# Freeze-Cast Porous Textured BaTiO 3 –Polymer Composites for Energy Harvesting Applications

Ajeet Kumar, Alex Tezcan, Zihe Li, Ruxue Yang, Florian Bouville, Guylaine

Poulin-Vittrant, Hamideh Khanbareh, James Roscow, Sylvain Deville, Chris

Bowen

► To cite this version:

Ajeet Kumar, Alex Tezcan, Zihe Li, Ruxue Yang, Florian Bouville, et al., Freeze-Cast Porous Textured BaTiO 3 -Polymer Composites for Energy Harvesting Applications. ACS Applied Energy Materials, 2025, 8 (15), pp.11437-11446. 10.1021/acsaem.5c01606. http://dx.doi.org/10.1021/acsaem.5c01606

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Freeze-casted porous textured BaTiO₃-polymer composites for energy harvesting applications

Ajeet Kumar¹*, Alex Tezcan¹, Zihe Li¹, Ruxue Yang², Bouville Florian², Guylaine Poulin-Vittrant³, Hamideh Khanbareh¹, James Roscow¹, and Sylvain Deville⁴ Chris Bowen¹

¹Department of Mechanical Engineering, University of Bath, Bath, BA27AY, United Kingdom

²Centre for Advanced Structural Ceramics, Department of Materials, Imperial College, London, United Kingdom

³GREMAN UMR 7347, CNRS, University of Tours, INSA-CVL, Blois, 41000, France

⁴Universite Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, UMR5306, F-69100, Villeurbanne, France

*E-mail: ak3829@bath.ac.uk
**Abstract:**

Porous piezoelectric ceramics exhibit a unique combination of high piezoelectric charge coefficients ($d_{ij}$) and low permittivity compared to their dense counterparts, which is desirable for achieving high piezo-sensing and energy-harvesting performance. Here, we report on a novel process demonstrating the use of a freeze-casting technique to fabricate lead-free porous textured $BaTiO_3$ ceramics with highly aligned porosity. A high degree of alignment of the piezoelectric $BaTiO_3$ platelets in the freezing direction was confirmed using scanning electron microscopy (SEM). The degree of texturing was quantified with X-ray diffraction yielding a Lotgering factor ($LF$) ~ 0.37. To enhance the mechanical strength and strain to failure for sensing and harvesting applications, the porous textured $BaTiO_3$ ceramics (~60 vol.% porosity, sintered at 1150 °C for 4h) were infiltrated with polymers (epoxy and polydimethylsiloxane (PDMS)) of contrasting elastic properties. The $BaTiO_3$-epoxy composite structure demonstrated a 0.93% strain to failure at a high failure stress of 71.6 MPa, with Young's modulus of 7.6 GPa. In contrast, the $BaTiO_3$-PDMS composite shows a flexible nature and exhibited a lower Young's modulus of 0.015 GPa and a higher strain to failure (>22%). The dielectric properties, polarization-electric field loops and piezoelectric properties were examined in detail, and the poled $BaTiO_3$-epoxy composite was used to fabricate a cantilever structure to demonstrate its energy harvesting and sensing performance. This work has shown that directional freeze casting can produce aligned porous and textured ferroelectric microstructure for sensing or energy harvesting applications.

**Keywords:** Lead-free, Porous, Piezoelectric, Composites, Mechanical properties, Energy harvesting
# 1. Introduction

Ferroelectrics are a group of highly polar materials that can generate electrical charge in response to mechanical stress through the direct piezoelectric effect, which makes them ideal for applications such as pressure sensors, SONAR, medical transducers, and vibration energy harvesters [1–6]. Ferroelectric (FE) single crystals exhibit superior properties, which is attributed to the enhanced piezoelectricity from the large intrinsic (lattice) contribution compared to the extrinsic domain wall motion observed in polycrystalline ceramic counterparts [7–9]. However, the necessity for complex fabrication processes, and the difficulty of controlling the size, shape, growth parameters, and homogeneity make single-crystal materials expensive and difficult to form in complex geometries.

As a result, polycrystalline ceramics have been fabricated with a preferred crystallographic orientation, namely a ‘texture’ [10–12]. Tape casting has been used to fabricate textured ceramics, whereby 5-10 vol.% of platelets are dispersed within a fine particle matrix and deposited in multiple thin layers onto a flat surface using a doctor blade to apply a shear stress to align the platelets parallel to the casting direction. The resulting sample is then heat treated to grow the aligned templates, where the degree of alignment of the platelets determines the texture quality of the final ceramics formed. As an example, an exceptionally high piezoelectric charge coefficient ($d_{33}$) of 755 pC/N was reported for textured dense (Ba,Ca)(Zr,Ti)O$_3$ (BCZT) ceramics, greater than the non-textured counterparts [13]. While textured ceramics possess piezoelectric coefficients lower than single crystals, they are higher than conventional polycrystalline ceramics. Nevertheless, most commercial piezoelectric material-based devices are fabricated from dense, lead-based, untextured ferroelectric ceramics [14] due to the complexities of fabricating single-crystal materials and the relatively slow layer-by-layer approach associated with the manufacturing of tape-cast textured ceramics. Despite having different microstructures, ferroelectric single crystals, textured ceramics, and dense polycrystalline ceramics share some common characteristics, namely a high permittivity ($\epsilon_{33}^T$). The high permittivity of these materials limit their performance figures of merit for piezoelectric sensing ($g_{33} = d_{33}/\epsilon_{33}^T$) and energy harvesting ($d_{33} \cdot g_{33} = d_{33}^2/\epsilon_{33}^T$) which results in low voltage generation in response to applied mechanical stress. The high density and stiffness of the materials also lead to poor acoustic impedance
matching with biological tissue and water, which is undesirable for medical ultrasound and SONAR applications.

Therefore, an effort has been made to lower the permittivity, stiffness and density of these materials by manufacturing porous piezoelectric ceramics and composites using a directional freeze-casting method [15–17]. The unique combination of low dielectric permittivity and high piezoelectric coefficients of aligned porous piezoelectric ceramics provides an opportunity to design new materials with attractive properties for piezoelectric-sensing (force, acceleration) and energy-harvesting (vibration, thermal) properties, compared to their dense counterparts [16,17].

To further increase relevant figures of merit (FoM), one approach is to produce porous textured materials which can exhibit improved piezoelectric coefficients, due to the presence of a texture, while also exhibiting a low permittivity, due to the presence of porosity. By inducing crystallographic texturing within the porous piezoceramic matrix, the piezoelectric activity of porous piezoelectric ceramics has potential to approach that of a single crystal. Freeze-casting has been used to create porous materials with highly aligned porosity. In this process, ceramic particles of the desired material are suspended in a slurry, which is subsequently frozen, and the ceramic particles are ejected from the moving solidification front, forcing them to segregate between the ice crystals that grow preferentially along temperature gradients. The freezing parameters such as particle size, the viscosity of the slurry, the velocity of sedimentation and the critical freezing-front velocity at which particle trapping occurs during solidification must be carefully controlled [18]. There have been limited reports to date that have explored the use of freeze-casting technique to align platelet-shaped particles [19,20]. Hunger et al. reported a self-assembly of platelets within a non-ferroelectric alumina composite via freeze-casting method to improve the mechanical properties [19,21]. These outperformed ceramics by a factor of 1.5 – 4 in terms of stiffness, strength and toughness materials that had the same amount of porosity, but did not exhibit the platelet microstructure. Bouville et al. developed a “freezing-under-flow” method. They successfully produced microporous materials (alumina and sodium-potassium niobate) with aligned, lamellar ceramic walls made up of crystallographically aligned grains using a combination of modified ice templating and templated grain growth (TGG). The piezoelectric and ferroelectric properties of the porous textured sodium potassium niobate ceramics were not reported [20].
To date, templated grain growth (TGG) and texturing methods have produced dense monolithic ceramics and composite microstructures with improved properties compared to non-textured ceramics (e.g., mechanical, piezoelectric) [22,23]. However, these processing techniques are unable to produce a composite architecture that combines controlled macroscopic porosity with textured ceramic layers. Therefore, we propose here a freeze-casting approach to fabricating porous, composite templated grain growth structures. We aim to texture the porous material to significantly reduce the permittivity while increasing the piezo-sensing and energy-harvesting performance. The process is attractive due to its simplicity and potential to produce various geometries, whilst the directional freezing produces highly aligned porosity to facilitate the polarisation of the ferroelectric materials. The key challenge is to use directional freezing to align ferroelectric seed crystal templates in a common orientation. In this study, platelets of lead-free BaTiO₃ piezoelectric ceramics were aligned vertically in the freezing direction by the freeze-casting technique. This porous templated structure of BaTiO₃ piezoelectric ceramics was then infiltrated with polymers of contrasting mechanical properties, namely an epoxy and polydimethylsiloxane (PDMS), to fabricate a composite structure, as shown in Fig. 1. The highly aligned BaTiO₃-epoxy and BaTiO₃-PDMS piezoelectric-polymer composite structures were characterised for their mechanical, dielectric, ferroelectric, and energy harvesting properties. This novel process of templating lead-free porous piezoelectric ceramics and fabrication of composite structures is reported for the first time. It provides a route towards crystallographic texturing of porous BaTiO₃ ceramic matrices.

## 2. Experimental Procedure

Highly aligned BaTiO₃ templated ceramics with ~60 vol.% porosity were fabricated using commercially available BaTiO₃ platelets with an aspect ratio in the range of ~5 – 13 (Entekno Ind. Tech. & Nano Mat. Corp., Turkey). The freeze-casting method was used to achieve the desired porosity, whereby the BaTiO₃ platelets were mixed with distilled water, 1 wt% of binder (polyvinyl alcohol, PVA, molar mass = 86.09 g/mol, density = 1.19 g/cm³), and a dispersant (acrylic acid, molar mass = 72.06 g/mol, density = 1.05 g/cm³) to form a slurry. The slurry was mixed for 2 hours using ball milling (Capco test equipment, UK) to produce a homogeneous mix and then poured into a mould (diameter = 1.3 cm, height = 2.3 cm) which was kept on the top of a metal cooling plate (20 cm x 20 cm) that was already cooled to -70 °C using an ultralow temperature circulator (RP2090, Lauda Scientific, Germany). After freezing, a freeze dryer (Mini LyoTrap, LTE Scientific Limited, UK) was
used to remove the ice crystals from the porous aligned BaTiO₃ platelets. The freeze-dried samples were then partially sintered at a temperature of 1150 °C for 4 hours (Elite Thermal Systems Limited, UK) to produce a small degree of sintering between the platelets to produce porous scaffold structures that were sufficiently robust for polymer impregnation. A schematic of the freeze-casting and freeze-drying process is shown in Figure 1 (a)-(e). The sintered porous BaTiO₃ textured ceramics were sectioned using a diamond wire saw (STX 202A, MTI Corporation, USA).

<figure><img src="image_2.png" /><figcaption>Fig. 1. Schematic of the process to fabricate BaTiO₃ platelets-polymer composite scaffold structure. (a) Preparation of slurry of BaTiO₃ platelets mixing in water, binder and dispersant. (b) BaTiO₃ platelets are aligned by ice templates in the freezing direction (freeze-casting). (c) Freeze drying was used to remove these ice crystals. (d) Porous scaffold structure of BaTiO₃ platelets is partially sintered to provide strength to the platelet assembly and to maintain their shape and size and (e) infiltration of the porous structure with different polymers to fabricate BaTiO₃ platelets-polymer composites. Schematic of the freezing direction (F.D.) and measurement direction of (f) X-ray diffraction, scanning electron microscopy (SEM) of the (g) top surface (h) cross-sectional, (i) mechanical properties and (j) electrical properties measurement of the porous textured ceramics.</figcaption></figure>

The freezing direction (F.D.) and measurement direction of each characterization are shown in Figure 1 (f)-(j). In this work, phase analysis, mechanical and electrical properties studies are carried out parallel to the freezing direction. X-ray diffraction (XRD, STOE
STADI P, STOE & Cie GmbH, Germany) patterns and scanning electron microscopy (SU3900, Hitachi, Japan) images were taken to investigate the crystal and micro-structures on the partially sintered porous BaTiO₃ textured ceramics. To measure the electrical properties and to fabricate an energy harvester, the partially sintered porous and textured BaTiO₃ ceramics were infiltrated with epoxy (EPO-TEK 301, Epoxy Technology, INC, USA) and polydimethylsiloxane (PDMS, SYLGARD™ 184 Silicone Elastomer, The Dow Chemical Company, UK) elastomer to enhance their mechanical strength and strain to failure. To impregnate the porous BaTiO₃ ceramic, samples were immersed in a mould filled with epoxy and PDMS (in vacuum) and cured at room temperature for 48 hours. The BaTiO₃-epoxy and BaTiO₃-PDMS composites were then sectioned, and electrodes were painted on opposite sides using silver paste. The frequency-dependent dielectric properties of unpoled BaTiO₃-epoxy and BaTiO₃-PDMS composites were measured using a precision impedance analyser (4294A, Agilent Technologies, USA). The ferroelectric polarisation vs electric field (P-E) hysteresis loops of the porous BaTiO₃-epoxy and BaTiO₃-PDMS composites were measured using a Radiant hysteresis loop tracer (Precision Premier II, Radiant Technologies, USA). The composite samples were poled using the corona poling technique where a high voltage (Glassman High Voltage, Inc., UK) was applied from a point source at ~60 °C for 60 minutes to pole the composite samples (distance between the sample and point source ~2.5 cm). The piezoelectric charge coefficient ($d_{33}$) of the poled composites was measured using a piezometer (Piezotest, Piezometer, UK). To measure the Young's modulus of the composites, a mechanical three-point bend test was performed using samples with dimensions (length x width x thickness) of 40 x 4 x 4 mm³ (Model 3369, Instron, Zwick-Roell, UK). A poled BaTiO₃-epoxy composite (1 x 0.5 x 0.1 cm³) was mounted onto an aluminium cantilever (10 x 0.5 x 0.1 cm³) attached to a shaker (Bruel & Kjaer VTS Ltd, UK) that provided a mechanical excitation to evaluate the sensing and energy harvesting performance. A function generator was used to provide a signal to the voltage amplifier. The output voltage of the voltage amplifier was then used as an input signal for the shaker. The generated voltage was measured as a function of load resistance and used to charge a storage capacitor.

# 3. Results and Discussion

## 3.1 Phase and microstructure analysis of porous scaffold of BaTiO₃ platelets

Figure 2(a) shows the XRD pattern for freeze-cast textured BaTiO₃ porous ceramics. Generally, for the polycrystalline BaTiO₃ ceramics, the peak intensities of all (hkl) planes are
<figure><img src="image_3.png" /><figcaption>Fig. 2. (a) X-ray diffraction pattern of textured BaTiO₃ porous ceramics, (b) schematic of the sintered sample, (c)-(d) top surface and (e)-(f) cross-sectional scanning electron microscopy images of porous scaffold structure of BaTiO₃ platelets, partially sintered at 1150 °C for 4 hours. The arrow in the (f) represents the alignment of platelets in the freezing direction (not to scale).</figcaption></figure>
## 3.2 Mechanical properties of textured BaTiO₃-polymer composites

<figure><img src="image_4.png" /><figcaption>Fig. 3. (a)-(b) Cross-sectional scanning electron microscopy image of textured BaTiO₃-PDMS composite. Yellow dotted rectangles show the PDMS-rich regions of the composites. Stress-strain curves for the textured (c) BaTiO₃-epoxy and (d) BaTiO₃-PDMS composites.</figcaption></figure>

To produce textured BaTiO₃ porous ceramics with sufficient mechanical stability to fabricate a sensing or harvesting device, two different polymers were infiltrated into the porous scaffolds [28,29]. Figure S2 shows textured BaTiO₃-epoxy and BaTiO₃-PDMS composites with different geometries. It was found that the pore channels were occupied by the polymers after infiltration, while also maintaining the scaffold structure and platelet alignment. Figure S3 shows a scanning electron microscopy image of the textured BaTiO₃-epoxy composite, while Figures 3(a)-(b) show cross-sectional SEM images of the textured BaTiO₃-PDMS composite. These images clearly show that the polymer not only infiltrates the columnar pore structure produced by the directional freeze-casting process but also covers the BaTiO₃ platelets so that they are effectively surrounded by the polymer matrix, thereby
providing mechanical stability composites. The regions related to the PDMS matrix and BaTiO₃ platelets are marked in Figures 3(a)-(b).

Three-point bend tests were conducted to determine the mechanical properties of the textured BaTiO₃-epoxy and BaTiO₃-PDMS composites. The failure stress, failure strain and Young's modulus for the textured BaTiO₃-epoxy composite were respectively found to be 71.6 ± 3.05 MPa, 0.93 ± 0.005% and 7.6 ± 0.02 GPa. The PDMS also occupies the pores in the composite and supports the composite structure well, resulting in a low Young's modulus (15 ± 1.2 MPa) along with an ability to be deformed to a high level of curvature and strain above 22 ± 1.5%. The mechanical properties of the textured BaTiO₃-epoxy and BaTiO₃-PDMS composites are summarised in Table 1.

### 3.3 Dielectric, ferroelectric, and piezoelectric properties of textured BaTiO₃-polymer composites

<figure><img src="image_5.png" /><figcaption>Fig. 4. Dielectric permittivity and dielectric loss for the textured (a) BaTiO₃-epoxy and (b) BaTiO₃-PDMS composites. Polarization vs electric field (P-E) hysteresis loops for the textured (c) BaTiO₃-epoxy and (d) BaTiO₃-PDMS composites.</figcaption></figure>
Figures 4(a) and (b) show the frequency-dependent dielectric properties of the textured BaTiO₃-epoxy and BaTiO₃-PDMS composite structure in the range of 1 kHz to 1 MHz. The relative permittivity (ε_r) and dielectric loss (tan δ) of the textured BaTiO₃-epoxy composite are ε_r = 73 and tanδ = 0.024, and for the textured BaTiO₃-PDMS are ε_r = 53 and tanδ = 0.007. Both the relative permittivity and loss of the composites are lower than the dense BaTiO₃ ceramic prepared using the BaTiO₃ platelets (ε_r = 612, tanδ = 0.039), summarised in Table 1. The lower dielectric permittivity and loss values are a result of the formation of aligned BaTiO₃ platelets that are suspended structures in a highly insulating and low-loss polymer matrix, where the epoxy and PDMS polymers infiltrated the BaTiO₃ platelets scaffold structure. This can be observed in Figure 3(b) for the PDMS-based composite and Figure S3 for the epoxy-based composite. The relative permittivity and dielectric loss of the pure epoxy are ε_r = 4 and tanδ = 0.016, and for the pure PDMS are ε_r = 2.7 and tanδ = 0.002, which is responsible for the lower dielectric properties of the textured BaTiO₃-polymer composites.

Polarization - electric field (P-E) ferroelectric hysteresis loops were measured for both the textured BaTiO₃-epoxy and BaTiO₃-PDMS composites to confirm the ferroelectric nature as shown in figures 4(c) and (d). The textured BaTiO₃-epoxy composite showed a maximum polarization of P_max ~ 0.97 µC/cm², remanent polarization of P_r ~ 0.18 µC/cm² and a coercive field of E_c ~ 13.24 kV/cm. The textured BaTiO₃-PDMS composite had a higher maximum polarization of P_max ~ 1.43 µC/cm², higher remanent polarization of P_r ~ 0.37 µC/cm² and a larger coercive field of E_c ~ 19.50 kV/cm. The ferroelectric properties of the textured BaTiO₃-epoxy and BaTiO₃-PDMS composite structures are influenced by the nature of the polymer infiltration and their properties. The higher P_max and P_r for the textured BaTiO₃-PDMS composite, compared to the epoxy-based composite is possibly related to the much lower Young's modulus (Y) of the PDMS (Y ~ 1.5 MPa) [30,31] compared to the epoxy (Y ~ 3.4 to 4.6 GPa) [32,33], which reduces the degree of polymer mechanical clamping that can reduce the degree of polarisation when subjected to an electric field [34]. The higher E_c of the textured BaTiO₃-PDMS composite, compared to the epoxy-based composites, is also likely to be a result of the low viscosity PDMS infiltrating the composites and coating the platelets to a greater extent, thereby necessitating a high applied electric field to induce switching of domains in the BaTiO₃ platelets. The PDMS also has lower relative permittivity (ε_r = 2.7), compared to the epoxy (ε_r = 4), which can lead to a larger fraction of the applied electric field being present in the low permittivity matrix phase [35].
The textured BaTiO₃-epoxy and BaTiO₃-PDMS composites were poled by applying a DC electric field at 60 °C for 60 minutes and show the piezoelectric charge coefficients of *d*₃₃ ~ 25 pC/N and 12 pC/N, respectively. The lower relative permittivity of the pure epoxy and PDMS are responsible for the lower dielectric, ferroelectric and piezoelectric properties of textured BaTiO₃-polymer composites. All electrical properties of the textured BaTiO₃-epoxy and BaTiO₃-PDMS composites are summarised in Table 1, where it can be observed that the low permittivity of the composites leads to relatively good *g*₃₃ voltage coefficients [23].

**3.4 Energy harvesting properties of BaTiO₃ platelets-polymer composites**

The mechanical stability (Figure 3) and robustness of textured BaTiO₃-epoxy and BaTiO₃-PDMS composites make them suitable for fabricating a piezoelectric cantilever structure to harvest energy from the mechanical vibrations. The textured BaTiO₃-epoxy composite shows higher values of *d*₃₃, *g*₃₃ and energy harvesting FoM (*d*₃₃ x *g*₃₃) than the textured BaTiO₃-PDMS composite used for the energy harvesting study. The schematic of the energy harvester is shown in Figure 5(a) and the actual set-up is shown in Figure S5. To demonstrate the energy harvesting capabilities, textured BaTiO₃-epoxy composite was used as a piezoelectric element (1 x 0.5 x 0.1 cm³) that was bonded on the aluminium cantilever (10 x 0.5 x 0.1 cm³) with a tip mass (~24 gms) to fabricate an energy harvester. This textured BaTiO₃-epoxy composite piezoelectric cantilever was attached to a shaker to provide mechanical vibrations which were controlled by the input signal voltage and frequency. During the measurement, the signal generator generates a sine wave signal with a voltage peak-to-peak amplitude (*V*<sub>pp</sub>) of 15V and a frequency of 15 Hz to drive the shaker. Under these periodic mechanical vibrations, the sample outputs periodic electrical signals, which are then collected by an electrometer.

As is shown in Figure 5(b), the textured BaTiO₃-epoxy composite generates an output voltage of ~3.2 *V*<sub>pp</sub>. Figure 5(c) shows the output voltage and power of the textured BaTiO₃-epoxy-based energy harvester as a function of the load resistance (*R*<sub>L</sub>). The output voltage shows an increase with an increase in *R*<sub>L</sub> and saturates at a maximum root mean square (RMS) voltage (*V*<sub>rms</sub>) of 1.51 V. The power of the textured BaTiO₃-epoxy-based energy harvester is calculated using the formula *P* = *V*²/*R* where *V* is the output *V*<sub>rms</sub> voltage and *R* is the load resistance (*R*<sub>L</sub>). At a load resistance of ~30 MΩ, the textured BaTiO₃-epoxy-based energy harvester generates the highest power of 0.033 μW. The output *V*<sub>rms</sub> voltage of the textured BaTiO₃-epoxy-based energy harvester was rectified using a bridge rectifier and
used to charge a capacitor, as shown in Figure 5(d). The capacitor of 10 µF capacitor is fully charged in less than 1 minute (42 seconds). These results show that the textured BaTiO₃ porous ceramics can be employed to fabricate a piezoelectric energy harvester with comparable energy-harvesting properties [23].

<figure><img src="image_6.png" /><figcaption>Fig. 5. (a) Schematic of the textured BaTiO₃-epoxy composite piezoelectric energy harvester used for this study. (b) Generated output voltage waveform from the cantilever with mechanical vibrations. (c) Output root mean square voltage and power as a function of load resistance (d) capacitor charging curve as a function of time from textured BaTiO₃-epoxy composite cantilever beam energy harvester by applying mechanical vibrations of 15 Hz with an amplitude of 15V.</figcaption></figure>
<table><caption>Table 1. Summary of dielectric, ferroelectric, piezoelectric and mechanical properties of textured BaTiO₃-polymer composites.</caption><thead><tr><th></th><th>BaTiO<sub>3</sub>-epoxy</th><th>BaTiO<sub>3</sub>-PDMS</th></tr></thead><tbody><tr><td>Pore fraction (vol.%)</td><td>60</td><td>60</td></tr><tr><td>σ<sub>f</sub> (MPa)</td><td>71.6</td><td>2.5</td></tr><tr><td>ε<sub>fail</sub> (%)</td><td>0.93</td><td>22</td></tr><tr><td>E (GPa)</td><td>7.6</td><td>0.015</td></tr><tr><td>ε<sub>r</sub> (RT, 1kHz)</td><td>73</td><td>53</td></tr><tr><td>tan δ (RT, 1kHz)</td><td>0.024</td><td>0.007</td></tr><tr><td>P<sub>max</sub> (μC/cm<sup>2</sup>)</td><td>0.97</td><td>1.43</td></tr><tr><td>P<sub>r</sub> (μC/cm<sup>2</sup>)</td><td>0.18</td><td>0.37</td></tr><tr><td>E<sub>max</sub> (kV/cm)</td><td>87</td><td>92</td></tr><tr><td>E<sub>c</sub> (kV/cm)</td><td>13.24</td><td>19.50</td></tr><tr><td>d<sub>33</sub> (pC/N)</td><td>25</td><td>12</td></tr><tr><td>g<sub>33</sub> (10<sup>-3</sup> Vm/N)</td><td>39</td><td>26</td></tr><tr><td>Harvesting FoM (d<sub>33</sub>.g<sub>33</sub>)</td><td>0.98</td><td>0.31</td></tr></tbody></table>

## 4. Conclusions

This paper presents a method to fabricate textured porous ceramics based on BaTiO₃ platelets using a freeze-casting approach. The potential to align ferroelectric platelets due to the forces generated by the developing ice-freezing front during directional freezing is reported. XRD patterns of textured BaTiO₃ porous ceramics indicate a degree of texturing, with a Lotgering factor (LF) of ~ 0.37, and alignment of the BaTiO₃ platelets in the freezing direction was also confirmed via scanning electron microscopy. The textured BaTiO₃ porous sintered ceramics were infiltrated with polymers (epoxy and polydimethylsiloxane) to improve the mechanical strength of the composite materials, where the textured BaTiO₃-epoxy composite structure exhibits failure stress (71.6 ± 3.05 MPa), failure strain (0.93 ± 0.005%) and Young's modulus (7.6 ± 0.02 GPa). The textured BaTiO₃-PDMS composite structure shows a flexible structure with higher strain (~22%). It was found that polymers infiltrate the porous columns of the structure, without impacting the aligned structure, and also make the BaTiO₃ platelets suspended in the polymer matrix. The lower value of dielectric, ferroelectric and piezoelectric properties of textured BaTiO₃-polymer composites is due to the low relative permittivity of the pure epoxy and PDMS. The energy harvesting
capabilities of the textured BaTiO₃-epoxy composite structure were demonstrated by fabricating a cantilever structure. Complete control over the template position and orientation in freeze casting will open new possibilities for synthesising low-cost, high-performance textured materials, and devices in various geometries for a range of applications.

## Acknowledgements

This work is supported by UKRI New Horizons on Porous Piezoelectric Single Crystal Sensors (POPSICALS); Project No. EP/X018679/1). CB acknowledges support of UKRI Frontier Research Guarantee on “Processing of Smart Porous Electro-Ceramic Transducers - ProSPECT”, project No. EP/X023265/1.

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