A device for measuring piezoelectricity

Abstract

A sample holder 200 holds a piezoelectric sample 116 in a measurement device for measuring its piezoelectric properties. The sample holder comprises first 202 and second 204 holders, each having a pair of jaws 205, 206 and 214, 216. The first 205 and fourth jaws 216 have electrically conductive portions to contact the sample. The jaws may have teeth 210, 212 formed from cylindrical rods or rollers. The second 206 and third 214 jaws may have electrically insulating rods rather than conductive ones. The conductive teeth, rods or rollers contact opposing surfaces of the sample. In use, the conductive teeth may be connected to charge amplifiers and the first and second holders are moved away from each other, to stretch the sample and measure its piezoelectric properties.

Description

FIELD OF THE INVENTION The present invention relates to the measurement of piezoelectric properties of materials, for example piezoelectric coefficients. BACKGROUND Piezoelectric materials are useful in various technical fields, such as robotics and mobile communications devices such as smartphones. Ways of accurately measuring the piezoelectric properties of materials are desired by manufacturers, designers, and users alike. The piezoelectric coefficient is a fundamental parameter which governs the piezoelectric activity of a material. SUMMARY OF THE INVENTION A material comprises a plurality of piezoelectric coefficients, each governing the piezoelectric activity in response to a deformation in a certain direction, e.g. pulling, compression, and twisting. The piezoelectric coefficients can be arranged into a piezoelectric tensor. Typically, the piezoelectric coefficients are determined using the Berlincourt Method, where the piezoelectric response of a sample is measured against the piezoelectric response of a reference material with a known piezoelectric coefficient. However, the determination of the piezoelectric coefficient of the reference material may itself be inaccurate. Furthermore, the piezoelectric coefficient of the reference material may change with time, e.g., due to material degradation. The present inventors have realised that the piezoelectric coefficients in the plane of the sample, i.e. d32, d23 components of the piezoelectric tensor or the transverse / pulling piezoelectric coefficients, are difficult to measure. Conventionally, to measure the piezoelectric properties in the plane of a sample (e.g. transverse or pulling piezoelectric coefficients), the sample is bent and the resultant generation of charge is measured. From the measured charge the piezoelectric properties of a sample can be determined. The present inventors have also realised that the conventional way of measuring piezoelectric coefficients in the plane of the sample by bending the sample is relatively inaccurate. This is because the charge generated by the piezoelectric sample from bending is relatively low and arises from a combination of compressive and tensile responses. As such, the signal to noise ratio of the measurements may be relatively large, and the bending geometry may not be adequately known, thereby making the measurements inaccurate. Bending the sample also introduces experimental artefacts into the measurements, for example, if the sample is held too tightly or too loosely. The bending of the sample will also result in the measurements of piezoelectric properties in the plane of the sample being influenced by the piezoelectric properties in the other dimensions of the sample. In a first aspect, the present invention provides a device for measuring a piezoelectric property of a sample, the device comprising: a first holder configured to hold the sample at a first position on the sample (preferably a planar or sheet sample), a second holder configured to hold the sample at a second position on the sample, and an actuator coupled to the first holder, the actuator configured to move the first holder away from the second holder, thereby to stretch a sample held by the first and second holders preferably within a plane of the sample, and more preferably without bending or twisting the sample. The device further comprises a charge measurement device electrically coupled to the first and second holders, the charge measurement device configured to output a charge signal indicative of a charge between the first and second holders, which may be a charge generated by the sample as a result of the sample being stretched. The first and second holders may define a line therebetween, and the actuator may be configured to move the first holder away from the second holder along the line. The movement of the first holder by the actuator may be restricted to be along the line. The device may further comprise one or more hinges flexibly coupling the actuator to the first holder, wherein the movement of the first holder is restricted by the one or more hinges. The actuator may be configured to move the first holder towards the second holder. The device may further comprise a controller configured to control the actuator by sending a drive signal to the actuator. The drive signal may comprise an oscillating waveform such that, when the actuator operates according to the drive signal, the actuator oscillates the first holder away from and towards to the second holder. The device may further comprise a force measurement device coupled to one or both of the first holder or the second holder, the force measurement device may be configured to output a force signal indicative of the force acting on one or both of the first holder or the second holder. The device may comprise a processor configured to determine the piezoelectric coefficient based on the force signal and the charge signal. The processor may be configured to determine the piezoelectric coefficient based on force, charge, and drive signals. The device may further comprise a fully analogue implementation configured to determine the piezoelectric coefficient based on the force signal and the charge signal. The actuator may be a piezoelectric actuator. The device may further comprise a frame, wherein opposing ends of actuator may be each coupled to the frame and the actuator may be configured to move the first holder by pushing against the frame. The device may further comprise a further actuator different from the actuator, the further actuator may be operable to adjust a distance between the first and second holders. The first holder may comprise a first jaw and a second jaw, the first jaw may comprise an electrically conductive portion that is arranged to contact the sample when the sample is held between the first and second jaws, the second holder may comprise a third jaw and fourth jaw, and the fourth jaw may comprise an electrically conductive portion that is arranged to contact the sample when the sample is held between the third and fourth jaws. The first jaw and the fourth jaw may be arranged to contact opposing surfaces of the sample when the sample is held by the first and second holders. One or both of the first and second holders may comprise one or more teeth made from elongate or substantially cylindrical rods arranged to contact the sample when the sample is held by the first and second holders. One or both of the first and second holders may comprise a pair of opposing teeth made from elongate or substantially cylindrical rods configured to hold therebetween the sample. In a further aspect, the present invention provides a method for measuring a piezoelectric property of a sample (preferably a planar or sheet sample), the method comprising: holding the sample, by a first holder, at a first position on the sample, holding the sample, by a second holder, at a second position on the sample, moving, by an actuator coupled to first holder, the first holder away from the second holder thereby stretching the sample e.g. within a plane of the sample, preferably without bending or twisting the sample, measuring, by a charge measurement device electrically coupled to the sample, an electrical charge generated by the stretching of the sample. The method may further comprise controlling, by a controller, the actuator by sending a drive signal to the actuator. The drive signal may comprise an oscillating waveform, whereby the drive signal causes the actuator to oscillate the first holder away from and towards the second holder. The method may further comprise measuring, by a force measurement device coupled to the first holder and / or the second holder, a force experienced by the sample. The method may further comprise determining, by a processor or suitable analogue electronics, a transverse or pulling piezoelectric coefficient based on the measured electrical charge and the measured force. The determining may comprise determining a transverse or pulling piezoelectric coefficient is also based on the drive signal. In a further aspect, the invention provides a sample holder of a piezoelectric measurement device, the sample holder being for holding a sample of a piezoelectric material, the sample holder comprising: a first holder comprising a first jaw and a second jaw, and a second holder comprising a third jaw and fourth jaw, wherein the first jaw comprises an electrically conductive portion that is arranged to contact the sample when the sample is held between the first and second jaw, and the fourth jaw comprises an electrically conductive portion that is arranged to contact the sample when the sample is held between the third and fourth jaws. The first jaw comprises a first tooth that is electrically conductive. The second jaw comprises a second tooth that is electrically insulative. The third jaw comprises a third tooth that is electrically insulative. The fourth jaw comprises a fourth tooth that is electrically conductive. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic illustration (not to scale) showing a top-down view of a measurement system comprising a measurement device for measuring a piezoelectric coefficient of a sample; Figure 2 is a schematic illustration (not to scale) showing a side view crosssection showing a sample holder; and Figure 3 is a process flowchart showing certain steps of a method of measuring a piezoelectric coefficient of a material DETAILED DESCRIPTION It will be appreciated that relative terms such as horizontal and vertical, top and bottom, upper and lower, above and below, front and back, and so on, are used below merely for ease of reference to the Figures, and these terms are not limiting as such, and any two differing directions or positions and so on may be implemented rather than truly horizontal and vertical, upper and lower, top and bottom, and so on. Figure 1 is a schematic illustration (not to scale) showing a top-down view of a measurement system comprising a measurement device 100 for measuring a piezoelectric coefficient of a sample. The device 100 comprises a frame 101, a first pin 102, a second pin 104, an actuator 106, a charge amplifier 108, a screw micrometer drive 110, a first compliant mechanism 112, a second compliant mechanism 113, a load cell 114, and a processor 115. In this embodiment, the first and second pins 102, 104 are positioned facing each other. A sample holder 200 (which holds a sample under test, and which will be described in more detail later below with reference to Figure 2) is held by or located on the first and second pins 102, 104. The first and second pins 102, 104 hold the sample holder 200 at different positions. The dashed lines in Figure 1 represent the sample holder 200 being positioned into the device 100 to be held by the first and second pins 102, 104. The first and / or second compliant mechanisms 112, 113 can be considered to be one or more linear guides. The first and / or second compliant mechanisms 112, 113 can comprise a flexure hinge. In this embodiment, the first and second pins 102, 104 are configured to be coupled with the sample holder 200, as discussed below in relation to Figure 2. The actuator 106 is coupled to the first pin 102. The actuator 106 is configured to actuate the first pin 102, i.e. move the first pin 102, both away from and towards the second pin 104. The actuator 106 is controlled by a controller (not shown). The controller outputs a drive signal to the actuator 106, thereby to drive the actuator 106. In this embodiment, the actuator 106 is a piezoelectric actuator. In this embodiment, the actuator 106 is coupled to the first pin 102 via the first compliant mechanism 112. The first compliant mechanism 112 is configured to restrict or limit the movement of the first pin 102 to be along a line of actuation. The line of actuation, shown in Figure 1 as line A-A, is defined as a line that passes through the first and second pins 102, 104. When the sample holder 200 is held by the first and second pins 102, 104, the line of actuation A-A is parallel to a plane of the sample held in the sample holder 200 and / or extends through the plane of the sample. In other words, the first compliant mechanism 112 converts the actuation of the actuator 106 into a movement of the first pin 102 towards or away from the second pin 104 along the line of actuation A-A. In this embodiment, one end of the actuator 106 is coupled to the frame 101 and an opposing end of the actuator 106 is coupled to the first compliant mechanism 112. The actuator 106 is configured to move the first pin 102, via the first compliant mechanism 112, by pushing against the frame 101. The charge amplifier 108 is electrically connected to the sample holder 200, by respective electrical connections, shown as dotted lines in Figure 1. The charge amplifier 108 is further communicatively coupled to the processor 115. The charge amplifier 108 is configured to convert the piezoelectric charge generated by the sample held by the sample holder 200 into a voltage. The charge amplifier 108 is further configured to output a charge signal indicative of the piezoelectric charge generated by the sample held in the sample holder 200. The charge amplifier 108 is further configured to transmit the charge signal to the processor 115 for calculation of the piezoelectric coefficient. The charge measurement may also be realized by measurement of the current and numerical integration. The screw micrometer drive 110 is coupled to the second pin 104 via the second compliant mechanism 113 and the load cell 114. The screw micrometer drive 110 is configured to move the second pin 104 away from and towards the first pin 102 along the line of actuation A-A so as to set the distance between the first and second pins 102, 104. Specifically, a thread of the screw micrometer drive is rotated to move the second pin 104 towards and away from first pin 102. The second compliant mechanism 113 converts the actuation of the screw micrometer drive 110 into a movement of the second pin 104 towards or away from the first pin 102 along the line of actuation A-A. The load cell 114 is coupled to the second pin 104. The load cell 114 is configured to measure the force applied to it by the second pin 104 thereby measuring the force experienced by the sample held in the sample holder 200. The load cell 114 is further configured to output a force signal indicative of the force experience by the sample held in the sample holder 200. In this embodiment, the load cell 114 is communicatively coupled to the processor 115. The load cell 114 is further configured to transmit the force signal to the processor 115. The output of the load cell 114 can be amplified, e.g. by a instrumentation differential amplifier, before transmission to the processor 115. In this embodiment, the load cell 114 is coupled to the second pin 104 via the second compliant mechanism 113. The second compliant mechanism 113 is configured to restrict or limit the application of force to be along a line of actuation A-A. The force on the load cell 114 is constrained to be along A-A by the fact that the pins 102, 104 both lie on A-A and the fact that the sample holder 200 can pivot around the pins 102, 104. The second compliant mechanism 113 ensures that the movement of the whole pin-cell-metal block arrangement is also along A-A, and also plays a part in the overload protection mechanism. In other words, the second compliant mechanism 113 converts the force experienced by the sample held in the sample holder 200 into a force along the line of actuation A-A. The force along the line of actuation A-A is measured by the load cell 114. The processor 115 is communicatively coupled to both the load cell 114 and the charge amplifier 108. The processor 115 configured to receive the charge signal from the charge amplifier 108 and the force signal from the load cell 114. The processor 115 is configured to determine the piezoelectric coefficient in the plane of the sample based on the received force and charge signals. Figure 2 is a schematic illustration (not to scale) showing a side view crosssection showing the sample holder 200 holding a sample 116. The sample holder 200 may be implemented in the measurement device 100 for holding the sample 116. In other words, the sample holder 200 may be coupled to, held by, or located by the first and second pins 102, 104 of the measurement device 100. The sample holder 200 comprises a first holder 202 and a second holder 204. The first holder 202 comprises a first pair of jaws, in particular a first jaw 205 and a second jaw 206. The first holder 202 is configured to hold the sample 116 between the first pair of jaws 205, 206. The first holder 202 further comprises a first screw 208 configured to adjust the distance between the first and second jaws 205, 206. A user can use the first screw 208 to increase the distance between the first and second jaws 205, 206 until the sample 116 can be positioned between the first and second jaws 205, 206. The user can then use the first screw 208 to reduce the distance between the first and second jaws 205, 206 until the sample 116 is firmly clamped between the first and second jaws 205, 206. In this embodiment, the first holder 202 further comprises a first tooth 210 positioned on the first jaw 205 and a second tooth 212 positioned on the second jaw 206. The first and second teeth 210, 212 are made from elongate rods, preferably cylindrical rods and are arranged opposing one another. The first tooth 210 is made of an electrically conductive material. The second tooth 212 is made of an electrically insulative material. In use, when the sample 116 is held by the first holder 202, the first tooth 210 contacts a first surface of the sample 116. Also, when the sample 116 is held by the first holder 202, the second tooth 212 contacts a second surface of the sample 116, the second surface of the sample 116 being opposite to the first surface. The first tooth 210 can be electrically coupled to the charge amplifier 108 as shown in Figure 1 by an electrical coupling. In this embodiment, the first holder 202 further comprises a first hole 213 configured to receive the first pin 102 of the frame 101. The second holder 204 comprises a second pair of jaws, in particular a third jaw 214 and a fourth jaw 216. The second holder 202 is configured to hold the sample 116 between the third and fourth jaws 214, 216. The second pair of jaws 214, 216 is positioned spaced apart from the first pair of jaws 205, 206 and facing the first pair of jaws 205, 206. The second holder 202 comprises a second screw 218 configured to adjust the distance between the third and fourth jaws 214, 216. A user can use the second screw 218 to increase the distance between the third and fourth jaws 215, 218 until the sample 116 can be positioned between the third and fourth jaws 215, 218. The user can then use the second screw 218 to reduce the distance between the third and fourth jaws 215,218 until the sample 116 is firmly clamped by the third and fourth jaws 215,218. In this embodiment, the second holder 204 further comprises a third tooth 220 positioned on the third jaw 214 and a fourth tooth 222 positioned on the fourth jaw 216. The third and fourth teeth 220, 222 are made from elongate rods, more preferably cylindrical rods and are arranged opposing one another. The third tooth 220 is made of an electrically insulative material. The fourth tooth 222 is made of an electrically conductive material. In use, when the sample 116 is held by the first and second holders 202, 204, the third tooth 220 contacts the same surface as the first tooth 210 and the fourth tooth 222 contacts the same surface as the second tooth 212. The fourth tooth 222 can be electrically coupled to the charge amplifier 108 as shown in Figure 1 by another electrical coupling. The electrically conductive teeth made from cylindrical rods, i.e. the first and fourth teeth made from cylindrical rods 210, 222, are arranged to contact opposing surfaces of the sample 116. The electrically conductive teeth made from cylindrical rods, i.e. the first and fourth teeth made from cylindrical rods 210, 222, are configured to carry the charge generated by the sample 116 to the charge amplifier 108. In this embodiment, the second holder 204 further comprises a second hole 223 configured to receive the second pin 104 of the frame 101. Figure 3 is a process flowchart showing certain steps of a method 300 of measuring a piezoelectric coefficient of a material. In this embodiment, the piezoelectric coefficient of the sample 116 is determined using the method 300. In this embodiment, the sample holder 200 holding the sample 116 is located in the device 100. Specifically, the first and second holders 202, 204 of the sample holder 200 are located on the first and second pins 102, 104 of the device 100. The first and second pins 102, 104 pass through the first and second holes 213, 223 respectively. At step 302, the first holder 202 holds the sample 116 at a first position on the sample 116. In this embodiment, the first and second jaws 205, 206 clamp the sample 116 therebetween. At step 304, the second holder 204 holds the sample 116 at a second position on the sample 116, the second position being different from the first position. In this embodiment, the third and fourth jaws 214, 216 clamp the sample 116 therebetween. In this embodiment, the sample is clamped by the first and second holders 202, 204 at opposite ends or sides of the sample 116. In this embodiment, the sample 116 is positioned such that the deformation defined by piezoelectric coefficient dsi or d32 is along the line of actuation A-A. At step 306, a user operates the screw micrometer drive 110 to adjust the distance between the first pin 102 and the second pin 104 (and thus the distance between the first holder 202 and the second holder 204) until the sample 116 is held just under tension. In this embodiment, the screw micrometer drive 110 is operated until the tension in the sample 116 changes from being below a threshold value to being equal to or greater than said threshold value. The threshold value may be, for example, 100mN. In this embodiment, the screw micrometer drive 110 is rotated whilst viewing the output from one or both of the charge amplifier 108 and the loadcell 114. The output from one or both of the charge amplifier 108 and the loadcell 114 is indicative of the force experienced by the sample 116. The thread of screw micrometer drive 110 is rotated until the sample 116 experiences a predetermined prestress or load, which is used to hold the sample 116 in place. This is particularly helpful in measurements of flexible samples (e.g. tensile devices). For flexible samples, the initially floppy sample is preferably placed under enough tension such that when the subsequent steps are carried out the sample never becomes loose, the subsequent steps comprising the application of a small AC load to the sample, which is explained in more detail below. The application of the predetermined prestress or load can be called a DC bias force. At step 307, the controller drives the actuator 106 by sending a drive signal to the actuator 106. In this embodiment, the drive signal includes an oscillating waveform such that, when the actuator 106 operates according to the drive signal, the actuator 106 oscillates the first pin 102 away from and towards to the second pin 104. At step 308, the actuator 106 oscillates the first pin 102 away from and towards to the second pin 104, according to the drive signal. Due to the oscillating movement of the first pin 102, the sample 116 alternates between being in a stretched state and an initial state of being held just under tension. As part of the oscillation, the actuator 106 moves the first pin 102 away from the second pin 104, thereby stretching the sample 116 along the line of actuation A-A. Preferably, the sample 116 is stretched or pulled along the line of actuation A-A without bending or twisting of the sample 116. Since the sample 116 is a piezoelectric material, this stretching of the sample 116 causes the sample 116 to generate an electrical charge. As another part of the oscillation, the actuator 106 moves the first pin 102 towards the second pin 104 along the line of actuation A-A, thereby relaxing the sample 116. The relaxation of the sample 116 back to the initial state of being held just under tension also generates an electrical charge. The amount of electrical charge generated is dependent on the force experienced by the sample 116 and the piezoelectric properties of the sample 116, for example the piezoelectric coefficient of the sample 116 along the line of actuation A-A. At step 310, the electrical charge generated by the sample 116 is received by the charge amplifier 108. The charge amplifier 108 outputs an amplified charge signal indicative of the charge generated by the sample 116. At step 312, the load cell 114 measures the force experienced by the sample 116, for example, by measuring the force acting on the load cell 114 by the second pin 104. Since the second pin 104 is coupled to the first pin 102 by the sample 116, when the actuator moves the first pin 102 away from or towards the second pin 104, the actuator 106 also exerts a force (via the sample 116) on the second pin 104. The force exerted on the second pin 104 is the same as the force experienced by the sample 116. As such, by measuring the force acting on the second pin 104, the force experienced by the sample 116 is also measured. The load cell 114 outputs a force signal indicative of the force experienced by the sample 116. At step 314, the processor 115 receives the charge signal and the force signal. At step 316, the processor calculates the piezoelectric coefficient from the received charge and force signals. In this embodiment, the piezoelectric coefficient of the sample 116, based on a piezoelectric sheet of material electrically poled in the ‘3’ direction which is a direction perpendicular to the electroded surfaces, and where the charge generated is constrained within the orthogonal (1, or 2) directions of the film, is determined from the formula: , QMi 13 FA42 where: d13 is the piezoelectric coefficient along the line of actuation of the sample 116 (the T direction), with electrodes deposited on the large flat surfaces (the ‘3’ direction); Q is the generated charge; F is the force exerted by the actuator 106 on the sample 116; A± is the area of the one of the charge generating electrodes which can cover both sides of the sample; and A2 is the area of the force generating cross section of the sample which is the width multiplied by thickness of the sample. At step 318, the processor outputs the determined piezoelectric coefficient for use by a user of the measurement device 100. For example, in some embodiments, the determined piezoelectric coefficient may be output to a display, which displays the piezoelectric coefficient. In this embodiment, d is one of the dsi, d32 components of the piezoelectric tensor, i.e. the longitudinal or the transverse coefficients, respectively. Thus, a method of measuring a piezoelectric coefficient of a material is provided. Apparatus, including the processor, for implementing the above arrangement, and performing the above steps, may be provided by configuring or adapting any suitable apparatus, for example one or more computers or other processing apparatus or processors, and / or providing additional modules. The apparatus may comprise a computer, a network of computers, one or more microcontrollers, or one or more processors, for implementing instructions and using data, including instructions and data in the form of a computer program or plurality of computer programs stored in or on a machine-readable storage medium such as computer memory, a computer disk, ROM, PROM, FRAM etc., or any combination of these or other storage media. Advantageously, the above device and method provides a way of measuring the piezoelectric properties of a sample along the line of actuation which tends to not be influenced by the piezoelectric properties of the sample in the other dimensions. This is done by directly pulling the sample. As such, measurements of the piezoelectric coefficient in the plane of the sample tend to be more accurate than the conventional methods discussed above. Advantageously, using the above device and method causes the piezoelectric sample to generate a relatively large charge (e.g. when compared with conventional methods) thereby improving the signal to noise ratio of the measurement. Advantageously, the use of teeth made from cylindrical rods in holding the sample provides a consistent way of holding the samples thereby reducing experimental artefacts when measuring the piezoelectric properties of a sample. Advantageously, teeth made from cylindrical rods can be manufactured very precisely. This allows more accurate comparisons between measurements of different samples using the same sample holder. Advantageously, the above-described sample holder is easier to use than conventional sample holders since the electrical couplings from the charge amplifier do not need to be attached to the sample, i.e. can be coupled to the sample holder. Advantageously, the use of the teeth made from cylindrical rods also allows for more accurate and precisely controlled measurements of the span and loading geometry. These parameters (e.g. the span) needs to be known quite precisely for accurate calculations of the piezoelectric coefficients to be made. As such, the use of the teeth made from cylindrical rods allows for further improvements in the accuracy of the calculations of the piezoelectric coefficients. In the above embodiments, the sample holder is held by the first and second pins which are coupled to the frame of the device. However, in other embodiments, the sample holder is coupled to or located in the frame by different holding means instead of or in addition to one or more pins, such as one or more clamps. In other embodiments, the pins may be omitted and the sample holder that holds or clamps the sample may be integral with the frame. In the above embodiments, the actuator is coupled to the first pin / holder via a compliant mechanism. However, in other embodiments, the actuator is coupled to the first pin / holder via a connector, other than a compliant mechanism, which is preferably configured to limit the movement of the first holder to be along a line of actuation. In other embodiments, the actuator is directly connected to the first pin / holder and the actuation of the actuator is along the line of actuation. In the above embodiments, the actuator oscillates the first pin / holder towards and away from the second pin / holder. However, in other embodiments, the actuator moves the first pin / holder in a different way, other than oscillation, such as a single linear motion towards / away from the second pin / holder. In the above embodiments, the device comprises a charge amplifier. However, in other embodiments, the device does not comprise a charge amplifier. In such embodiments, the device may comprise a current amplifier instead of the charge amplifier. In the above embodiments, the device comprises a screw micrometer drive. However, in other embodiments, the device does not comprise a screw micrometer drive. In some embodiments, the screw micrometer drive may be omitted, and may be replaced by a different type of actuator which may be operable to adjust or set the distance between the first and second pins / holders. For example, the actuator coupled to the first holder may be actuated using a DC static preload or static voltage to move the first holder away from the second holder until the sample is held just under tension. This allows the actuator to shrink by a tiny amount which would place the sample in tension thereby providing the DC bias load. Subsequent cyclic actuation / loading by the actuator may be done on top of or in addition to the DC bias load. The preload (via a DC voltage being applied to the stack actuator) can be used as a parameter in the determination of piezoelectric coefficient. This is particularly useful in samples where the piezoelectric coefficient is preload dependent. In the above embodiments, the screw micrometer drive is coupled to the second pin / holder via the load-cell and the second compliant mechanism. However, in other embodiments, the screw micrometer drive is not coupled to the second pin / holder via the compliant mechanism, e.g. the screw micrometer drive may be directly coupled to the second pin / holder. In the above embodiments, the frame comprises two pins configured to be coupled to the first and second holders. However, in other embodiments, the frame comprises means, other than the two pins, for coupling with the first and second holders. In the above embodiments, a load cell is used to measure the force experienced by the sample. However, in other embodiments, a different type of force measurement device, other than a load cell, is implemented to measure the force experienced by the sample. Such a force measurement device may be configured to output a force signal indicative of the force experienced by the second holder. In the above embodiments, the actuator is a piezoelectric actuator. However, in other embodiments, the actuator is a different type of actuator, such as a pneumatic or electric actuator. In the above embodiments, the actuator is configured to move the first holder both away from and towards the second holder. However, in other embodiments, the actuator is configured to move the first holder only away from or only towards the second holder. In the above embodiments, the actuator oscillates the sample between a stretched state and an initial state of being held just under tension. However, in other embodiments, the actuator oscillates the sample between a stretched state and a compressed state, or between a compressed state and an initial noncompressed and non-stretched state. For certain piezoelectric materials compression of that piezoelectric material generates an electrical charge. Certain samples are made from thin flexible materials and cannot be easily put into compression (as they would tend to buckle or bend). However, for stiffer samples, the sample can be put under tension and compression. The oscillating frequency of the actuator can range from almost quasi static to a few kHz (e.g. 1kHz to 10kHz) in the present system. This is important as tiny charges can be developed at the same frequency as the driving frequency. The use of built-in digital lock in amplifiers can extract those tiny charges. The extracted tiny charges can be used to further improve the accuracy of the piezoelectric coefficients. In the above embodiments, the piezoelectric coefficient of the sample is determined based on the force and charge signals. However, in other embodiments, the processor is also communicatively coupled to the controller and the processor is configured to determine the piezoelectric coefficient of the sample based on the force, charge, and drive signals. For some samples, the piezoelectric coefficient can be also dependent on at least one of a frequency of oscillation, a preload, and a cyclical AC load. In some embodiments, the sample may comprise an electrode (a metallic electrode, such as a copper electrode) printed or etched on to a surface of the sample. The electrode can be shaped dependent on application. For example, the electrode can be shaped such that the electrode covers areas of interest on the sample, e.g. area of which is desirable to determine piezoelectric characteristics. The electrode may, for example comprise circular or annular portion. The conductive portions of the first and second holders may be arranged to contact respective portions of the electrode. Advantageously, this allows for more accurate determination of the piezoelectric properties of specific portions of samples. As long as the top and bottom electrodes can be accessed using the teeth, the charge from the area electrode can be extracted / conducted to the charge amplifier. This is particularly useful for investigating the distribution of piezoelectric coefficients of a sample. For example, in some samples the edges of the sample have lower piezoelectric coefficients than the central section. The electroding of only a limited portion of the sample allows for a determination of the distribution of the piezoelectric coefficients. In the above embodiments, each holder comprises a respective screw for adjusting the distance between the clamp jaws of that holder. However, in other embodiments, one or both of the holders comprises a different device for adjusting the distance between the clamp jaws of that holder, other than a screw. In the above embodiments, each of the first and second holders comprises a respective electrically conductive tooth made from a cylindrical rod. In other embodiments, each of the first and second holders comprises a conductive portion that is not a tooth made from a cylindrical rod, e.g. one or more of the first and second holders may comprise a conductive portion that is a wedge-shaped portion, which may be known as a “knife edge”. In the above embodiments, each of the first and second holders comprises a respective electrically insulative tooth made from a cylindrical rod. In other embodiments, each of the first and second holders comprises an electrically insulative portion that is not a tooth made from a cylindrical rod, e.g. one or more of the first and second holders may comprise an electrically insulative portion that is a wedge-shaped portion, which may be known as a “knife edge”. In the above embodiments, the electrically conductive teeth of the first and second holders contact opposing surfaces of the sample. However, in other embodiments, the electrically conductive teeth of the first and second holders do not contact opposing surfaces of the sample, e.g. the electrically conductive teeth of the first and second holders contact the same surface of the sample. In this embodiments, the first and second holders each comprises a hole configured to receive one of the pins of the frame. However, in other embodiments, the first and second holders comprise means, other than holes, for coupling with the frame. Further aspects of the invention are provided by the subject matter of the following clauses: 1. A device for measuring a piezoelectric property of a sample, the device comprising: a first holder configured to hold the sample at a first position on the sample; a second holder configured to hold the sample at a second position on the sample; an actuator coupled to the first holder, the actuator configured to move the first holder away from the second holder; and a charge measurement device electrically coupled to the first and second holders, the charge measurement device configured to output a charge signal indicative of a charge between the first and second holders. 2 The device of clause 1, wherein the first and second holders define a line therebetween, and the actuator is configured to move the first holder away from the second holder along the line. 3. The device of clause 2, wherein the movement of the first holder by the actuator is restricted to be along the line. 4. The device of any preceding clause, further comprising one or more hinges flexibly coupling the actuator to the first holder, wherein the movement of the first holder is restricted by the one or more hinges. 5. The device of any preceding clause, wherein the actuator is further configured to move the first holder towards the second holder. 6. The device of any preceding clause, further comprising a controller configured to control the actuator by sending a drive signal to the actuator. 7. The device of clause 6, wherein the drive signal comprises an oscillating waveform such that, when the actuator operates according to the drive signal, the actuator oscillates the first holder away from and towards to the second holder. 8. The device of any preceding clause, further comprising a force measurement device coupled to one or both of the first holder or the second holder, the force measurement device configured to output a force signal indicative of the force acting on one or both of the first holder or the second holder. 9. The device of clause 8, further comprising a processor configured to determine the piezoelectric coefficient based on the force signal and the charge signal and, preferably, the drive signal. 10 The device of clause 8, further comprising a fully analogue implementation configured to determine the piezoelectric coefficient based on the force signal and the charge signal. 11. The device of any preceding clause, wherein the actuator is a piezoelectric actuator. 12. The device of any preceding clause, further comprising a frame, wherein opposing ends of actuator are each coupled to the frame and the actuator is configured to move the first holder by pushing against the frame. 13. The device of any preceding clause, further comprising a further actuator different from the actuator, the further actuator operable to adjust a distance between the first and second holders. 14. The device of any preceding clause, wherein: the first holder comprises a first jaw and a second jaw; the first jaw comprises an electrically conductive portion that is arranged to contact the sample when the sample is held between the first and second jaws; the second holder comprises a third jaw and fourth jaw; and the fourth jaw comprises an electrically conductive portion that is arranged to contact the sample when the sample is held between the third and fourth jaws. 15. The device of clause 14, wherein the first jaw and the fourth jaw are arranged to contact opposing surfaces of the sample when the sample is held by the first and second holders. 16. The device of any preceding clause, wherein one or both of the first and second holders comprises one or more teeth made from cylindrical rods arranged to contact the sample when the sample is held by the first and second holders. 17. The device of clause 16, wherein one or both of the first and second holders comprises a pair of opposing teeth made from cylindrical rods configured to hold therebetween the sample. 18. A method for measuring a piezoelectric property of a sample, the method comprising: holding the sample, by a first holder, at a first position on the sample; holding the sample, by a second holder, at a second position on the sample; moving, by an actuator coupled to first holder, the first holder away from the second holder thereby stretching the sample; measuring, by a charge measurement device electrically coupled to the sample, an electrical charge generated by the sample as a result of the sample being stretched. 19. The method of clause 18, wherein the sample is stretched without being bent or twisted. 20. The method of clause 19, further comprising controlling, by a controller, the actuator by sending a drive signal to the actuator, wherein optionally the drive signal comprises an oscillating waveform, whereby the drive signal causes the actuator to oscillate the first holder away from and towards the second holder. 21. The method of any of clauses 18 to 20, further comprising measuring, by a force measurement device coupled to the first holder and / or the second holder, a force experienced by the sample. 22. The method of clause 21, further comprising determining, by a processor, or suitable analogue electronics, a transverse or pulling piezoelectric coefficient based on the measured electrical charge and the measured force. 23. The method of clause 22 when dependent on clause 19, wherein the determining comprises determining a transverse or pulling piezoelectric coefficient is also based on the drive signal. 24. A sample holder of a piezoelectric measurement device, the sample holder being for holding a sample of a piezoelectric material, the sample holder comprising: a first holder comprising a first jaw and a second jaw; and a second holder comprising a third jaw and fourth jaw; wherein the first jaw comprises an electrically conductive portion that is arranged to contact the sample when the sample is held between the first and second jaws; and the fourth jaw comprises an electrically conductive portion that is arranged to contact the sample when the sample is held between the third and fourth jaws. 25. The sample holder of clause 24, wherein: the first jaw comprises a first roller that is electrically conductive; the second jaw comprises a second roller that is electrically insulative; the third jaw comprises a third roller that is electrically insulative; the fourth jaw comprises a fourth roller that is electrically conductive.

Claims

1. A sample holder of a piezoelectric measurement device, the sample holder being for holding a sample of a piezoelectric material, the sample holder comprising:a first holder comprising a first jaw and a second jaw; anda second holder comprising a third jaw and fourth jaw; whereinthe first jaw comprises an electrically conductive portion that is arranged to contact the sample when the sample is held between the first and second jaws;the fourth jaw comprises an electrically conductive portion that is arranged to contact the sample when the sample is held between the third and fourth jaws;the first jaw comprises a first rod that is electrically conductive;the second jaw comprises a second rod that is electrically insulative;the third jaw comprises a third rod that is electrically insulative; andthe fourth jaw comprises a fourth rod that is electrically conductive.

2. The sample holder of claim 1, wherein one or both of the first and second holders comprises one or more teeth made from cylindrical rods or rollers arranged to contact the sample when the sample is held by the first and second holders.

3. The sample holder of claim 2, wherein one or both of the first and second holders comprises a pair of opposing teeth made from cylindrical rods configured to hold therebetween the sample.

4. The sample holder of any preceding claim, wherein the first jaw and the fourth jaw are arranged to contact opposing surfaces of the sample when the sample is held by the first and second holders.

5. A device for measuring a piezoelectric property of a sample, the device comprising the sample holder of any preceding claim.

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