ES2773935B2 - PROCEDURE FOR OBTAINING CERAMIC COATINGS OF LEAD ZIRCONATE-TITANATE (PZT) - Google Patents

Description

DESCRIPTION

PROCEDURE FOR OBTAINING CERAMIC COATINGS FROM

LEAD ZIRCONATE-TITANATE (PZT)

Technical sector

The invention relates to a process for obtaining ceramic materials based on lead zirconate-titanate (PZT) with a perovskite crystal structure.

State of the art

Solid solutions of lead zirconate PbZrO3 and lead titanate PbTiO3, that is, lead zirconate titanate (PZT) Pb (ZrxTi1-x) O3, are one of the most widely used ferroelectric perovskite-type materials due to their excellent dielectric properties , piezoelectric and ferroelectric. This material is used in the form of thick films in a wide variety of devices for applications such as micromechanical systems (MEMS), relatively large force microactuators, ultrasonic transducers, micropumps, pyroelectric infrared detectors or acoustic wave filters.

The films used in the production of these devices are made up of PZT particles, which are obtained by classical techniques, such as mixed oxide technology, or alternatively by chemical methods whose main characteristics are low reaction temperature, uniform mixture of reagents, precise component control and high product purity.

After obtaining the PZT particles, they are deposited on different substrates by means of non-contact inkjet printing techniques, such as screen printing and lithography, which involve the reproduction of a pattern on the substrate.

After the deposit of the PZT particles on the substrate, there are several treatments that lead to a thin or thick film of PZT. Heat treatment is the most typical even when the sintering temperature of the PZT is above 800 ° C, which limits the type of substrates that can be used. Much effort has been put into reducing the temperature required to obtain PZT films, for example by using a PZT precursor instead of PZT particles. Another way to avoid the use of high temperatures is to use polymers to bond the particles, allowing you to print on flexible or polymeric substrates. But, obviously, the presence of polymers will reduce the piezoelectric properties of the film. All the possibilities mentioned above must be taken into account to prepare a suitable ink.

Inkjet printed PZT particle ink can be thermally or optically sintered. Thermal treatments in an oven have fewer limitations because, regardless of the ink formulation, this process will remove the solvent and cause the PZT particles to sinter, but the use of high temperatures and oxidative atmospheres precludes the use of flexible substrates, polymeric and metallic. Something similar happens with precursor-based inks. The precursor must be calcined above 500 ° C to generate the desired material.

Therefore, an alternative process is necessary to obtain ceramic coatings of lead zirconate-titanate (PZT) that avoids heat treatment for sintering the PZT particles.

Object of the invention

The invention relates to a process for obtaining lead zirconate-titanate (PZT) ceramic coatings that uses a laser treatment (laser sintering / laser pyrolysis) as an alternative to heat treatment (sintering / calcination).

Thus, the procedure for obtaining ceramic coatings of lead zirconate-lead titanate (PZT) comprises:

- get a PZT ink,

- deposit the PZT ink on a substrate,

- sintering the PZT ink by laser treatment, and

- subjecting the substrate with the sintered PZT ink to a polarization process.

Preferably the substrate on which the ink is deposited is a metallic substrate. Even more preferably the metallic substrate is steel.

Preferably the PZT ink is a PZT nanoparticle ink.

The advantage of laser sintering is the application of heat to small surfaces, drastically reducing the effect of heat on the substrate. Laser treatment will sinter the PZT nanoparticles and produce a consistent printed film without the need for polymers. Another advantage of laser treatment is that the number of different substrates that can be used is greater than that of heat treatment in an oven.

A 1064 nm wavelength laser is used to sinter the PZT ink. The laser treatment uses a power of between 200-300 mW and a speed of between 0.1 and 0.5 mm / s. Preferably it is a power of 300 mW and a speed of 0.1 mm / s.

The PZT ink is deposited onto the substrate by inkjet printing. The use of inkjet printing allows a pattern to be built directly on the substrate by sequential deposition of ink drops. Inkjet printing also has the advantage of its scalability and the fact that all the ink used in the process is in the substrate pattern, which means that the amount of wasted ink is drastically reduced.

The invention employs piezoelectric variable drop "Dropon-Demand" (DoD) inkjet printers that are less restrictive than thermal DoD Inkjet printers.

Description of the figures

Figure 1 shows 6 SEM images of laser sintered PZT nanoparticle coatings with powers of 200mW, 2000mW and 3000mW on alumina substrates.

Figure 2 shows 6 SEM images of laser sintered PZT nanoparticle coatings with powers of 200mW, 600mW and 1000mW on steel substrates.

Figure 3 shows an X-ray spectrum of two PZT nanoparticle coatings treated with different laser powers (600mW and 100mW) and where peaks are illustrated. of PZT, PbO and ZrO2.

Figure 4 shows 6 SEM images of laser sintered PZT precursor coatings with powers of 200mW, 400mW and 600mW on steel substrates.

Figure 5 shows 6 SEM images of laser sintered nanoparticles with powers of 800mW, 1000mW and 1200mW on steel substrates.

Figure 6 shows an X-ray spectrum of various PZT precursor coatings treated with different laser powers (200mW, 400mW, 600mW, 800mW, 1000mW, and 1200mW).

Figure 7 shows 4 SEM images of PZT precursor coatings and PZT nanoparticles deposited on a steel substrate and sintered at a laser power of 400mW.

Figure 8 shows 4 SEM images of PZT precursor coatings and PZT nanoparticles deposited on a steel substrate and sintered at a laser power of 800mW.

Figure 9 shows 4 SEM images of PZT nanoparticle coatings sintered at a laser power of 600mW and deposited on steel and alumina.

Figure 10 shows 4 SEM images of PZT nanoparticle inkjet coatings on steel substrates under different laser power and speed conditions.

Figure 11 shows the electrical response of polarized coatings at different voltages under compression test.

Detailed description of the invention

The process for obtaining lead zirconate-titanate (PZT) ceramic coatings comprises the steps of obtaining a PZT ink, depositing the PZT ink on a substrate, sintering the PZT ink by means of a laser treatment, and subjecting the substrate with PZT ink sintered to a polarization process.

Each of the steps for obtaining PZT ceramic coatings is described below by way of non-limiting example.

PZT Particle Ink Formulation

PZT ink is obtained from a PZT nanoparticle ink or an ink based on a PZT precursor.

The PZT to obtain each of the inks is synthesized from organometallic compounds used in chemical processes based on metal alkoxides, carboxylates and beta-diketones. Those organometallic compounds will be formed with three different metals: lead, zirconium, and titanium.

In the case of PZT nanoparticle ink, the chemical process used to synthesize the PZT nanoparticles includes adding water to the mixture of organometallic compounds to form a gel. This gel hybridizes to form a perovskite PZT nanoparticle powder. The powder is obtained in agglomerated form, so it must be ground with balls to generate separate nano-sized particles.

In the case of the PZT precursor ink, a sol is needed instead of the gel because the ink must flow through the ink jet printer. This means that the sun must be stable against ambient water vapor. As a result, diamines are used as chelating agents to ensure the stability of the precursor.

Subsequently, vehicles and binders are added to the PZT nanoparticles or the PZT precursor, so that in both cases inks with a viscosity lower than 30 cps and a surface tension lower than 70 N / m are obtained, which are suitable for printing. by ink jet.

Printing process

Both the PZT nanoparticle ink and the PZT precursor ink are filtered (with a 5 pm Whatman and Cameo syringe filter, respectively) before printing to avoid clogging of the nozzle with which the ink is deposited. This filtering process is of great importance for PZT nanoparticle inks due to its tendency to flocculation. However, the PZT precursor ink is free of particles, making filtering unnecessary, but filtering is still preferable as the precursor is unstable enough to cause gel chunks that tend to clog. the mouthpiece.

Both PZT inks are printed on alumina substrates ("Ceramtec" type) and on steel substrates, specifically stainless steel ("Hasberg" type 304).

For this, inkjet printing tests were performed with a "Microfab" jet device . A piezoelectric voltage of 43 V was used for adequate droplet formation and a maximum injection frequency of 1 kHz. During printing, the substrate to be printed was at room temperature. Inkjet printing parameters, such as speed and drop spacing, were optimized for each type of substrate and ink.

Laser sintering

After printing of the substrates with the PZT ink, the printed samples were thermally cured on a hot plate at 40 ° C to remove the vehicle, subjected to laser sintering and microstructurally characterized by scanning electron microscopy and diffraction. X-ray. For sintering, a pulsed laser with a wavelength 1064 nm was used, setting the frequency at 10 KHz, a speed of 1 mm / s and a laser pulse of 200 ns. The influence of potency in both processes was studied using the PZT nanoparticle ink and the PZT precursor ink.

Polarization

The PZT ceramic coatings were then polarized using a 10 kV power supply ( "Keithley 2290-10" type). Each sample was immersed in silicon oil at 120 ° C for 30 minutes and a current of less than 1 mA was passed through the sample. The samples were polarized at 50V and 75V. The first was chosen to achieve 5000 V / mm for samples with a height of 10 pm.

Compression test

After polarization, the PZT ceramic coatings were characterized by a compression test (performed with an "Instron Mini 44"). A data acquisition system ( "Agilent 34970") was used to select the desired signal. The conditions used in the compression test were: a crosshead speed of 200 mm / min and a load limit of -100 N.

Characterization

The crystalline structure and morphology of the PZT ceramic coatings were characterized by X-ray diffraction using a diffractometer ( "Philips XPERT MRD" type) (Cu Ka1A = 1.54059 Á) from 20 ° to 110 °, with a step size of 0.02 ° at 20 and 10s / ° scanning speed In addition, a field emission scanning electron microscope ( "FEI Quanta 3D FEG model") was used in low vacuum configuration using acceleration voltages between 10 and 20 kV.

Results using PZT nanoparticle ink

Different processing conditions were carried out on alumina and steel substrates in order to find suitable conditions to sinter PZT nanoparticles on each substrate. The laser power used to sinter the PZT nanoparticles deposited on the alumina substrate changes the appearance of the deposition and the shape of the sintered particles.

Figure 1 shows 6 SEM images of laser sintered PZT nanoparticle coatings with powers of 200mW (images a and b), 2000mW (images c and d) and 3000mW (images e and f) on alumina substrates.

In the images a) and b) of figure 1 the case of a coating treated with low laser power (200 mW) is shown. It can be seen that the binder still remains (dark gray phase indicated with an arrow), while the effect of increasing the power (see images c) and d) of figure 1), stimulates sintering and causes growth of the neck between the particles (surrounded in image d) of figure 3). On the other hand, the use of powers of 3000mW is not owed because the PZT degrades as seen in the images e) and f) of Figure 1.

The same was done on a steel substrate obtaining similar results. See Figure 2 which shows 6 SEM images of laser sintered PZT nanoparticle coatings with powers of 200mW (images a and b), 600mW (images c and d) and 1000mW (images e and f) on steel substrates.

The samples treated with a low laser power of 200mW (images a) and b) of figure 2) show the presence of organic material, while the samples treated with a higher power of 600mW (images c) and d) of figure 2) appear having eliminated organic material and favoring sintering and the growth of PZT particles. This is related to the amount of power used in the laser treatment. On the contrary, image f) of figure 2 shows the degradation of PZT, which indicates the need to take into account the effect that laser treatment can have on the crystalline phase of the coating.

The composition of the coating is measured by X-ray diffractometry, which provides useful information, such as the interaction between the coating and the substrate, or the degradation of PZT that is directed to other types of oxides.

Figure 3 shows an X-ray spectrum of two coatings of PZT nanoparticles treated with different laser power and where peaks of PZT, PbO and ZrO2 are illustrated. PZT peaks are represented by asterisks, PbO peaks are represented by the plus sign, and ZrO2 peaks are represented by triangles.

Specifically, figure 3 shows the difference between a coating without degradation of the PZT like that of image e) of figure 2, and a coating like that of image f) of figure 2 where the PZT begins to degrade. The degradation is associated with the peak near the main peak of the PZT and is marked with a triangle in figure 3. The PZT is unstable with respect to temperature, which produces decomposition when the temperature rises above a certain limit or is maintained for too long a time. This decomposition is accompanied by the formation of individual oxides such as PbO. Therefore, the laser power must be chosen to avoid PZT degradation while the PZT particles are still sintered.

Results using PZT precursor ink

PZT precursor-based ink coatings were treated with lasers of different powers to study the formation of PZT ceramic coatings. In this case, only steel substrates were used.

Figure 4 shows 6 SEM images of laser sintered PZT precursor coatings with powers of 200mW (images a and b), 400mW (images c and d) and 600mW (images e and f) on steel substrates and Figure 4 shows 6 SEM images of coatings of laser sintered PZT precursor with powers of 800mW (images a and b), 1000mW (images c and d) and 1000mW (images e and f) on steel substrates.

As seen in Figures 4 and 5, the shape of the laser-treated coatings changes depending on the power used. The coatings treated with low power of 200mW (images a) and b) of figure 4) show a continuity that is related to the absence of the crystalline phase of PZT (see figure 6). While the transformation between the amorphous precursor and perovskite occurs, the deposited coating undergoes great volume shrinkage, only about 20 percent of the inkjet printed material remains in the laser sintered ceramic coating.

As can be seen in images c) and d) of figure 4, an increase in laser power causes the formation of larger craters (holes) and the appearance of cracks, as more material is removed. Going to a higher power (images e) and f) of Figure 4), the coatings begin to separate and some ceramic particles appear, as shown in Figure 6. Between laser powers of 600mW and 800mW, the PZT perovskite is fully formed, as can be seen in images a and b of figure 5, from this point on, the coatings form islands due to the large cracks that extend along them. The coatings treated with higher laser power show irregular shapes (images c) and e) of figure 5) associated with rapid volume contraction and these irregular shapes are composed of continuous rigid structures but full of pores (images d) and f) of figure 5). Furthermore, the X-ray spectrum of coatings treated with laser power greater than 800 mW shows the presence of some unwanted oxides (PbO and ZrO2).

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Comparison between coatings using PZT Nanoparticles and PZT precursor

PZT nanoparticle ceramic and PZT hammer coatings were compared to verify that PZT ceramic coating shows better properties for laser sintering treatment.

For this, the PZT coatings printed with inkjet on steel substrates and treated with laser were compared under the same processing conditions, with laser powers of 400 mW and 800 mW.

In figure 7 4 SEM images are shown, images a) and b) show coatings of PZT precursor and images c) and d) show coatings of PZT nanoparticles, both coatings deposited on a steel substrate and processed with a laser power of 400mW.

The coating based on PZT nanoparticle ink processed with a laser power of 400 mW shows the presence of several pores and some cracks due to the contraction of the volume suffered and the elimination of the binder (see images c) and d) of figure 7) . However, the PZT precursor-based ink coating shows some pores and cracks due to the partial removal of the binder (pictures a) and b) of figure 1). However, it should be noted that the laser power used is not sufficient to transform the precursor into PZT perovskite (see figure 8), therefore the coating based on the PZT precursor is not comparable with the coating of PZT nanoparticles, since it is not PZT. crystalline.

On the other hand, figure 8 shows 4 SEM images, images a) and b) show coatings of PZT precursor and images c) and d) show coatings of PZT nanoparticles, both coatings deposited on a steel substrate and processed with a 800mW laser power.

The ink coating based on the PZT precursor processed with a laser power of 800 mW shows small pieces of the coating broken due to the extension of the cracks associated with the volume contraction that occurred during the formation of PZT (see images a) and b ) of figure 8). In contrast to the coating processed with 400 mW, perovskite PZT is present in the coating processed with 800 mW, as is shown in the X-ray spectrum of figure 6. The PZT nanoparticle coating that is treated under the same conditions is much more continuous, even if it shows pores and cracks. Its X-ray spectrum shows the presence of PZT perovskite but it appears to have no unwanted oxides like the PZT precursor coating treated under the same conditions (see figure 6).

Comparison between steel and alumina substrates

The coatings were inkjet printed onto alumina ceramic substrates and onto metallic steel substrates, thermally cured to remove vehicle, and laser sintered under the same conditions.

Figure 9 shows 4 SEM images of coatings generated by inkjet of PZT nanoparticles sintered at a laser power of 600mW and a laser speed of 1mm / s, images a) and b) show coatings deposited on steel substrates and images c) and d) show coatings deposited on alumina substrates.

On the one hand, the coating deposited on alumina (see image c) of figure 9) shows the presence of organic material that has not been removed (dark gray phase indicated by arrows) by laser treatment, while there is no organic material on the film printed on a steel substrate (see image a) of figure 9).

This factor may be related to the temperature reached during the laser treatment, which, at the same time, is associated with the coefficient of thermal diffusivity. The coefficient of thermal diffusivity is represented by the following equation:

where p is the density of the material; Cp is the specific capacity and k is the thermal conductivity.

In this case, steel is a worse heat diffuser than alumina (see table), which makes it possible to use less aggressive sintering conditions. This happens because steel maintains the laser-induced temperature rise longer than alumina and, to The same laser treatment, the temperature in the steel substrate is maintained longer than that of alumina. This is illustrated in images b) and d) of Figure 3 where sintered PZT nanoparticles can be clearly recognized. Sintering of particles on the steel substrate is, therefore, preferable to that of an alumina substrate.

The following table illustrates the relevant theoretical properties of 304 stainless steel and alumina.

Optimization of the ceramic coating of PZT

PZT nanoparticle coatings printed on steel substrates are laser treated under different conditions to generate a sintered, continuous film. It is important to take into account the fact mentioned above that too high a laser power leads to degradation of the perovskite PZT phase. Because of this, lower laser power and slower laser speed movement is preferable.

In the tests carried out, the highest power used to optimize the coating properties was 600 mw and the fastest speed was 1 mm / s while the lowest power used was 200 mW and the slowest speed was 0.1 mm / s.

Various laser treatment conditions are tested to minimize the presence of cracks and holes. The combination of low stage speed and relatively high power (500-600 mW) can lead to degradation of the PZT phase, so those conditions are avoided.

Figure 10 shows 4 SEM images of PZT nanoparticle inkjet coatings on steel substrates under different laser power and speed conditions. Image a) shows a coating of PZT nanoparticles sintered at a power of 200mW and a speed of 0.1 mm / s, image b) a

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coating sintered at a power of 200mW and a speed of 0.5 mm / s, image c) a coating sintered at a power of 300mW and a speed of 0.1 mm / s and image d) a coating sintered at a power of 300mW and a speed of 0.5 mm / s.

Best results are obtained for low stage speed (0.1-0.5 mm / s) and low power (200-300 mW).

As can be seen in Figure 10, the sintered coating with a power of 300 mW and a speed of 0.1 mm / s seems to be more homogeneous (see image c) Figure 10). The coating treated with the same power but with a faster speed shows cracks and more holes in the surface of the film (see image d) of figure 10). At the same time, the coatings treated with an even lower power show several marks associated with the movement of the laser on the coating, revealing a more heterogeneous surface (see images a) and b) of figure 10).

Optimized end conditions for laser treatment allow to obtain continuous coatings without cracks due to the controlled and gradual removal of the binder, avoiding the formation of cracks and the growth of holes associated with the rapid evaporation of the binder and volume contraction due to the sintering of particles.

Polarization

The previously optimized coatings were polarized at different voltages and their response under the compression test was compared to the response of a non-polarized sample (see Figure 11).

As shown below, the response of a sample that has not been polarized is negligible compared to the response of a polarized sample. The effect of increasing the applied voltage is presented in Figure 11.

The results are related to the orientation of the PZT lattice. In the case of a non-polarized coating, its domains are randomly oriented, so its piezoelectric behavior is low. After the polarization process, a certain number of domains is oriented in the same direction that is associated with the increase in electrical response under the compression test. Using a higher bias voltage leads to increased response until saturation is reached.

In accordance with all this, in the tests carried out, PZT nanoparticle inks and PZT precursor inks were prepared, which were ink jet printed on ceramic and metallic surfaces to generate piezoelectric thick film coatings. A 1064 nm wavelength laser was used to sinter the ceramic PZT nanoparticles and transform the PZT precursor into PZT perovskite.

Due to the volume shrinkage suffered by the PZT precursor-based coating during laser treatment, this type of coatings show the worst continuity. Furthermore, the X-ray spectrum shows the presence of phases other than the transformed perovskite. Taking these facts into account, it is assumed that coatings based on PZT nanoparticles have better properties for polarization.

Laser treatment allows the generation of thick film coatings of sintered PZT nanoparticles, so it is important to choose a precise substrate material to promote particle sintering. Metallic substrates appear to be more suitable for laser sintering than alumina substrates.

The PZT nanoparticle coatings printed on metal substrate were optimized by selecting the appropriate laser treatment. After testing various treatment conditions, the best PZT nanoparticle coating was found for a laser power of 300 mW and a speed of 0.1 mm / s.

Finally, the PZT nanoparticle coatings were polarized and characterized by a compression test. Depending on the polarization conditions, the measured response ranged from 20 mV to more than 150 mV.

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Claims (9)

1. - Procedure for obtaining ceramic coatings of lead zirconate-titanate (PZT), characterized in that it comprises: - get a PZT ink, - deposit the PZT ink on a substrate, - sintering the PZT ink by laser treatment, and - subjecting the substrate with the sintered PZT ink to a polarization process. 2. - Procedure for obtaining ceramic coatings of lead zirconate-titanate (PZT), according to the preceding claim, characterized by using a metallic substrate. 3. - Procedure for obtaining ceramic coatings of lead zirconate-titanate (PZT), according to the preceding claim, characterized by using a steel substrate. 4. - Process for obtaining ceramic coatings of lead zirconate-titanate (PZT), according to any one of the preceding claims, characterized in that a PZT nanoparticle ink is used. 5. - Process for obtaining ceramic coatings of lead zirconate-titanate (PZT), according to any one of the preceding claims, characterized in that a laser with a wavelength of 1064 nm is used to sinter the PZT ink. 6. - Procedure for obtaining ceramic coatings of lead zirconate-titanate (PZT), according to any one of the preceding claims, characterized in that the laser treatment uses a power of between 200-300 mW and a speed of between 0.1 and 0.5 mm / s. 7. - Process for obtaining ceramic coatings of lead zirconate-titanate (PZT), according to the previous claim, characterized in that the laser treatment uses a power of 300 mW and a speed of 0.1 mm / s. 8. - Process for obtaining ceramic coatings of lead zirconate-titanate (PZT), according to any one of the preceding claims, characterized in that the ink PZT is deposited by ink jet printing. 9. Process for obtaining ceramic coatings of lead zirconate-titanate (PZT), according to the preceding claim, characterized in that the PZT ink is deposited by variable drop ink jet printing.