  2  of the type commonly used to actuate a valve needle of a fuel injector  5  (shown in  FIG. 2 ) for a compression-ignition internal combustion engine. The actuator  2  includes a poled piezoelectric stack  2   a  having a plurality of piezoelectric layers  4  separated by a plurality of internal electrodes forming positive and negative electrode groups  6   a  and  6   b,  respectively.  FIG. 1  is illustrative only and in practice the stack  2   a  would include a greater number of layers  4  and electrodes  6   a,    6   b  than those shown. Arrows, exemplified by arrows  4   a  and  4   b,  between adjacent interdigitated electrodes  6   a,    6   b  indicate the dominant direction of remanent polarisation of the dipoles contained in the piezoelectric layers  4 ; wherein, the arrow-head of each arrow  4   a,    4   b  indicates the position of the negative pole of each dipole, and the arrow-tail indicates the position of the positive pole of each dipole. The arrows are illustrative only and in practice there would be many more dipoles than indicated in the figures. 
         [0003]    The electrodes of the positive group  6   a  are interdigitated with the electrodes of the negative group  6   b,  with the electrodes of the positive group  6   a  connecting with a positive external electrode  8   a  and the electrodes of the negative group  6   b  connecting with a negative external electrode  8   b.  The positive and negative external electrodes  8   a,    8   b  receive an applied voltage, in use, that produces an intermittent electric field between adjacent interdigitated electrodes  6   a,    6   b.  The intermittent electric field rapidly varies with respect to its strength. In turn, this causes the stack  2   a  to extend and contract along the direction of the applied field. 
         [0004]    A lower end cap  10   b  is adjacent to the lowermost piezoelectric layer  4  of the stack  2   a  and an upper end cap  10   a  is adjacent to the uppermost piezoelectric layer  4  of the stack  2   a.  The lower end cap  10   b  is coupled to an injector valve needle  7  (shown in  FIG. 2 ), either directly or through an intermediate mechanical and/or hydraulic coupling. Thus, as the stack  2   a  extends and contracts upon application of the electric field, the injector valve needle  7  is caused to move to control injection of pressurised fuel into an associated engine cylinder (not shown). 
         [0005]    Referring to  FIG. 2 , in order to prevent injection of fuel into the cylinder, the injector valve needle  7  securely abuts an injector nozzle seating  15   b;  thereby preventing fuel from passing through fuel channels  15   a  in the nozzle  15 . This is achieved by applying a voltage of, for example, 200V to the electrodes of the positive group  6   a  which causes the stack  2   a  to extend. The electrodes of the negative group  6   b  are maintained at 0V. Due to fuel injection taking a relatively short period of time, the fuel injector valve needle  7  is engaged with the associated seating  15   b  in the aforementioned manner for approximately 95% of the fuel injector&#39;s operating cycle. 
         [0006]    To inject fuel into the cylinder, the voltage applied to the electrodes of the positive group  6   a  is rapidly reduced, thereby causing the stack  2   a  to contract. The amount that the voltage is reduced is dependent on the pressure of the fuel. For example, at a minimum pressure of around 200 bar (such as when the engine is idling) the voltage applied to the electrodes of the positive group  6   a  will drop to 20V, and at a maximum pressure of around 2000 bar the voltage applied to the electrodes of the positive group  6   a  will drop to −20V, briefly making the electrodes of the positive group  6   a  negative. 
         [0007]    In order for the piezoelectric actuator  2  to behave in the aforementioned manner, it is necessary to pole the piezoelectric stack  2   a.  One known process of poling the piezoelectric stack  2   a  will be described with reference to  FIGS. 3 to 5 . 
         [0008]    An example of an unpoled multilayer ferroelectric sample of which the piezoelectric stack  2   a  is comprised is shown schematically in  FIG. 3 . A multilayer structure  3  is formed from a plurality of relatively thin ferroelectric ceramic layers  4 , as in  FIG. 1 . An example of a ferroelectric material is lead zirconate titanate, also known by those skilled in the art as PZT. 
         [0009]    The multilayer structure  3  is poled by applying a potential difference across the positive and negative external electrodes  8   a,    8   b  which, in turn, apply the potential difference across the internal groups of positive and negative electrodes  6   a,    6   b.  In order to achieve poling of the dipoles contained within the piezoelectric material, the dipoles must be exposed to an electric field large enough to cause permanent crystallographic realignment and dipole reorientation. The minimum electric field strength necessary to affect this change is referred to as the “coercive” field strength. Due to the alternating polarity of the internal electric field, the poling direction of the dipoles within the piezoelectric material alternates throughout the structure, as indicated by the arrows, exemplified by arrows  4   c  and  4   d,  orthogonal to the internal groups of positive and negative electrodes  6   a,    6   b  in  FIG. 4 . 
         [0010]    A ferroelectric multilayer will only be poled where it is exposed to the coercive electric field. As shown in  FIG. 5 , once the coercive field has been applied to the multilayer structure  3 , the central regions of the ferroelectric layers  4  contained between adjacent oppositely charged electrodes  6   a,    6   b  (shown as central region  14 ) are poled. The central region  14  is also known as the active zone. 
         [0011]    In order to avoid a short circuit, each electrode of the internal groups of positive and negative electrodes  6   a,    6   b  is terminated such that it does not contact the respective external electrode of the opposite polarity  8   a,    8   b.  Thus, on either side of the central region  14  there is defined a side region  12  which forms an isolation zone (or margin). The side regions  12  are also known as the inactive zone. Accordingly, the ends of the piezoelectric layers  4  contained in the side regions  12  are not subjected to an electric field because adjacent electrodes in these regions are at the same potential; thus, the piezoelectric material in the side regions  12  remains unpoled. This gives rise to a ferroelectric strain discontinuity between the regions of poled and unpoled ceramic, placing the unpoled material in tension and the poled material in compression. This is because, when a ferroelectric material is poled, the crystallographic realignment creates a permanent elongation along the axis of the applied field, known as “ferroelectric strain”. In the multilayer structure  3  this elongation only occurs within the interdigitated central region  14 . As a result, the poled material is clamped, which may adversely affect the actuator stroke, i.e. some clamping force may increase the stroke relative to zero load conditions, whereas a very high clamping force will reduce the stroke. Longitudinal distortion may occur if the clamping of the poled material by the unpoled material in the side regions  12  is not uniform. Furthermore, the unpoled material tends to fracture and crack due to the tensile forces created within it by the elongation of the central region  14 . These cracks are called “poling cracks”. 
         [0012]    The poling cracks are of different types. Some poling cracks form parallel to the plane of inner electrodes  6   a,    6   b,  and are known as “parallel poling cracks”, whereas some deviate from the inner electrode plane direction, and are known as “deviated poling cracks”. The parallel poling cracks  13  are the most common type of poling cracks and a number of such cracks are identified in  FIG. 1 . Typically, the parallel poling cracks  13  form at the interface between an inner electrode and the adjacent ferroelectric layer  4 . The deviated poling cracks  16  are the most dangerous cracks since they can potentially link inner electrodes of opposite polarities. This can lead to failure of the piezo stack by dielectric breakdown in the deviated crack. 
         [0013]    When the actuator is in use, the deviated cracks  16  are worsened due to the repeated tensile strain imposed by the rapidly intermittent electric field which further fatigues the composite structure in the side regions  12 . In addition to the temporary elongation of the stack  2   a  produced by the intermittent field, the permanent and temporary straining of the poled central region  14  causes the end caps  10   a,    10   b  to experience lateral compression and bending which manifests itself as doming of the end caps  10   a,    10   b;  illustrated in  FIG. 1 . 
         [0014]    Ferroelectric materials and their associated poling are discussed further in the Applicant&#39;s granted European patent number 1 516 373. 
         [0015]    It is an object of the present invention to provide a method of poling a sample of ferroelectric material which removes or alleviates the aforementioned problems. 
       SUMMARY OF INVENTION  
       [0016]    According to a first aspect of the invention, there is provided a method of poling a ferroelectric sample suitable for use in a fuel injector of an internal combustion engine, the method comprising; 
         [0017]    providing a ferroelectric sample having a stack of ferroelectric layers, wherein adjacent layers are separated by internal electrodes, forming a first group and a second group of electrodes; 
         [0018]    applying a multiaxial pressure to the ferroelectric sample; 
         [0019]    reducing the bonding strength between respective adjacent pairs of ferroelectric layers and internal electrodes; and 
         [0020]    generating a first electric field between the first and second group of electrodes to pole each ferroelectric layer in a first poling direction. 
         [0021]    Thus, by reducing the ferroelectric layer/electrode bonding strength prior to poling the ferroelectric sample, the generation of deviated poling cracks can be significantly reduced or eliminated. Accordingly, the possibility of a short circuit occurring between adjacent inner electrodes is reduced. 
         [0022]    In one embodiment, the step of reducing the bonding strength may include applying a first negative voltage to said first group of electrodes; removing said first negative voltage; applying a second negative voltage to said second group of electrodes; and removing said second negative voltage before generating said first electric field between the first and second groups of electrodes. 
         [0023]    Alternatively, the step of reducing the bonding strength may include generating a second electric field between the first and second group of electrodes to pole the each ferroelectric layer in an operational poling direction, opposite to said first poling direction. 
         [0024]    Advantageously, the method comprises positioning the ferroelectric sample within the fuel injector before generating the electric field between the first and second groups of electrodes. This provides a convenient way of applying a multiaxial pressure to the sample, for example by way of a hydrostatic load generated by high pressure fluid in which the sample is immersed. It is preferred, although not essential, that the fluid is fuel, e.g. diesel fuel which is used with the injector. 
         [0025]    In order to ensure that the generation of cracks is minimised, said multiaxial pressure may be greater than 1500 bar. However, a higher multiaxial pressure, for example approximately 2000 bar, should ensure a higher degree of crack elimination. 
         [0026]    Conveniently, the first and second groups of electrodes are interdigitated. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0027]    Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which; 
           [0028]      FIG. 1  illustrates a poled piezoelectric actuator for use in a fuel injector comprising piezoelectric layers separated by interdigitated electrodes; 
           [0029]      FIG. 2  illustrates a fuel injector for an internal combustion engine of the type in which the piezoelectric actuator of  FIG. 1  may be employed; 
           [0030]      FIG. 3  illustrates an unpoled multilayer ferroelectric sample of which the actuator of  FIG. 1  is comprised; 
           [0031]      FIG. 4  illustrates the multilayer ferroelectric sample of  FIG. 3  after poling, with the poling direction of the dipoles shown schematically throughout the sample; 
           [0032]      FIG. 5  is an enlarged view of a part of the multilayer sample of  FIGS. 3 and 4 ; 
           [0033]      FIG. 6  illustrates a piezoelectric actuator, similar to the actuator of  FIG. 1 , during poling according to an embodiment of the present invention, with the poling direction of the dipoles shown schematically throughout the sample; 
           [0034]      FIG. 7  is a flowchart showing an embodiment of the invention; and 
           [0035]      FIG. 8  is a flowchart showing an alternative embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0036]    It has been ascertained that an important factor which determines the type of crack formed during the poling is the relationship between the strength of the piezo ceramic material itself, and the strength of the bonding between the piezo ceramic material and the inner electrodes  6   a,    6   b.  More specifically, the deviated cracks  16  occur mainly when the strength of the adhesion between inner electrodes and the piezo ceramic is in the same range as the strength of the ceramic itself. Therefore, the method according to the first embodiment of the present invention seeks to decrease the adhesion strength of the inner electrodes  6   a,    6   b  to the piezo ceramic layers  4 . 
         [0037]    Adhesion of each inner electrode to the adjacent ceramic layer  4  is a result of the link or bond between a metal atom of the inner electrode (Silver and/or Palladium) and an oxygen atom of the PZT material. More specifically, the links are Pd—O—Pb or Ag—O—Pb. Thus, a decrease in the number of these links will lower the strength of the electrode bonding and will decrease/eliminate the generation of the deviated cracks during poling. This can be achieved by applying a voltage to an inner electrode, facilitating the rupture of the Ag—O or Pd—O bonding by an electrochemical reaction. Such a reaction occurs when the inner electrode is at the negative side, thereby bringing the necessary electrons. 
         [0038]    In a first embodiment of the present invention, as shown in  FIGS. 6 and 7 , the piezoelectric stack  2   a  is subjected to a multiaxial force over its entire surface during the poling process, as indicated by arrows  22  in  FIG. 6 , and poled using a single-stage poling process. This multiaxial force may be applied to an encapsulated actuator by way of housing the actuator within a chamber containing pressurised diesel fuel within an assembled fuel injector. In this way, it is possible to subject the stack  2   a  to a hydrostatic loading up to the maximum system pressure (typically ˜2000 bar) of the fuel injection system in which the injector is installed. The hydrostatic loading applied to the stack  2   a  is chosen to be above a given level, where poling cracks are not generated. From testing, this threshold level has been found to be in the range of 1000±300 bar. Accordingly, the poling method according to the first embodiment may be carried out with a hydrostatic pressure in excess of 1500 bar. Note that this first step of applying a multi axial load to the actuator which, in this embodiment, is attained via a hydrostatic pressure, is illustrated by step  100  in the flowchart of  FIG. 7 . 
         [0039]    Next, a negative voltage is applied sequentially to each of the two groups of inner electrodes  6   a,    6   b  in order weaken the bonds between the inner electrodes  6   a,    6   b  and the respective adjacent piezoelectric ceramic layers  4 . The negative voltage must be applied sequentially since it is not possible to apply the negative voltage to both groups of inner electrodes  6   a,    6   b  at the same time. Note that step  102  represents the application of the negative voltage to the first group of inner electrodes  6   a,  and step  104  represents the application of a negative voltage to the second group of internal electrodes. 
         [0040]    The loading condition and the magnitude and duration of the negative voltage applied to each of the groups of electrodes  6   a,    6   b  are selected such that the electrode bonding weakening occurs for both polarities of inner electrode before any poling cracks are generated. 
         [0041]    Subsequently, and as illustrated at step  106  in  FIG. 7 , the stack  2   a  is poled conventionally. That is to say, a potential difference large enough to exceed the coercive electric field is applied across the internal positive and negative electrodes  6   a,    6   b.  The coercive electric field strength is typically between 1 and 2 kV/mm. However, raising the temperature of the stack  2   a  during poling reduces the coercive field strength below 1 kV/mm. 
         [0042]    Thus, by weakening the bonds between each of the inner electrodes and the respective adjacent piezoelectric ceramic layers  4  prior to poling the stack  2   a,  any poling cracks which are generated by the poling process will be of the of the type parallel to the plane of the inner electrodes. Accordingly, the risk of short circuit caused by a deviated crack propagating between two adjacent inner electrodes of opposite polarity is substantially eliminated. 
         [0043]    In the above-described method, it is important that the axial loading on the stack  2   a  is such that the total strain-stress in all zones (active or inactive) will remain below the tensile rupture limit of the ceramic of the electrode bonding level. The application of a high uniaxial load (e.g. between 5000 and 10000 N) would be likely too high for the stack geometry, resulting in a risk of stack buckling or mechanical depoling. 
         [0044]    In the case of mechanical depolarisation, the axial load will tend to flip all the PZT dipoles to the position where they have the minimum length, so that the system finds its state of lowest energy. When the dipoles are oriented at 90 degrees to the axis of electrical field there is no longer any strain due to the piezo effect (intrinsic piezo effect), because the dipole is not aligned with the field and thus the field cannot “stretch” the crystal. Furthermore, if the load is high enough, the electrical field will not be strong enough to flip the dipoles back in the direction of the field. The dipoles will remain mechanically locked at 90 degrees to the field direction. The result is that it is not possible to change the alignment of the dipoles in order to pole the sample. For the above reasons at least, applying an increased uniaxial load to the stack  2   a  as the sole technique to avoid cracks is not an option. 
         [0045]    Accordingly, in the method according to the first embodiment at step  100 , a multiaxial force in the form of a hydrostatic pressure (or isostatic) is applied to the stack  2   a.  With hydrostatic loading, it is possible to reach very high compression and still have the possibility of physically poling the stack  2   a.  This is because, with the hydrostatic loading, there is no preferential direction for the dipoles to take under loading alone. Thus, the application of a hydrostatic load above a given threshold level, in the range of 1000 to 1500 bar, ensures that no poling cracks are generated during the process of sequentially applying the negative voltage to the inner electrodes in order to weaken the bonds with the piezoelectric ceramic layers  4 . 
         [0046]    A second embodiment of a method of poling a ferroelectric sample will now be described with reference to  FIG. 6 , and the flowchart of  FIG. 8 . As in the case of the first embodiment, the piezoelectric stack  2   a  is subjected to a high hydrostatic pressure over its entire surface during the poling process as illustrated at step  200 . 
         [0047]    Next, as illustrated at step  202 , a reverse poling voltage is applied to the stack  2   a.  More specifically, the reverse poling voltage has the same magnitude as the conventional poling voltage, but with the opposite polarity. Thus, a positive voltage is applied to the negative electrode  8   b  or, to the same effect, a negative voltage is applied to the positive external electrode  8   a.  The result is that the stack  2   a  is poled in reverse compared to the voltage it will see in operation. 
         [0048]    Subsequently, as illustrated at step  204 , a conventional poling voltage is applied to the stack  2   a,  i.e. a positive voltage is applied to the positive electrode  8   a.  When the conventional poling voltage is applied, the stack  2   a  is completely depoled and then repoled in the correct “final” polarity, which is the same as the polarity used in operation of the injector. 
         [0049]    In the above-described method, it is important that the hydrostatic (or isostatic) pressure is sufficiently high in order to obtain the correct result. Isostatic poling at too low pressure, e.g. 500 bar, does not bring any advantage. However, pressure above a threshold level, in the range of 1000 to 1500 bar, has been found to be high enough to prevent crack generation. 
         [0050]    Typically, the poling voltage applied is around 200 V and, accordingly the reverse poling voltage is around −200 V. This gives a nominal field in the PZT layers  4  of about 2.3 kV/mm. This is significantly higher than the voltage at which poling occurs (the coercive field voltage level), which is ˜70 to 80 V at room temperature. However, by applying a higher poling voltage it is possible not only to pole the stack  2   a,  but also to elongate the stack  2   a  to the maximum stroke, while subjected to the high isostatic loading level, in order to generate internal poling cracks of only one type, i.e. poling cracks parallel to the inner electrodes. 
         [0051]    The time for which the reverse poling voltage is applied to the stack may be as little as 50 milliseconds. 
         [0052]    By means of the method according to the second embodiment described above, it has been found that the generation of deviated poling cracks is significantly reduced or eliminated, although the exact physical mechanism by which this result is achieved is not fully understood. 
         [0053]    An additional poling technique is substantially the same as the above-described second embodiment, with the exception that the step of applying the reverse poling voltage to the stack  2   a  is omitted. Accordingly, in the third embodiment, a very high isostatic load is applied to the stack  2   a  and, subsequently, the stack is poled in a conventional way.