Abstract:
In a method for polarizing a piezoceramic material, first, a base is provided that is made of non-polarized piezoceramic material and has at least two opposite planar electrodes and at least one predetermined breaking point because of which a stress relief fracture forms when a voltage having a first voltage value is applied. A number of voltage pulses, the amplitudes of which follow a time-related envelope curve, is applied to the electrodes of the base. The amplitudes of the voltage pulses in a first section of the envelope curve are higher than the first voltage value, and the amplitudes of the voltage pulses in a second section of the envelope curve that follows the first section have a second voltage value which is sufficient to permanently polarize the piezoceramic material.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage Application of International Application No. PCT/EP2009/051253 filed Feb. 4, 2009, which designates the United States of America, and claims priority to German Application No. 10 2008 011 414.6 filed Feb. 27, 2008, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to a method for polarizing a piezoceramic material. 
       BACKGROUND 
       [0003]    Piezoceramic materials, such as e.g. lead-zirkonate-titanate, stretch when an electrical voltage is applied in a direction parallel to the electric field generated by the electrical voltage. Piezoceramic materials are used inter alia for piezoelectric actuators, by means of which the injection of fuel into a combustion engine is controlled, for example. 
         [0004]    Piezoceramics have electric dipoles which initially are unpolarized. In order to enable the piezoelectric effect to be used the piezoceramic material must be polarized. 
         [0005]    DE 100 28 335 B4 discloses a method for polarizing a piezoceramic material wherein, starting from a main body made from unpolarized ceramic material having at least two electrodes embodied with flat surfaces and disposed opposite each other, a number of voltage pulses are applied to the electrodes. The pulse heights of the voltage pulses follow a time-dependent envelope curve which in a first section increases during a rise time from a minimum electrical voltage to a maximum electrical voltage and which in a second section holds the maximum electrical voltage during a hold time. The minimum electrical voltage has a value such that when the electrodes are charged the maximum compatible charging of the still unpolarized piezoceramic material is undershot. The maximum electrical voltage is suitable for producing a permanent polarization of the piezoceramic material. 
       SUMMARY 
       [0006]    According to various embodiments, a method for polarizing a piezoceramic material can be provided which enables the unpolarized piezoceramic to be polarized faster. 
         [0007]    According to various embodiments, a method for polarizing a piezoceramic material, may comprise the following method steps of: —providing a main body made from unpolarized piezoceramic material having at least two electrodes embodied with at least two flat surfaces and disposed opposite each other, and having at least one predetermined fracture joint due to which a strain-relieving crack forms when an electrical voltage having a first electrical voltage value is applied, and—applying a number of electrical voltage pulses to the electrodes, the pulse heights of which voltage pulses follow a time-dependent envelope curve, the pulse heights of the electrical voltage pulses in a first section of the envelope curve being greater than the first electrical voltage value and the pulse heights of the electrical voltage pulses in a second section of the envelope curve following the first section having a second electrical voltage value which is sufficient to produce a permanent polarization of the piezoceramic material. 
         [0008]    According to a further embodiment, the first electrical voltage value can be greater than the second electrical voltage value. According to a further embodiment, the second electrical voltage value can be equal to the maximum permissible electrical voltage of a piezoelectric actuator for which the main body is provided. According to a further embodiment, the time period of the first section can be less than the time period of the second section, in particular the time period of the first section can be less than or equal to a quarter of the time period of the second section. According to a further embodiment, the envelope curve may have a third section preceding the first section, the pulse heights of the electrical voltage pulses of the third section having a third voltage value which is less than the second voltage value and is sufficient to incinerate a contaminant in particular in a passivation layer of the main body. According to a further embodiment, the time period of the first and the third section together can be less than the time period of the second section, in particular the time period of the first and the third section together can be less than or equal to a quarter of the time period of the second section. According to a further embodiment, the method may comprise: determining the electrical conductivity of the main body made from unpolarized piezoceramic material, deducing the degree of contamination and setting the third voltage value based on the determined degree of contamination. According to a further embodiment, the method may comprise: applying an electrical voltage of in particular around 10V to the electrodes in order to determine the electrical conductivity of the main body made from unpolarized piezoceramic material. According to a further embodiment, the method may comprise: applying to the main body, while the electrical voltage pulses are being applied, a force acting against the main body, which force corresponds in particular to a mean value of a force against which a piezoelectric actuator for which the main body is provided works at a mean stroke during operation. According to a further embodiment, the envelope curve may have a fourth section following the second section, the pulse heights of the electrical voltage pulses of the fourth section having a fourth voltage value which corresponds to a mean value of an electrical voltage that is applied during the operation of a piezoelectric actuator for which the main body is provided. According to a further embodiment, the time period of the fourth section can be less than the time period of the second section, in particular the time period of the fourth section can be less than or equal to half of the time period of the second section. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Exemplary embodiments are shown by way of example in the attached schematic drawings, in which: 
           [0010]      FIG. 1  shows a piezoelectric stack having unpolarized piezoceramic material, 
           [0011]      FIG. 2  shows the piezoelectric stack after its piezoceramic material has been polarized, 
           [0012]      FIG. 3  shows a characteristic curve of an electrical voltage that is applied to the piezoelectric stack, 
           [0013]      FIG. 4  shows an envelope curve of the electrical voltage, 
           [0014]      FIG. 5  shows an alternative characteristic curve of an electrical voltage that can be applied to the piezoelectric stack of  FIG. 1 , 
           [0015]      FIG. 6  shows an envelope curve of the electrical voltage of  FIG. 5 , 
           [0016]      FIG. 7  shows a further characteristic curve of an electrical voltage that can be applied to the piezoelectric stack of  FIG. 1 , and 
           [0017]      FIG. 8  shows an envelope curve of the electrical voltage of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    As stated above, according to various embodiments, a method for polarizing a piezoceramic material, may comprise the following method steps of:
       providing a main body made from unpolarized piezoceramic material having at least two electrodes embodied with flat surfaces and disposed opposite each other, and having at least one predetermined fracture joint, due to which a strain-relieving crack forms when an electrical voltage having a first electrical voltage value is applied, and   applying a number of electrical voltage pulses to the electrodes, the pulse heights of which voltage pulses follow a time-dependent envelope curve, the pulse heights of the electrical voltage pulses in a first section of the envelope curve being greater than the first electrical voltage value and the pulse heights of the electrical voltage pulses in a second section of the envelope curve following the first section having a second electrical voltage value which is sufficient to produce a permanent polarization of the piezoceramic material.       
 
         [0021]    The main body made from unpolarized piezoceramic material is provided e.g. in order to be used for a piezoelectric actuator. For this to be possible the unpolarized piezoceramic material must be polarized. Furthermore the main body has one or more predetermined fracture joints on account of which one or more strain-relieving cracks form when the first electrical voltage is applied. By the selective guidance of said strain-relieving cracks through e.g. porous ceramic layers or porous electrode layers it is possible to increase the service life of a piezoelectric actuator using the main body. Predetermined fracture joints are disclosed inter alia in DE 10 2004 031 404 A1 and DE 102 34 787 C1. 
         [0022]    Owing to the predetermined fracture joints it is possible, prior to the polarization of the unpolarized piezoceramic material by means of the electrical voltage pulses of the second section of the envelope curve, to apply the electrical voltage pulses of the first section of the envelope curve to the main body made from unpolarized piezoceramic material. The electrical voltage of the first section (first voltage value) is greater, in particular considerably greater, than an electrical voltage which, when applied, results in the formation of the strain-relieving crack or cracks. 
         [0023]    The first voltage value can be in particular greater than the second voltage value, thereby increasing the efficiency of the method according to various embodiments. 
         [0024]    Guiding the strain-relieving crack or cracks through the porous layers then ensures a selective propagation of the strain-relieving crack or cracks to a depth as of which the strain-relieving crack or cracks no longer tend toward a detrimental kinking. 
         [0025]    The second section of the envelope curve can begin immediately after the electrical voltage pulses of the first section of the envelope curve have been applied to the main body. The second voltage value is high enough to be sufficient to bring about a permanent polarization of the piezoceramic material. The second voltage value is e.g. dependent on the piezoceramic material and/or the distance between two electrodes. 
         [0026]    The second electrical voltage value can be equal to the maximum permissible electrical voltage of a piezoelectric actuator for which the main body is provided. The second voltage value is then the higher value from the electrical voltage by means of which a charging for permanent polarization is achieved or else the maximum permissible electrical voltage of the piezoelectric actuator. 
         [0027]    All that is necessary in order for the strain-relieving cracks to form is to apply the voltage pulses having the first voltage value for a relatively short period of time to the main body made from unpolarized piezoceramic material. According to one embodiment variant of the method the time period of the first section is therefore less than the time period of the second section. In particular the time period of the first section can be less than or equal to a quarter of the time period of the second section. The time period of the second section of the envelope curve, during which the piezoceramic material is polarized, amounts to e.g. 60 seconds. During this time e.g. 6000 voltage pulses are applied to the main body. The time period of the first section, during which the strain-relieving crack or cracks form, can then amount, for example, to 15 seconds, during which e.g. 1500 electrical voltage pulses are applied to the main body. 
         [0028]    The main body may have contaminants, in particular in a passivation layer. Said contaminants do not necessarily result in the piezoelectric actuator for which the main body is used being functionally unserviceable. According to another variant of the method the envelope curve therefore has a third section preceding the first section, the pulse heights of the electrical voltage pulses of the third section having a third voltage value which in particular is less than the second voltage value and is sufficient to incinerate said contamination. The electrical voltage pulses of the third section are provided for the purpose of selectively generating an electrical flashover at the site of the contamination. A spark produced as a result of the flashover can incinerate the contamination without leaving behind a conductive burn site. 
         [0029]    The time period that is necessary to ensure the contamination is burned off can likewise be selected to be relatively short. According to an embodiment variant of the method the time period of the first and third section together is less than the time period of the second section. In particular the time period of the first and third section together can be less than or equal to a quarter of the time period of the second section. 
         [0030]    The third voltage value (voltage for incinerating the contamination) can be calculated e.g. by first determining the electrical conductivity of the main body made from unpolarized piezoceramic material, as a result of which deductions can be made concerning the extent of the contamination in particular in the passivation layer. The third voltage value can then be set on the basis of the determined extent of the contamination. 
         [0031]    The conductivity of the main body made from unpolarized piezoceramic material can be determined, for example, by applying an electrical voltage of in particular around 10V to the electrodes. 
         [0032]    If the third electrical voltage value is greater than or equal to the second voltage value, then the third section is not required. 
         [0033]    The absolute stability of the piezoelectric actuator for which the main body is used can be increased if, during the polarization of the piezoceramic material, the mean force during the mean stroke to which the piezoelectric actuator is subjected during operation acts in addition. According to an embodiment variant of the method the main body is therefore subjected during the application of the electrical voltage pulses to a force acting against the main body, which force corresponds in particular to a mean value of a force against which a piezoelectric actuator for which the main body is provided works during operation at a mean stroke. This can be achieved, for example, by the main body working against an application-specific spring stiffness or a defined force while the voltage pulses are being applied. 
         [0034]    The envelope curve can also have a fourth section following the second section, in which case the pulse heights of the electrical voltage pulses of the fourth section have a fourth voltage value which corresponds to a mean value of an electrical voltage that is applied during the operation of a piezoelectric actuator for which the main body is provided. The time period of the fourth section can be less than the time period of the second section. In particular the time period of the fourth section can be less than or equal to half of the time period of the second section. 
         [0035]    The method according to various embodiments can be performed particularly effectively in respect of time if a poling system for the method is operated in such a way that a mechanism of the poling system is implemented such that when the voltage pulses are applied during the second section the effect of the force corresponds only to an injection mean value of the piezoelectric actuator despite maximum voltage. 
         [0036]      FIG. 1  shows a piezoelectric stack  1  (main body), which in the case of the present exemplary embodiment is implemented in a cuboid shape, in a partially cutaway and perspective representation. The piezoelectric stack  1  has an unpolarized piezoceramic material  2  embodied in a layer structure and a plurality of internal electrodes  3 ,  4  arranged within the unpolarized piezoceramic material  2  and was produced in a generally known way, for example by means of the process steps of stacking, separating, debinding and polishing. The piezoceramic material  2  has for example lead-zirkonate-titanate and in the case of the present exemplary embodiment the internal electrodes  3 ,  4  are electrically conductive metal layers which permeate the piezoelectric stack  1 . 
         [0037]    In the case of the present exemplary embodiment the internal electrodes  3 ,  4  are electrically connected in alternation to external electrodes  5 ,  6  arranged on opposite external surfaces of the piezoelectric stack  1  and mounted directly on the lateral surface of the piezoceramic material  2 . The internal electrodes  3 ,  4 , which are electrically connected to one of the two external electrodes  5 ,  6 , are therefore brought out as far as the outside at which said external electrode  5 ,  6  is arranged for the purpose of electrical connection to the external electrode  5 ,  6 . To ensure that the internal electrodes  3 ,  4  are electrically insulated from the other external electrodes  5 ,  6 , however, the internal electrodes  3 ,  4  do not reach as far as the outside of the piezoelectric stack  1  at which the other external electrodes  5 ,  6  are arranged. 
         [0038]    In the case of the present exemplary embodiment the lateral surface of the piezoelectric stack  1  is additionally provided with a passivation layer  8  which may contain contaminants  9 . 
         [0039]    In the case of the present exemplary embodiment the piezoelectric stack  1  also has a predetermined fracture joint  7  due to which a strain-relieving crack  10  shown in  FIG. 2  forms if an electrical voltage is applied to the internal electrodes  3 ,  4 . 
         [0040]    The piezoceramic material  2  of the piezoelectric stack  1  shown in  FIG. 1  is still unpolarized and must be polarized in order to have a piezoelectric effect. This is achieved by applying an electrical voltage U to the internal electrodes  3 ,  4  or, as the case may be, the external electrodes  5 ,  6 , the characteristic curve of the voltage being shown in  FIG. 3 . 
         [0041]    In the case of the present exemplary embodiment the electrical voltage U applied to the piezoelectric stack  1  consists of electrical voltage pulses  34 ,  35  whose pulse heights follow a time-dependent envelope curve  31 . The envelope curve  31 , which is shown again in  FIG. 4 , has a first section  33 , indicated by an unbroken line in  FIG. 4 , and a second section  32  indicated by a dashed line in  FIG. 4 . 
         [0042]    In the case of the present exemplary embodiment the second section  32  of the envelope curve  31  comprises 6000 individual pulses  35 , each of which has the same pulse height at an electrical voltage U 2 . The time period TH of the second envelope curve  32  amounts to e.g. 60 seconds. The electrical voltage U 2  is sufficient to polarize the unpolarized piezoceramic material  2  and in the case of the present exemplary embodiment has a value of approx. 160V. If the maximum permissible electrical voltage of a piezoelectric actuator (not shown in further detail) for which the piezoelectric stack  1  is provided is sufficient for polarizing the unpolarized piezoceramic material  2 , then the pulse heights of the second section  32  can be equal to said maximum permissible voltage. 
         [0043]    In the case of the present exemplary embodiment the time period TV of the first section  33  is significantly shorter than the time period TH of the second section  32  and amounts to e.g. 15 seconds and comprises for example 1500 individual pulses  34 . The electrical voltage U 1  for the pulse heights of the pulses  34  of the first section is selected such that it is greater than a voltage due to which the strain-relieving crack  10  forms. In the case of the present exemplary embodiment the voltage U 1  amounts to approx. 200V. 
         [0044]    After the electrical voltage has been applied to the piezoelectric stack  1  there is thus produced a piezoelectric stack  1 ′ having polarized piezoceramic material  2 ′, as shown in  FIG. 2 . 
         [0045]      FIG. 5  shows an alternative voltage curve for polarizing the unpolarized piezoceramic material  2  of the piezoelectric stack  1 . The voltage curve shown in  FIG. 5  consists of the electrical voltage pulses  34 ,  35  and in addition voltage pulses  53  whose pulse heights together follow a time-dependent envelope curve  51 . The envelope curve  51 , which is shown again in  FIG. 6 , has, in addition to the envelope curve  31  shown in  FIG. 4 , a third section  52 , indicated by a dashed line in  FIG. 6 , which precedes the first section  33 . 
         [0046]    In the case of the present exemplary embodiment the time periods TV′ of the first and third section  33 ,  52  together are considerably shorter than the time period TH of the second section  32  and amount to e.g. 15 seconds. The first section  32  and the third section  52  each have e.g. 750 individual pulses  34 ,  53 . The pulse heights of the first section  34  have a voltage value U 1 , with the result that the strain-relieving crack  10  forms. 
         [0047]    In particular the passivation layer  8  may have the contaminants  9 . In the case of the present exemplary embodiment these are incinerated by means of the voltage pulses  53  of the third section  52 . The pulse heights at an electrical voltage U 3  of the electrical voltage pulses  53  of the third section  52  are in this case lower than the voltage U 2  of the second section  32 . The height of the voltage U 3  of the voltage pulses  53  of the third section  52  is determined e.g. by initially determining the electrical conductivity of the piezoelectric stack  1  made of unpolarized piezoceramic material  2 , as a result of which deductions can be made concerning the extent of the contamination  9  in the passivation layer  8 . The voltage U 3  can then be set based on the determined extent of the contamination  9 . The conductivity of the piezoelectric stack  1  made from unpolarized piezoceramic material  2  can be determined for example by applying an electrical voltage of in particular around 10V to the internal electrodes  3 ,  4 . 
         [0048]    After the electrical voltage has been applied to the piezoelectric stack  1  there is thus produced the piezoelectric stack  1 ′ having polarized piezoceramic material  2 ′, as shown in  FIG. 2 . 
         [0049]      FIG. 7  shows an alternative voltage curve for polarizing the unpolarized piezoceramic material  2  of the piezoelectric stack  1 . The voltage curve shown in  FIG. 7  relates to the electrical voltage pulses  34 ,  35 ,  53  and further voltage pulses  73  whose pulse heights together follow a time-dependent envelope curve  71 . The envelope curve  71 , which is shown again in  FIG. 8 , has, in addition to the envelope curve  51  shown in  FIG. 6 , a fourth section  72 , indicated by an unbroken line in  FIG. 8 , which follows the second section  32 . 
         [0050]    In the case of the present exemplary embodiment the fourth section  72  of the envelope curve  71  consists of 3000 individual pulses  75 , the pulse heights of which have an electrical voltage U 4 . The time period TH 2  of the fourth section  72  amounts to e.g. 30 seconds. The voltage U 4  for the voltage pulses  73  of the fourth section  72  corresponds to the mean value of an electrical voltage that is applied during the operation of the piezoelectric actuator for which the piezoelectric stack  1  is provided. 
         [0051]    After the electrical voltage has been applied to the piezoelectric stack  1  there is thus produced the piezoelectric stack  1 ′ having polarized piezoceramic material  2 ′, as shown in  FIG. 2 . 
         [0052]    In the case of the present exemplary embodiment, during the polarization of the unpolarized piezoceramic material  2 , i.e. while the electrical voltage is being applied to the piezoelectric stack  1 , this is acted upon during operation by the mean force F at the mean stroke to which the piezoelectric actuator for which the piezoelectric stack  1  is provided is subjected. This can be achieved, for example, by the piezoelectric stack  1  working against an application-specific spring stiffness or a defined force while the voltage pulses  34 ,  35 ,  53 ,  73  are being applied.