WO2023214441A1

WO2023214441A1 - Polarisation and measurement cell for piezoelectric ceramic materials

Info

Publication number: WO2023214441A1
Application number: PCT/IT2023/050121
Authority: WO
Inventors: Antonio IACOMINI; Davide SANNA; Sebastiano GARRONI; Andrea MELIS; Pier Nicola LABATE; Alberto Mariani; Stefano ENZO; Gabriele MULAS
Original assignee: Università Degli Studi Di Sassari
Priority date: 2022-05-06
Filing date: 2023-05-08
Publication date: 2023-11-09
Also published as: IT202200009386A1

Abstract

The present invention concerns a polarisation and measurement cell (1) for piezoelectric ceramic materials, said cell (1) comprising a support (10) comprising a housing (5), wherein said housing (5) is adapted to contain a sample of piezoelectric ceramic material, said housing (5) is bounded on two opposite sides, respectively by a first electrode (2) and by a second electrode (3), said cell (1) comprising an opening (6) adapted to allow a ray to pass through for X-ray diffraction measurement of said sample of piezoelectric ceramic material, said cell (1) being characterised in that it comprises adjustment means (4) for adjusting the relative position of said second electrode (3) with respect to said first electrode (2). The invention also concerns an apparatus (11) for measurement and polarisation of piezoelectric ceramic materials comprising said polarisation and measurement cell (1).

Description

POLARISATION AND MEASUREMENT CELL FOR PIEZOELECTRIC CERAMIC

MATERIALS

The present invention concerns a polarisation and measurement cell for piezoelectric ceramic materials.

In particular, the present invention relates to a polarisation and measurement cell for bulk piezoelectric ceramic materials.

As is known, piezoelectricity is the property of some materials to generate an electric field following a mechanical deformation (direct piezoelectric effect) and, vice versa, when said materials are subjected to an electric field, to exhibit a mechanical deformation (inverse piezoelectric effect).

In recent years, piezoelectric ceramic materials have been used in a wide range of industrial and consumer products, including sonar, ultrasound, microphones and injectors.

The ability of the piezoelectric ceramic or piezoceramic materials of converting electrical energy into mechanical energy and vice versa depends on their crystalline structure. The necessary condition for the piezoelectric effect to be obtained is the absence of a centre of symmetry of the unit cell, which is responsible for the separation of charge between positive and negative ions, and for the formation of domains with parallel orientation.

In said piezoelectric ceramic materials, piezoelectricity is induced through the polarisation process, which consists in the application of a strong electric field at a certain temperature, lower than a threshold temperature value defined as Curie temperature, which allows to align the dipoles in the same direction of the applied field. The dipole moment thus remains unchanged after the electric field has been removed and the ceramic material exhibits piezoelectric properties without an excessively high voltage or high deformation being imposed.

Furthermore, the final piezoelectric properties are influenced by parameters such as the density of the final compacted material, which is in turn influenced by the structural characteristics of the piezoelectric ceramic material, such as crystalline structure, detectivity and chemical composition.

In conclusion, it is evident that the optimization of the electromechanical properties of the piezoceramics is related to an efficient polarisation process and to the microstructure of the material itself, which is investigated through the use of different characterization techniques. Among them, X-ray diffraction is one of the most important and widespread techniques for studying the microstructural properties of the crystalline and non-crystalline solids.

From what has been described, it emerges that the study of the polarisation - structure - property relationship is of fundamental importance for the implementation of such materials both from an industrial and basic science point of view. For this reason, only recently, measurement cells capable of polarising the sample during X-ray diffraction measurement on thin films have been developed.

In particular, Patent Publication No. CN111912705A shows an apparatus for measuring the electrical and mechanical properties of a piezoelectric ceramic material. Said apparatus comprises a piezoelectric analysis cavity which allows measurements to be made for piezoelectric materials. Inside said piezoelectric analysis cavity there are two copper electrodes of substantially cylindrical shape, between which the piezoelectric material to be measured is arranged. In said apparatus there is also located a heating cartridge configured to heat the interior of the cavity by heating the silicone oil contained in the outer walls of the cavity itself. In particular, the temperature inside the cavity is controlled by means of a thermocouple. Said cavity further comprises a network cable interface configured to connect strain gauges present to a measurement detection system external to the apparatus itself. Said cavity has two openings: a viewing window provided with a quartz plate, and a through hole defined in the upper part of the cell, which serves as a channel for the insertion of heating silicone oil.

Said configuration is not simple to apply in a research laboratory and is particularly expensive for applied research centres and companies. Furthermore, said solution requires a very bright source, such as synchrotron or neutron lights, having a very high average cost and present only in specialized laboratories.

Yet, said configuration allows to apply voltages only on materials prepared as thin films, which hardly find application in the piezoceramic industry with high market value.

Furthermore, said measurement cells present additional criticalities, not allowing analysis on bulk systems and having significant dimensions.

In particular, the polarisation process using these measurement cells is very long and not very effective.

Furthermore, said cells are not suitable for allowing the simultaneous measurement and polarisation of a piezoelectric material.

There is, therefore, a need in the specific sector to have a measurement cell of reduced size and costs, to carry out experiments on laboratory instrumentations.

This need is met by the measurement cell according to the present invention which offers, moreover, further advantages that will become clear hereafter.

The solution according to the present invention fits into this context, which aims to carry out microstructural investigation experiments as the electric field and temperature vary.

In particular, said polarisation and measurement cell is a device with compact volume and made of printable materials.

These and other results are obtained according to the present invention by proposing a polarisation and measurement cell for piezoceramic materials that allows to optimize the electromechanical properties of the materials.

The aim of the present invention is therefore to provide a polarisation and measurement cell which makes it possible to overcome the limits of the measurement cells according to the prior art and to obtain the previously described technical results.

A further aim of the invention is that said polarisation and measurement cell can be realized with substantially low costs, both with regard to the production costs and with regard to the operating costs.

Still another aim of the invention is the analysis of the microstructure of the material by means of different techniques, including X-ray diffraction analysis. In particular, said operation allows to make the material effectively piezoelectric, hence to test the fundamental properties to be then used in commercial applications and to test how the microstructure thereof varies as a function of the temperature and of the applied electric field, in such a way as to optimize the preparation thereof and therefore the final properties thereof.

Yet another aim of the invention is to propose a polarisation and measurement cell that is simple, safe and reliable.

In particular, the cell according to the present invention integrates perfectly with various measuring instruments, such as for example diffractometers. This allows to obtain qualitative and quantitative information on the polarisation process of the piezoelectric ceramic material, so as to optimize the polarisation process or understand the alignment mechanism of the ferroelectric dipoles. Furthermore, said cell is easily adaptable even on low volume chambers, such as that of a scanning electron microscope. In fact, the device according to the present invention has adjustable sizes, which allow access within extremely limited volumes.

Furthermore, said device according to the present invention allows the simultaneous measurement and polarisation of a piezoelectric ceramic material.

In addition, the device according to the present invention allows the analysis of bulk materials that need high voltages in order to be able to be polarised. In particular, said bulk materials are used in most applications in the field of the technological devices.

The subject-matter of the present invention therefore relates to a polarisation and measurement cell for bulk materials in particular for piezoelectric ceramic materials, said cell comprising a support comprising a housing, wherein said housing is adapted to contain a sample of piezoelectric ceramic material, said housing is bounded on two opposite sides, respectively by a first electrode and by a second electrode, said cell comprising an opening adapted to allow a ray to pass through for X-ray diffraction measurement of said sample of piezoelectric ceramic material, said cell being characterised in that it comprises adjustment means for adjusting the relative position of said second electrode with respect to said first electrode. In particular, said adjustment means being movable inside said housing in such a way as to bring said sample of material to be polarised on the diffraction plane. Furthermore, this feature allows samples of bulk material that may have variable thickness to be brought on the diffraction plane.

In particular, said first electrode and said second electrode may be configured to apply an electric field comprised between 10kV/cm and 25kV/cm. In particular, according to the invention said opening can pass through said first electrode, and in particular said first electrode can be substantially U-shaped.

Still according to the invention said adjustment means for adjusting the relative position of said second electrode with respect to said first electrode can be chosen from: a spring, screw, pneumatic or clamp system.

Still according to the invention said cell may comprise temperature control means comprising temperature detection means and temperature adjustment means. In particular, said temperature control means are adapted to minimize the temperature variations in the cell. In fact, said temperature control means can be directly in contact with said housing which, being in a completely open environment, does not require cooling systems.

In particular, according to the invention said temperature control means may comprise a heating cartridge inserted in a defined cavity within said adjustment means.

Furthermore, according to the invention said second electrode may correspond to said adjustment means for adjusting the relative position of said second electrode with respect to said first electrode.

Still according to the invention, said temperature detection means may comprise a temperature sensor, such as for example a thermocouple or a thermistor.

In particular according to the invention, said temperature sensor can be configured to be in contact with said sample of piezoceramic material to be measured or it can be inside a metallic portion, interposed between said support on one side and said second electrode and said adjustment means for adjusting the relative position of said second electrode with respect to said first electrode on the other side.

Still according to the invention said support can be made of plastic material, in particular it can be a thermoplastic or thermosetting polymer.

The subject-matter of the following invention further relates to an apparatus for measurement and polarisation of piezoelectric ceramic materials characterised in that it comprises said polarisation and measurement cell, a current generator, first connecting cables between said cell and said generator, a temperature control system, second connecting cables between said system and said cell, a generator of the heating system and electronic processing means adapted to remotely control said cell.

Finally, the subject-matter of the following invention further relates to the use of a polarisation and measurement cell for piezoelectric ceramic materials for the polarisation of a sample of ceramic material.

Still, the subject-matter of the following invention further relates to the use of said cell for application in morphological and structural analysis instruments, in particular diffractometers, electron scanning microscopes, spectroscopes or fluorescence chambers.

The present invention will now be described by way of non-limiting illustration according to a preferred embodiment thereof, with particular reference to the figures in the appended drawings and the examples, wherein:

- figure 1 shows a perspective view of a polarisation and measurement cell according to a first embodiment of the present invention,

- figure 2 shows a sectional view of the polarisation and measurement cell according to a second embodiment of the present invention,

- figure 3 shows a top view of the device of figure 2,

- figure 4 shows a bottom view of the device of figure 2,

- figure 5 shows a perspective view of the spring system of the polarisation and measurement cell and the respective cell according to a third embodiment,

- figure 6 shows a bottom view of the polarisation and measurement cell according to a third embodiment,

- figure 7 shows an apparatus comprising the polarisation and measurement cell according to the present invention,

- figure 8 shows the piezoelectric coefficients d33 obtained from the polarisation tests of example 1 and 2, and

- figure 9 shows the results of the application of said cell in a diffractometer.

With reference to the figures, reference numeral 1 is assigned to a polarisation and measurement cell for piezoelectric ceramic materials, in which a sample of material to be polarised must be made conductive before each measurement. Figure 1 shows the essential elements of a first embodiment of said cell 1 , said cell 1 comprising:

- a support 10 in plastic material that is heat-resistant at least up to the temperature of 200°C, comprising a housing 5 for the samples of material to be polarised and an opening 6, said opening 6 allowing a ray to pass through for X-ray diffraction measurement from a diffraction device to said housing 5 in such a way as to allow the simultaneous measurement and polarisation of said sample, said housing 5 being bounded above by a first electrode 2 and below by a second electrode 3, and

- adjustment means 4 for adjusting the relative position of said second electrode 3 with respect to said first electrode 2, which are positioned inside said housing 5, said adjustment means 4 being movable inside said housing 5 in such a way as to bring said sample of material to be polarised on the diffraction plane. In particular, in said first embodiment said adjustment means 4 consists of a spring system.

Furthermore, in the case shown in figure 1 , said first electrode 2 and said second electrode 3 are made of copper. In particular, said first electrode 2 is II- shaped in such a way as to allow said ray to pass through for X-ray diffraction measurement.

Figure 2 shows a second embodiment of said cell 1. In particular, with reference to figure 2, said cell comprises a support 10 in plastic material that is heat- resistant at least up to a temperature of 200°C, said support 10 comprising a housing 5 for the samples of material to be polarised and an opening 6. In particular, said opening 6 allows a ray to pass through for the X-ray diffraction measurement from a diffraction device to said housing 5. Furthermore, said housing 5 is bounded at the top by a first electrode 2 and at the bottom by a second electrode 3. Still with reference to figure 2, said cell 1 comprises adjustment means 4 for adjusting the relative position of said second electrode 3 with respect to said first electrode 2 which are positioned inside said housing 5. In particular, said adjustment means 4 is slidably removable inside said housing 5 in such a way as to bring said sample of material to be polarised on the diffraction plane.

In addition, said cell 1 comprises temperature adjustment means, said adjustment means comprises temperature detection means and temperature adjustment means. In the embodiment shown in figure 2, said temperature adjustment means is a heating cartridge 7 insertable in a defined cavity within said adjustment means 4. In particular, said heating cartridge 7 allows an increase in temperature up to 180°C, in such a way as to facilitate the movement of the dipoles and therefore the polarisation process, decreasing the timings and the voltage to be applied. Still with reference to figure 2, said temperature detection means comprises a temperature sensor 8 inserted, in a lateral cavity of said support 10, laterally to said housing 5 and in contact with said sample of said sample of material to be polarised.

In particular, in said second embodiment said adjustment means 4 for adjusting the relative position of said second electrode 3 with respect to said first electrode 2 consists of a metallic cylinder or screw and serve as a second electrode 3 allowing the current to pass through from the first electrode 2 to the adjustment means 4, passing through said sample of piezoceramic material to be polarised that will be arranged above the head of the cylinder or screw.

In particular, said support 10 is made of plastic material, preferably said material is a metallic or ceramic material, more preferably it is a thermoplastic or thermosetting polymer. Said support 10 can be made by 3D printing, so as to be adaptable to different instrumentations based on diffractometry, but also to other types of morphological/structural analysis techniques. Furthermore, said support 10 is adjustable based on the different design needs such as the working temperatures and mechanical properties.

Furthermore, said opening 6 of said support 10 allows different types of signals to reach the sample, thus expanding the possible analysis techniques to which said cell 1 can be applied simultaneously to the polarisation process.

Figure 5a shows the adjustment means 4 for adjusting the relative position of said second electrode 3 with respect to said first electrode 2 in a third embodiment of said polarisation and measurement cell. In particular, in said embodiment, said adjustment means 4 for adjusting the relative position of said second electrode 3 with respect to said first electrode 2 is a spring system for anchoring the sample and for applying the voltage. Figure 5b shows said polarisation and measurement cell 1 according to said third embodiment. In other and not shown embodiments, said adjustment means 4 for adjusting the relative position of said second electrode 3 with respect to said first electrode 2 may also be pneumatic or clamp systems.

In a fourth embodiment, shown in figure 6, said temperature sensor can be positioned inside an additional metallic portion 17, interposed between said support 10 on one side and said second electrode 3 and said adjustment means 4 on the other side. Said metallic portion 17 comprises a copper nut that facilitates the movement of said adjustment means 4 for adjusting the relative position of said second electrode 3 with respect to said first electrode 2, the current to pass through and moreover it allows to be able to insert said temperature sensor closer to said heating cartridge compared to the previous embodiments that envisaged said temperature sensor in contact with said sample of piezoceramic material.

Figure 7 shows an apparatus 11 , said apparatus 11 comprising:

- a polarisation and measurement cell 1 according to the present invention,

- a current generator 12,

- first connecting cables 13 between said cell 1 and said generator 12,

- a temperature control system comprising a screen 14, - second connecting cables 15 between said system and said cell 1 ,

- a generator 16 of the heating system, and

- electronic processing means, not shown, wherein said processing means allows remote control of said cell 1 by means of mobile application.

In particular, said cell 1 may comprise magnets, not shown, arranged on the sides of said cell 1 , said magnets allowing said cell 1 to be fixed to a sample holder support of known type, inside an X-ray diffraction instrument, or to other types of instrumentation.

The invention will be described below by way of non-limiting illustration, with particular reference to several illustrative examples.

Example 1. Polarisation test at room temperature

In order to perform a polarisation test with the polarisation and measurement cell 1 according to the present invention, said first electrode 2 and said second electrode 3 were applied on the surface of a sample of the material to be polarised. Said material was a piezoceramic material of known composition, belonging to the category of the so-called KNN (ceramic material based on sodium and potassium niobate) and having a disc shape with sizes of 11.5mm in diameter and 1.2mm in thickness. This shape has made it possible to have two surfaces, one exposed to the external environment and one directed towards the adjustment means 4, in particular a copper screw present in said polarisation and measurement cell 1 that serves as a second electrode 3. The surface of said material in contact with said second electrode 3 (the copper screw) was electroded with silver paste to ensure a low resistance to charge transfer. Furthermore, said surface did not interact with the signal coming from the instrument since it was not exposed to the X-ray beam. The upper surface of the material to be polarised, that is, the one concerned by the scanning of the X-ray beam, was electroded with graphite. Said first (upper) electrode was chosen in graphite because of its cost-effectiveness and because it did not alter the analysis of the sample. The experiment was carried out at room temperature (about 20°C). In particular, said experiment comprised the following steps in succession:

- inserting the sample into said housing 5,

- applying an electric field with increasing values comprised between 10-25 kV/cm for about 20 - 30 minutes,

- removing the electric field, - measuring the piezoelectric coefficient d33.

The coefficient d33 expresses the amount of charge generated per unit of force applied and is generally expressed as pC/N. d33 is the main coefficient that is evaluated in order to have indications on the piezoelectric properties of a material. Said coefficient depends on factors that are characteristic of the material such as composition, microstructure, density but it also depends on the intensity of the applied electric field. Therefore, the success of the polarisation process is often evaluated by measuring the piezoelectric coefficient d33 at the end of said process. The results, shown in the lower curve of figure 8, highlighted an increasing piezoelectric response as the electric field values increased, in particular the coefficient d33 varied between 105 and 145pC/N.

Example 2. Polarisation test at high temperature

In order to be able to perform a polarisation test at high temperature, the experimental set-up of the previous example was considered. In particular, said test comprised the following steps:

- inserting the sample into said housing 5,

- increasing the temperature of the sample up to the target value of 100°C with a heating ramp of 3-5°C/min,

- applying the electric field with increasing values comprised between 10- 25kV/cm for about 20-30min,

- decreasing the temperature naturally,

- removing the electric field, and

- measuring the piezoelectric coefficient d33.

The results, shown in the upper curve of figure 8, highlighted an increasing piezoelectric response as the electric field values increased, in particular the coefficient d33 varied between 135 and 150pc/N.

Example 3. Comparison between polarisation tests as temperature varies

By comparing the results obtained from the experiment at room temperature of example 1 and the experiment at high temperature of example 2, it was observed that, with the same electric field, the experiment at high temperature had higher piezoelectric coefficients d33. This result was particularly evident at low electric field values. Furthermore, the two curves converged towards a piezoelectric coefficient value d33 of about 150pC/N, which is in excellent agreement with the literature data of this system (146pC/N). This confirms the correct operation of the cell and of the electrodes in the step of charge transfer to the sample.

Example 4. Tests of application of the cell in a measuring instrument, in particular a diffractometer

In order to be able to apply said cell in a measuring instrument, in particular a SmartLab diffractometer (Rigaku), an in-situ experiment was set up by means of X-ray diffraction on a ceramic pad with the same composition with respect to the previous examples. Also in this case said first electrode 2 and said second electrode 3 were applied on both surfaces of the pad. Subsequently, the complete X-ray pattern of the sample inside the cell was obtained. The subsequent step was to test the heating means and the polarising system, under the same conditions with which the previous examples were carried out. In particular, the temperature was increased up to 100°C and an electric field was applied with increasing values (10, 15 and 20kV/cm). From the results of said in-situ experiments, visible in figure 9, in which the signals coming from the diffraction are indexed with the Miller indices, i.e. from a triad of numbers representative of the family of crystalline planes typical of a given crystalline structure, it can be inferred that the diagnostic diffraction peaks of the material under examination undergo a variation in intensity following the application of the electric field and the set temperature. This is due to a process in which the ferroelectric dipoles align along preferential crystallographic directions following the application of the potential difference. Specifically and as a consequence thereof, some diffraction peaks undergo an increase in intensity (002 and 022) while others undergo a decrease (111 and 200). The observation of this phenomenon highlights how the system integrates perfectly with the instrumentation and is actually useful to obtain qualitative and quantitative information on the polarisation process of the material that is useful, for example, to optimize the polarisation process or to understand the alignment mechanism of the ferroelectric dipoles.

The present invention has been described, in an illustrative but non-limiting manner, according to preferred embodiments thereof, but it is to be understood that variations and/or modifications may be made by those skilled in the art without thereby departing from the relative scope of protection, as defined by the attached claims.

Claims

1 ) A polarisation and measurement cell (1 ) for bulk materials in particular for piezoelectric ceramic materials, said cell (1 ) comprising a support (10) comprising a housing (5), wherein said housing (5) is adapted to contain a sample of piezoelectric ceramic material, said housing (5) is bounded on two opposite sides, respectively by a first electrode (2) and by a second electrode (3), said cell (1 ) comprising an opening (6) adapted to allow a ray to pass through for X-ray diffraction measurement of said sample of piezoelectric ceramic material, said cell (1 ) being characterised in that it comprises adjustment means (4) for adjusting the relative position of said second electrode (3) with respect to said first electrode (2).

2) The cell (1 ) according to claim 1 , characterised in that said first electrode (2) and said second electrode (3) are configured to apply an electric field comprised between 10kV/cm and 25kV/cm.

3) The cell (1 ), according to claim 1 or 2, characterised in that said opening (6) passes through said first electrode (1 ), and in particular said first electrode (1 ) is substantially U-shaped.

4) The cell (1 ), according to any one of the preceding claims, characterised in that said adjustment means (4) for adjusting the relative position of said second electrode (3) with respect to said first electrode (2) is chosen from: a spring, screw, pneumatic or clamp system.

5) The cell (1 ), according to any one of the preceding claims, characterised in that it comprises temperature control means comprising temperature detection means and temperature adjustment means.

6) The cell (1 ), according to claim 5, characterised in that said temperature adjustment means comprises a heating cartridge (7) inserted into a defined cavity within said adjustment means (4) for adjusting the relative position of said second electrode (3) with respect to said first electrode (2).

7) The cell (1 ), according to claim 6, characterised in that said second electrode (3) corresponds to said adjustment means (4) for adjusting the relative position of said second electrode (3) with respect to said first electrode (2).

8) The cell (1 ), according to any one of claims 5-7, characterised in that said temperature detection means comprises a temperature sensor (8).

9) The cell (1 ), according to the preceding claim characterised in that said temperature sensor (8) is configured to be in contact with said sample of piezoceramic material to be measured or is inside a metallic portion (17), interposed between said support (10) on one side and said second electrode (3) and said adjustment means (4) for adjusting the relative position of said second electrode (3) with respect to said first electrode (2) on the other side.

10) The cell (1 ), according to any one of the preceding claims, characterised in that said support (10) is made of plastic material, in particular it is a thermoplastic or thermosetting polymer.

11 ) An apparatus (11 ) for measurement and polarisation of piezoelectric ceramic materials characterised in that it comprises a polarisation and measurement cell (1 ) according to any one of claims 1 -10, a current generator (12), first connecting cables (13) between said cell (1 ) and said generator (12), a temperature control system, second connecting cables (15) between said system (14) and said cell (1 ), a generator (16) of the heating system and electronic processing means adapted to remotely control said cell (1 ).