Laser sintered ceramic coatings of PZT nanoparticles deposited by Inkjet Printing on metallic and ceramic substrates

Author’s Accepted Manuscript Laser sintered ceramic coatings of PZT nanoparticleS deposited by Inkjet Printing on metallic and ceramic substrates Itziar Fraile, Maitane Gabilondo, Nerea Burgos, Mikel Azcona, Francisco Castro www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(18)31370-1 https://doi.org/10.1016/j.ceramint.2018.05.225 CERI18400

To appear in: Ceramics International Received date: 18 April 2018 Accepted date: 26 May 2018 Cite this article as: Itziar Fraile, Maitane Gabilondo, Nerea Burgos, Mikel Azcona and Francisco Castro, Laser sintered ceramic coatings of PZT nanoparticleS deposited by Inkjet Printing on metallic and ceramic substrates, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.05.225 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Laser sintered ceramic coatings of PZT nanoparticleS deposited by Inkjet Printing on metallic and ceramic substrates Itziar Fraile1,2, Maitane Gabilondo1,2, Nerea Burgos1, Mikel Azcona1 and Francisco Castro1. 1 Ceit, Manuel Lardizabal 15, 20018 Donostia / San Sebastián, Spain. 2 Universidad de Navarra, Tecnun, Manuel Lardizabal 13, 20018 Donostia / San Sebastián, Spain. E-mail: [email protected]

ABSTRACT Lead zirconate titanate (PZT) Pb(ZrxTi1-x)O3 is one of the most studied perovskite type ferroelectric materials due to its excellent dielectric, piezoelectric and ferroelectric properties. PZT particles and a PZT precursor were synthesized using a chemical method. A vehicle was added to the synthesized particles and precursor for obtaining two inks with appropriate rheological properties to be printed by Inkjet Printing. The use of an 80μm diameter nozzle made necessary the utilization of an energetic ball milling for assuring the dispersion of small PZT particles in the ink. After ball milling nanoparticles of 150 nm diameter were obtained. These inks were deposited on alumina and steel substrates followed by sintering using a pulsed laser of 1064 nm wavelength. The work shows the effect laser sintering has on, both inks, the one containing PZT nanoparticles and that one based on the PZT precursor. Laser processing was optimized in order to generate suitable films to be subsequently poled. The effect of poling on these films was also studied and their piezoelectric properties were measured by a compression test. The microstructural characteristics of these films were obtained by SEM and X ray diffractometry.

Keywords: Inkjet, laser sintering, PZT, piezoelectric effect, poling

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1. INTRODUCTION Lead zirconate titanate (PZT) Pb(ZrxTi1-x)O3 is one of the most studied perovskite type ferroelectric materials due to its excellent dielectric, piezoelectric, and ferroelectric properties [1]. This material in the form of thick films is used in a variety of devices for applications such as micromechanical systems (MEMS), microactuators with relatively large force, ultrasonic transducers, micropumps, pyroelectric infrared detectors, and bulk acoustic wave pass filters [2]. Chemical routes are presented as an alternative to obtain PZT particles instead of the classical techniques like Mixed Oxides Technology [3]. The principal characteristics of chemical methods are their low reaction temperature, uniform mixing of reactants, precise control of components, and high product purity [4]. Inkjet printing is a contactless printing technique, which employs a principle that makes it different from other printing techniques. Conventional printing methods, like screen printing and lithography, involve the reproduction of a pattern on the substrate. Instead, using inkjet printing allows the pattern to be directly built on the substrate by the sequential deposition of ink drops [5]. Other characteristics of inkjet printing are scalability and reduced amount of wasted ink since all the ink used in the process lies in the pattern on the substrate. There are different kinds of inkjet printers but this work will be limited to piezoelectric Drop-on-Demand (DoD) inkjet printers which are less restrictive than thermal DoD Inkjet printers [6]. After deposition of PZT nanoparticles there are several treatments that lead to a PZT thin or thick film. Thermal treatment is the most typical one even when PZT sintering temperature

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is above 800ºC [2,7], however, this limits the kind of substrates that can be used. Many efforts have been devoted to reduce the temperature needed for obtaining PZT films, for example, using a PZT precursor instead of PZT particles [8]. Another way of avoiding the use of high temperatures is using polymers to bond the particles, which allows printing PZT on flexible or polymeric substrates [9]. However, obviously the presence of polymers will reduce the piezoelectric properties of the film. All the possibilities mentioned above have to be taken into account for preparing an adequate ink. Nanoparticles-containing inks can be thermally or optically sintered. Thermal treatments in a furnace have fewer limitations because, independently of ink formulation, this process is aimed at removing the solvent and produce NPs sintering. Nevertheless, treatments at high temperatures and under oxidative atmospheres do not allow the use of, both, flexible polymeric and metallic substrates. The advantage of laser sintering is the application of heat to small local areas, thus drastically reducing the effect of heat on the substrate [10]. The mechanism of photonic sintering is very similar to the one obtained in a furnace except for the fact that light absorption provides heating of the sample [11]. That is the reason why powder absorbance is of great importance to laser sintering as it requires selecting an accurate wavelength for obtaining a suitable and uniform coating [12]. It is also crucial to control laser energy per volume and time [13]. Something similar happens with precursor-based inks. As the precursor must be decomposed above 500ºC for generating the desired material [14–17], damage to the substrate is a concern. Therefore, aiming to use substrates with different temperature sensitivity as well as intending to prevent substrate damage, this work was conducted varying laser sintering conditions for optimizing the delivered laser energy.

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The aim of this work was therefore centered on an innovative method using laser treatment pursuing, either, precursor decomposition or particle sintering depending on the type of ink used. Thus, the laser treatment produced particles sintering or precursor decomposition for finally obtaining a consistent printed film. Another advantage of laser treatment is that the number of different substrates that could be used is larger than those for thermal treatment in a furnace. 2. EXPERIMENTAL PROCEDURE As described in the introduction, PZT particles are synthetized from precursors used in chemical processes based on metal alkoxides, carboxylates and beta-diketones. Those organometallic compounds are formed with three different metals: lead, zirconium and titanium. The chemical process followed to synthesize PZT particles include the addition of water to the mix of organometallic compounds in order to form a gel. This gel is subsequently annealed to form perovskite PZT nanosize powder, which is nevertheless obtained in agglomerated form. Hence, this product needs to be ball-milled in order to generate separated nanosize particles (Fig. 1). In the case of the precursor-based ink, a sol is needed instead of the gel because the ink needs to flow through the inkjet printer nozzle. The stability of this sol must be also guaranteed in the presence of ambient water vapor for which a diamine-chelating agent was used. Vehicles and binders were added to the PZT nanoparticles and precursor for obtaining an ink with a viscosity below 30 cps and surface tension below 70 N/m, which are adequate for inkjet printing.

In order to avoid nozzle clogging PZT nanoparticles and precursor inks had to be filtered before printing. This operation was carried out with 5μm Whatman and Cameo syringe

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filters, respectively. This filtering process is particularly important for nanoparticlecontaining inks due to their tendency to flocculate. As PZT precursor ink is particle-free, filtering would seem unnecessary, however, it is still recommended as the precursor may be unstable thus causing the appearance of a dispersion of gel agglomerates which tend to clog the nozzle. The PZT nanoparticle ink was printed on commercial alumina and stainless steel substrates 1

and the PZT precursor-based ink was printed on stainless steel substrates, thus allowing

poling of the film. Inkjet printing tests were carried out with a Microfab jetting device. A piezo voltage of 43 V and a maximum jetting frequency of 1 kHz were used for proper drop formation. During printing, the substrate plate of the printer was at room temperature. Inkjet printing parameters like velocity and spacing between drops were optimized for each kind of substrate and ink. After printing, the samples were thermally cured in a hot plate at 40ºC for 30 min in order to eliminate the vehicle before laser sintering. In this study a 1064nm wavelength pulsed laser was used fixing frequency at 10 KHz, a maximum velocity of 1 mm/s and laser pulse of 200 ns. The influence of laser power on, both, PZT nanoparticles sintering and precursor ink decomposition was studied. Samples were microstructurally characterized using scanning electron microscopy and X-ray diffraction. For poling the printed layers a 10kV power supply 2was used. The samples were introduced in silicon oil at 120ºC for 30min and passing a current lower than 1mA through them. Specimens with a height of 10μm were poled using a voltage per unit length between 5000 and 7500V/mm. After poling the PZT layers were characterized by a compression test using an Instron Mini 44 press. With this purpose, a special set-up 1 2

CeramTec Alumina Hasberg Schneider Gmbh Stainless Steel 304 Keithley 2290-10

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consisting of two alumina plates (to electrically insulate the sample) and two copper electrodes was implemented, for allowing measurement of the voltage generated by the sample upon mechanical deformation. A data acquisition system called Agilent 34970A, was used to capture the desired signal. The compression tests were performed fixing a crosshead velocity of 200mm/min and a compressive load limit of 100N. The crystalline structure and morphology of the PZT sintered films were characterized by X-Ray diffraction using a Philips XPERT MRD diffractometer (Cu Ka1λ= 1.54059 Å) from 20º to 110º with a step size of 0.02º in 2θ and 10 s/º of scan speed. Additionally, a field emission gun scanning electron microscope (FEI model Quanta 3D FEG) was used in low vacuum configuration using accelerating voltages between 10 and 20 kV.

3. RESULTS AND DISCUSSION

For finding suitable conditions to sinter inks containing PZT nanoparticles on alumina and steel substrates different processing conditions were sought. After printing, the system is thermally activated by the laser thus encouraging sintering, hence the development of necks between particles, that produces changes in the appearance of the deposited layer as a function of laser power (Fig. 2). Fig. 2 and 3 include several micrographs of layers deposited on alumina and steel substrates sintered using different laser powers at a constant velocity of 1mm/sec. Micrographs 2a and b show the case of a film treated with low power (200mW). In this case some remnants of the organic binder are still observed (dark grey phase surrounding the particles and indicated with an arrow) and only incipient necking is developed between solid particles. In contrast, the effect of increasing power (micrographs

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c to h) encourages sintering thus causing neck formation between particles (encircled on Fig. 2f) and grain growth. On the other hand, the use of 3000mW (Figs. 2g and h) is not advised because, as explained further ahead, it causes degradation of PZT.As appreciated by the sequence of micrographs in Fig.3 the same trend is followed for the films sintered on steel substrates. As shown by the XRD traces in Fig. 4 the degradation of PZT is clearly detected by the presence of PbO and ZrO2 therefore indicating the dissociation of PZT to form stable single oxides from its constituents. Expectedly, these oxides are the same independently of the substrate used and their presence is more noticeable at the highest laser powers used. This effect arises as the result of a thermal degradation mechanism originated from a too high temperature generated on the PZT film when excessive power is delivered by the laser beam. It must also be emphasized that the laser power at which the degradation of PZT is observed is completely different for both substrates, being much lower for the films on steel substrates. From these observations, it can be concluded that, proper elimination of the organic binder, neck growth and degradation during laser sintering are thermally activated phenomena dependent on the combination of temperature and time provided during the laser treatment. Consequently, differences observed between the two substrates, must be related to their different capacity to retain heat. A possible explanation may be associated to the temperature reached during the laser treatment, which in turn depends on the thermal diffusivity of the substrate used. The thermal diffusivity coefficient is represented by the following equation:

(1)

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where ρ is the density of the material;

is the specific capacity and k is the thermal

conductivity. In this case, stainless steel with a smaller  value than aluminium oxide, is a worse thermal diffuser (Table 1), thus allowing the use of less aggressive sintering conditions to reach the same temperature. In addition, under the same laser treatment steel maintains the increase of temperature induced by the laser for a longer time than alumina. Sintering of particles on the steel substrate is therefore preferred to that on an alumina substrate. Working now with PZT precursor-based layers a new set of inkjet printed films was laser treated using only steel substrates. These experiments were aimed at identifying the optimum conditions for which the PZT crystalline phase could be obtained. The results, illustrated by the XRD traces in Fig. 5, show the differences obtained from 400 to 1200mW. Clearly, the crystallization process gradually advances from 400mW, where the material remains almost fully amorphous to the extreme case at 1200mW where three clearly identifiable crystalline phases coexist in equilibrium. The presence of the PZT perovskite, (main peaks around 30 and 45 degrees) starts to be identifiable at around 400mW, remains as the predominant crystalline phase as laser power is increased, and coexists with ever-increasing amounts of lead and zirconium oxides, as products of PZT degradation at the highest laser powers. The morphology of the films obtained in these conditions are shown in Fig. 6. As appreciated, under the laser processing, the films are subjected to thermal stresses and volume changes accompanying burning of the organic binders and the formation of crystalline phases. These lead to extensive pore and crack formation, which results in severe fragmentation of the films. The final product was therefore obtained broken in small

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porous pieces as the film loses continuity. This is illustrated in Figs. 6 c, e and g, by the letters F and S corresponding, respectively, to the broken film and the substrate. Additional experiments carried out at 400 and 800 mW on steel substrates revealed that the films obtained using the PZT nanoparticles-containing-ink were much more continuous than those shown in Fig. 6, but still exhibited cracking and noticeable amounts of porosity. Under these circumstances, aiming at obtaining films minimizing the presence of cracks and porosity, alternative processing conditions for the PZT nanoparticles-containing-ink were explored slowing down the process to a minimum velocity of 0.1mm/s and limiting the laser power up to 600 mW. Nevertheless, laser powers above 400 mW with stage velocities between 0.1 and 0.5 mm/s, still resulted in PZT degradation. On the other hand, as shown in Fig. 7 combining low velocities with lower laser power (200-300 mW) allowed obtaining films with a more homogeneous appearance. Trials carried out at 300 mW with a velocity of 0.1mm/s were considered optimum (Fig. 7c) for preventing or minimizing pores formation and cracking. This comes as a result of a more adequate kinetics of binder burning at the time that PZT degradation is avoided and volumetric shrinkage during particle sintering occurs gradually and relatively free of distortion. Film samples obtained by the previously presented method were poled at different voltages and their response under the compression test compared with the response of a non-poled sample (Fig. 11). As shown the piezoelectric response of a sample that has not been poled (0 V/mm) is negligible compared with the response of poled samples. It is well known that the direct piezoelectric effect of PZT crystals is strongly affected by the anisotropy of the crystal structure.

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Additionally the extent of such anisotropy may be influenced by poling as the number of electric dipoles oriented along the applied field direction increases with the strength of such field. For the experimentally obtained films, from PZT nanoparticles-containing inks, this effect is verified for samples previously subjected to poling at increasing applied voltages (Fig. 8).

These results may be expected, as they are a function of the number of oriented perovskite unit cells. In the case of the non-poled sample, the domains are orientated at random so its piezoelectric behavior is low. On the other hand, after poling, a number of electric domains are oriented in the same direction, being therefore associated to the increase of an electric response under the mechanical stress applied during the compression test. The use of a higher poling electric field increases the response until saturation is reached. Furthermore, as illustrated in Fig. 8 this effect is reproducible as the samples are subjected to cyclic compression testing.

4. CONCLUSIONS

PZT nanoparticle and precursor inks were prepared and inkjet printed on ceramic and metallic surfaces in order to generate piezoelectric thick films. A 1064nm wavelength laser was used as a heat source to sinter ceramic PZT nanoparticles and transform PZT precursor to PZT perovskite. Due to the thermal diffusivity of the substrates used, steel substrates appeared to be more appropriate for laser sintering the experimental thick PZT films.

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Due to the volume contraction suffered during laser treatment by the films obtained from PZT precursor based inks, this kind of films may not be adequately obtained without extensive cracking. From this point of view PZT nanoparticle based films were observed to exhibit a more attractive behavior for obtaining continuous and manageable films. To this end the optimum laser treatment must allow properly burning the organic binder, prevent degradation of the PZT perovskite and minimize or avoid completely the generation of extensive porosity and cracking of the deposited films. Limiting laser power to 300 mW combined with sufficiently slow scan velocities, around 0.1mm/s, was observed to render quite adequate and manageable films. Such films may be satisfactorily poled. Depending on the poling conditions, the materials generate between 20 and 150 mV as a response to the stress imposed by the compression test. ACKNOWLEDGEMENT The authors thankfully acknowledge the economical support from the Basque Government through the ELKARTEK ACTIMAT 2018 project. Referencees

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Figure 1. (a) PZT nanoparticle size distribution and (b) SEM image.

Figure 2. SEM images of nanoparticles laser sintered on alumina substrate with (a and b) 200 mW, (c and d) 600mW, (e and f) 2000 mW and (g and h) 3000 mW.

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Figure 3. SEM images of nanoparticles laser sintered on steel substrate with (a and b) 200 mW, (c and d) 600 mW and (e and f) 1000 mW.

Figure 4. X ray spectrum of a PZT film where the perovskite structure is degraded (above) and of a film with crystalline PZT (below).

Figure 5. X ray spectrum of several PZT precursor films treated with different laser power.

Figure 6. SEM images of PZT precursor laser processed on steel substrates with (a and b) 400 mW, (c and d) 600 mW, (e and f) 800 mW and (g and h) 1200 mW.

Fig. 7. SEM images of thick films generated by Inkjet Printing of PZT NP ink and laser sintered on steel substrate with (a) a power of 200 mW and a velocity of 0.1 mm/s, (b) 200 mW and 0.5 mm/s, (c) 300 mW and 0.1 mm/s and (d) 300 mW and 0.5 mm/s.

Figure 8. Electric response of samples under the compression test after poling at different voltages.

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Table 1. Relevant theoretical properties of Stainless steel 304 [18] and alumina [19]. Substrate Density (g/cm3) Specific capacity (J/kg

)

Thermal conductivity (W/m Thermal diffusivity (mm2/s)

)

Stainless steel 304

Alumina

7.9

3.85

477

775

14.9

35

3.95

11.73

16

17

18

19

20

21

22

23

24

25

26

27