Fabrication of bulk piezoelectric and dielectric BaTiO3 ceramics using paste extrusion 3D printing technique

A simple and facile method was developed to fabricate functional bulk barium titanate (BaTiO₃, BT) ceramics using the paste extrusion 3D printing technique. The BT ceramic is a lead-free ferroelectric material widely used for various applications in sensors, energy storage, and harvesting. There are several traditional methods (eg, tape casting) to process bulk BT ceramics but they have disadvantages such as difficult handing without shape deformation, demolding, complex geometric shapes, expansive molds, etc. In this research, we utilized the paste extrusion 3D printing technique to overcome the traditional issues and developed printable ceramic suspensions containing BT ceramic powder, polyvinylidene fluoride (PVDF), N,N-dimethylformamide (DMF) through simple mixing method and chemical formulation. This PVDF solution erformed multiple roles of binder, plasticizer, and dispersant for excellent manufacturability while providing high volume percent and density of the final bulk ceramic. Based on empirical data, it was found that the maximum binder ratio with good viscosity and retention for desired geometry is 1:8.8, while the maximum BT content is 35.45 vol% (77.01 wt%) in order to achieve maximum density of 3.93 g/cm³ (65.3%) for 3D printed BT ceramic. Among different sintering temperatures, it was observed that the sintered BT ceramic at 1400°C had highest grain growth and tetragonality which affected high performing piezoelectric and dielectric properties, 200 pC/N and 4730 at 10³ Hz respectively. This paste extrusion 3D printing technique and simple synthesis method for ceramic suspensions are expected to enable rapid massive production, customization, design flexibility of the bulk piezoelectric and dielectric devices for next generation technology.     

SLA 3D printing of high quality spine shaped β-TCP bioceramics for the hard tissue repair applications

β-TCP has excellent biodegradability and bioabsorption properties, and is regarded as an ideal hard tissue repair material. In the present study, 3D printing β-TCP green bodies was realized using the stereo lithography apparatus (SLA) technology. The effects of the KH-560 dispersant and solid loading on the slurry properties were investigated systematically. The optimized KH-560 addition amount and the solid loading of the slurry were 2.0 wt% and 48 wt%, respectively, and the corresponding slurry for the subsequent SLA 3D printing exhibited good fluidity, uniform dispersion and good stability. The sintering schedule of the printed β-TCP green bodies was optimized by the DSC-TG analysis. By sintering the green bodies at 1050 °C for 8 h, high quality β-TCP bioceramics without crack or deformation were fabricated. It was found that increasing the solid loading of the slurry would decrease the porosity while reducing the shrinkage degree of the β-TCP ceramics. However, the slurry could hardly be printed when its solid loading was higher than 50 wt%.     

Fabrication of barium titanate ceramics via digital light processing 3D printing by using high refractive index monomer

A digital light processing (DLP) technology has been developed for 3D printing lead-free barium titanate (BTO) piezoelectric ceramics. By comparing the curing and rheological properties of slurries with different photosensitive monomer, a high refractive index monomer acryloyl morpholine (ACMO) was chosen, and a design and preparation method of BTO slurry with high solid content, low viscosity and high curing ability was proposed. By further selecting the printing parameters, the single-layer exposure time was reduced and the forming efficiency has been greatly improved. Sintered specimens were obtained after a nitrogen-air double-step debinding and furnace sintering process, and the BTO ceramics fabricated with 80 wt% slurry shows the highest relative density (95.32 %) and piezoelectric constant (168.1 pC/N). Furthermore, complex-structured BTO ceramics were prepared, impregnated by epoxy resin and finally assembly made into hydrophones, which has significance for the future design and manufacture of piezoelectric ceramic-based composites that used in functional devices.     

DLP 3D printing porous β-tricalcium phosphate scaffold by the use of acrylate/ceramic composite slurry

Digital light processing (DLP) 3D printing has been utilized to fabricate controlled porous β-tricalcium phosphate (β-TCP) scaffolds, which promote cell adhesion and angiogenesis during bone regeneration. However, the current limitation of DLP 3D printing for the fabrication of β-TCP scaffold is how to prepare a low viscosity ceramic slurry and remove the toxicity of residual non-polymerized slurry. The present study has developed a low viscosity ceramic slurry system by mixing β-TCP with photosensitive acrylate resin, and the viscosity of slurry is about 3 Pa s and the solid content of β-TCP can be as high as 60 wt%. After optimizing the ratio of slurry, printing, degreasing and sintering processes, the maximum compressive strength of the DLP printed scaffolds reaches up to 9.89 MPa, while the porosity keeps ca. 40%. According to the proliferation of cells, it confirms the preserved biocompatibility of DLP-fabricated β-TCP scaffolds. These porous scaffolds made by DLP 3D printing technology is of great significance for bone regeneration, and will also help to expand the application of DLP technology in biomedical field.     

Influence of Microstructure on the Dielectric and Piezoelectric Properties of Lead Zirconate-Polymer Composites

Composites with 0–3 connectivity were fabricated from lead zirconate titanate (PZT) and phenolic resin powders. These composites were investigated for dielectric and piezoelectric properties with variations in active particle density and PZT-polymer interface porosity. The dependence of dielectric and piezolectric properties on interface porosity is discussed, especially in terms of porosity factors. The dependence of the piezoelectric constant on interface porosity was greater than that of the dielectric constant. The interface pores play the role of a stress buffer. Thus local stress applied on PZT particles in the composites was remarkably diminished. When particle porosity was high, the dependence of the dielectric and piezoelectric constants on interface porosity decreased.     

Additive manufacturing of SiC-Sialon refractory with excellent properties by direct ink writing

Additive manufacturing of SiC-Sialon refractory with complex geometries was achieved using direct ink writing processes, followed by pressureless sintering under nitrogen. The effects of particle size of SiC powders, solid content of slurries and additives on the rheology, thixotropy and viscoelasticity of ceramic slurries were investigated. The optimal slurry with a high solid content was composed of 81 wt% SiC (3.5 µm+0.65 µm), Al₂O₃ and SiO₂ powders, 0.2 wt% dispersant, and 2.8 wt% binder. Furthermore, the accuracy of the structure of specimens was improved via adjustment of the printing parameters, including nozzle size, extrusion pressure, and layer height. The density and flexural strength of the printed SiC-Sialon refractory sintered at 1600 °C were 2.43 g/cm³ and 85 MPa, respectively. In addition, the printed SiC-Sialon crucible demonstrated excellent corrosion resistance to iron slag. Compared to the printed crucible bottom, the crucible side wall was minimally affected by molten slag.     

Influence of bimodal particle distribution on material properties of BaTiO3 fabricated by paste extrusion 3D printing

Barium titanate (BaTiO₃) is a lead-free piezoelectric ceramic widely used in sensors and actuators applications. However, there are many manufacturing challenges to process BaTiO₃ due to the brittle nature of ceramics. Most current sensors based on piezoelectricity are limited to mold shapes or flat 2D structures, which narrow their applications. Paste extrusion (PE) 3D printing technique allows the fabrication of complex geometry ceramics with less design limitations. However, the piezoelectric property of 3D printed ceramics is typically lower than those fabricated using traditional means due to lower density. Herein, a study to evaluate the influence of bimodal particle distribution on improving density and piezoelectricity of BaTiO₃ ceramics fabricated using PE 3D printing is presented. 3D printed and compression pressed samples under the same mixing ratios were compared. The highest packing density was obtained using 50-50% vol. fraction of bimodal particles for both types of samples. A predictive model for packing density was validated by experimental results. The highest piezoelectric coefficient of 350 pC/N was obtained using 50-50% vol. bimodal particle distribution. This piezoelectric coefficient is 40% higher than the monodispersed sample using 100 nm particles with a piezoelectric coefficient of 250 pC/N.     

A new and simple way to prepare monolithic solid oxide fuel cell stack by stereolithography 3D printing technology using 8 mol% yttria stabilized zirconia photocurable slurry

Yttria (8 mol%) stabilized zirconia (8YSZ) photocurable slurry is the basis for stereolithography-based 3D (SLA) printed structured electrolyte support for monolithic solid oxide fuel cell (SOFC) stack. The curing resin with trifunctional trimethylolpropane triacrylate and 1,6-hexanediol diacrylate (TMPTA/HDDA) mass ratio of 1.5:8.5 and 1 wt% of photoinitiator provided excellent curing performance and low viscosity of 2.1 mPa·s. Stable 8YSZ photocurable slurry possessing high solid content of 43 vol% and low viscosity of 3.6 Pa·s at 30 s^?1 shear rate were obtained, without particle sedimentation after 180-day stability test. The activation energy of 8YSZ fabricated by 3D printing method was 0.87 eV, similar to that by dry-pressing method. The 3D printed monolithic 3-tube SOFC stack exhibited a peak power density of 230 mW·cm^?2 at 850 °C. This research proves the great potential of 3D printing technology to prepare monolithic SOFC stack, paving the way to develop SOFCs for practical applications.     

Additive manufacturing of lead-free KNN by binder jetting

Additive manufacturing of lead-free piezoceramics is of great interest, given the large request of application-oriented designs with optimal performances and reduced material consumption. Binder Jetting (BJ) is an additive manufacturing technique potentially suited to the production of ceramic components, however the number of feasibility studies on BJ of piezoceramics is extremely limited and totally lacking in the case of sodium-potassium niobate (KNN). In this work, as-synthesised powders are employed in the BJ 3D printing process. Microstructural properties, such as porosity, grain size distributions, and phase composition are studied by SEM, XRD and MIP (Mercury Intrusion Porosimetry) and compared to die-pressed pellets. Analyses reveal considerable residual porosity (~40%) regardless of the printing parameters, with a weak preferential orientation parallel to the printing plane. The piezoelectric characterization demonstrates an outstanding d₃₃ value of 80–90 pC N^?1. Finally, Figures of Merits for the employment as porous piezoceramics in the direct mode are presented.     

LCD-SLA 3D printing of BaTiO3 piezoelectric ceramics

Stereolithography-based 3D printing is a promising method to produce complex shapes from piezoceramic materials. In this study, LCD-SLA 3D printing was used to create lead-free piezoceramics based on barium titanate (BaTiO₃, BT). Three types of BT powders (micron, submicron and nanoscale) were tested in LCD-SLA 3D printing, and a technique for the preparation of a ceramic slurry suitable for LCD-SLA printing has been developed. Using TGA-DSC analysis, the thermal debinding parameters to obtain crack-free samples were determined, followed by further sintering and the study of the piezoelectric properties (ε_r = 1965, d₃₃ = 200 pC/N, tan = 1,7 %). The results of the study demonstrate high potential for the production of complex piezoceramic elements that can be used in aviation, in particular, aviation radio equipment; in the marine industry for transceiver modules of hydroacoustic antennas; and in the nuclear industry for pressure control sensors in the steam–water path.     

Tape cast porosity-graded piezoelectric ceramics

In this work a functionally graded porous Nb-doped PZT material (PZTN) was produced by tape casting. Each step of the production process (slurry formulation, lamination and thermal treatments) was thoroughly investigated. Tapes with different carbon black (CB) amounts were produced. The conditions necessary to laminate 6 layers of different CB concentration were optimized by tailoring the binder to plasticizer volume ratio of each single green layer. Cracks and delaminations were eliminated by gradually increasing CB content and adjusting the binder burn-out procedure. The optimization process led to a well developed, crack-free porosity-graded multilayer, less than 400 μm thick and with porosity along the thickness ranging from 10 to 30 vol.%. The application of a load during the heating treatments was absolutely required to obtain warpage-free planar multilayer specimens.     

Fabrication and characterization of a tiny PZT thick-film longitudinal-bending coupled drive vibrator using electrohydrodynamic jet printing

This paper describes a fabrication method of a novel type of micro piezoelectric thick-film longitudinal-bending coupled (LBC) vibrator with a length of 8 mm and thickness of 0.34 mm using electrohydrodynamic jet (E-jet) printing. The LBC micro-vibrator was designed, and a frequency sensitivity analysis of the structural parameters was implemented. When the thickness of the lead zirconate titanate (PZT) thick-film element of the vibrator reached 76 μm, tuning of the composite-mode frequency consistency could be achieved. The micro-vibrator was then fabricated by depositing the PZT thick film directly on the surface of the elastic body using E-jet printing, and the film thickness was flexibly adjusted by tuning the number of prints, thus avoiding the problems of insufficient PZT thin film thickness (≤1 μm) and excessive bulk PZT ceramic volume. Micro-morphological observations showed that the printed thick film was dense and smooth, with a thickness of approximately 76 μm. Furthermore, the vibration mode frequency of the vibrator differed from the test resonant frequencies by only 0.92%, and the vibrator could achieve driving motion with a volume and an excitation voltage approximately one-tenth of that of a bulk piezoelectric motor. Moreover, the unit power density was 0.36 W kg⁻¹ V⁻¹, which is 1.6 times higher than that of a large bulk piezoelectric motor, indicating that the vibrator has good potential for small-space and low-voltage applications.     

Flexible piezoelectric energy harvesters with graphene oxide nanosheets and PZT-incorporated P(VDF-TrFE) matrix for mechanical energy harvesting

Flexible piezoelectric energy harvesters (PEHs) have attracted extensive interest because of their ability to transform mechanical energy into electric power. Here, PEHs were fabricated using P(VDF-TrFE)-based piezoelectric composite films containing lead zirconate titanate (PZT) powder and –OH-functionalized graphene (HOG) nanosheets (HOG-P/P). Among all composites, a high open-circuit voltage (Voc) of approximately 50 V_p-p and a maximum power density of 1.4 μW/cm² were obtained from a HOG-P/P PEH with 0.10 wt% HOG nanosheets and 15 wt% PZT under bending–releasing mode. Moreover, the PEH exhibited a stable voltage output after 3000 bending–releasing cycles. In addition, the PEH harvested mechanical energy from human body movements and generated an output voltage and current of 60 V and 8 μA during the finger bending–releasing process, lighting up 30 commercial white LEDs. The enhanced piezoelectric performance can be attributed to the introduction of HOG nanosheets and PZT powder. This work provides an effective strategy for improving the output performance of P(VDF-TrFE)-based PEHs.     

Investigation on properties of Al2O3–SiO2 complex shaped refractory fabricated by layered extrusion forming

As one of the 3D printing methods, layered extrusion forming (LEF) has distinct advantages to form complex configuration ceramics directly. The feasibility of using LEF to make refractory products with complex shapes was explored by this work, using water-based Al₂O₃–SiO₂ ceramic slurry and specially equipped device. By measuring rheological parameters, the effects of binder addition, dispersant addition and volume proportion of the solid portion composed of α-Al₂O₃ ultrafine powder (92 wt%) and silica fume (8 wt%) on rheological behavior of the slurry were investigated. The green body specimens prepared by the LEF were fired at 1400°C–1600 °C for 3h. The influence of firing temperature on phase composition, microstructure, sintering degree and comprehensive properties of the specimens was investigated. At 2.5 wt% addition of aluminum dihydrogen phosphate as binder, 0.2 wt% addition of sodium hexametaphosphate as dispersant and with solid portion between 56 vol% and 58 vol%, required pseudoplastic behavior of the slurry can be achieved, suitable for the LEF. With the increase of heating temperature, mullitization by the reaction between the α-Al₂O₃ ultrafine powder and silica fume becomes stronger and sintering gets enhanced, leading to improved comprehensive properties of the specimens. Fired at 1600 °C, properties in terms of bulk density 3.03g/cm³, cold compressive strength 190.5 MPa and refractoriness under load 1598 °C are achieved. Crucible slag test shows a good resistance to the glass melt corrosion. Good feasibility of fabricating some complex shaped refractory products by LEF as a novel forming approach has been confirmed by the present work.     

Additive manufacturing of B4C-W-based composites via fused deposition molding using highly filled granular feedstocks

The miniaturization and mobility of nuclear reactors have become an important trend in the development of nuclear energy. In order to simplify the design of shielding materials with improved complexity and reduced weight, 3D B₄C-W-based composites were fabricated via fused deposition molding using highly-filled granular feedstocks containing 62 vol% B₄C-W powders (boron carbide accounted for 30 wt% and tungsten for 70 wt%) and 38 vol% polymer binders (60 wt% Carnauba wax, 22 wt% polypropylene, 13 wt% polystyrene, and 5 wt% stearic acid). The rheological properties and microstructure of the feedstock and extruded filaments were clarified. Roles of the printing parameters including extrusion temperature, platform temperature and deposited-layer height in the morphology of 3D composites were investigated in detail. Extruded filaments with good shape retention, dense fracture surface, and uniformly dispersed B₄C-W grains were achieved, benefiting from the smooth printing and shear thinning behaviour of the feedstock. Defects including warping, small pores or stacking pores could be formed under improper printing parameters, owing to the poor bonding strength between deposited layers induced by thermal internal stress or decomposition of wax. 3D composites with large size of 130 × 130 × 5 mm were fabricated, which showed a satisfactory compressive strength of 34.8 MPa. This work showed a facile route to fabricate 3D radiation shielding materials based on highly-filled polymers.     

EVA-PVA binder system for polymer derived mullite made by material extrusion based additive manufacturing

Low processing temperature of preceramic polymers (PCPs) makes them attractive for material extrusion based additive manufacturing (MEX-AM), earlier called fused deposition modeling (FDM). Fabrication of bulk polymer derived ceramics is challenging due to gas evolution during crosslinking leading to pores and cracks in final product. Mixture of ethylene vinyl acetate (EVA) and polyvinyl alcohol (PVA) was successfully used to generate open porosity before crosslinking step. For 3D printing, a pellet extruder was used and a PVA binder content of 50 vol% was essential for succcessful solvent debinding process in water. The effect of PVA content and different EVA grades on printability and debinding behavior was studied. EVA with a lower melt flow index (MFI) showed better compatibility with PVA additive in terms of mixing and printing. EVA with higher vinyl acetate content seems to be more favorable for later thermal debinding processes because of its higher gas permeability.     

Effects of slurry composition on the properties of 3-1 type porous PZT ceramics prepared by ionotropic gelation

In this study, 3-1 type lead zirconate titanate (PZT) ceramics with one-dimensional pore channels were produced by ionotropic gelation process of alginate/PZT suspensions. By increasing the sodium alginate concentration from 1.0wt% to 3.0 wt%, the alginate/PZT suspensions turned from Newtonian to non-Newtonian behavior with substantial increase in apparent viscosity. Accordingly, 3-1 type PZT ceramics with porosity decreasing from 56.78% to 41.44% were obtained, while the pore size distribution became non-uniform gradually. Based on the structural features, the 3-1 type PZT ceramics possessed much higher relative permittivity (ε_r) than that of 3-0 or 3-3 type PZT ceramics with similar porosities. Increase in the porosity led to a moderate decline in the longitudinal piezoelectric strain coefficient (d₃₃), a reduction in the dielectric loss factor (tan δ), and a high value of hydrostatic strain coefficient (d_h). As a result, the 3-1 type PZT ceramics possessed a maximal hydrostatic figure of merit (HFOM) value of 5597×10^–15 Pa⁻¹ when the porosity was 56.78%, which may be of help for low frequency hydrophones applications.     

3D printing of high solid loading zirconia feedstock via screw-based material extrusion

Zirconia ceramic (3Y-TZP) feedstocks with solid loadings from 50 vol% to 68 vol%, in a 60:40 paraffin wax to LDPE ratio binder system, were prepared and printed using a screw-based material extrusion printer. A two-step debinding process involving solvent debinding (cyclohexane + ethanol) and thermal debinding (140 °C–600 °C at 0.2 °C/min) followed by sintering at 1500 °C for 2 h was employed. Tests performed include TGA, density test, Vickers hardness and fracture toughness, XRD, and SEM. The TGA result showed two significant drops in weight starting at 180 °C and 380 °C, which corresponds to the decomposition of paraffin wax and LDPE, respectively. A minimum of 40 wt% of soluble binder was removed from the green sample after solvent immersion for 3 h at 40 °C for solid loadings ≥55 vol%. High solid loading feedstocks produced samples with comparable density, Vickers hardness and fracture toughness, which are 97.5%, ∼12.3 GPa, and ∼5.5 MPa m^1/2, respectively; while XRD and SEM shows no adverse tetragonal to monoclinic phase transformation and grain growth, respectively. This study demonstrates that 3D printing of granular 3Y-TZP ceramic feedstock via screw-based material extrusion technique is feasible even with high solid loadings, which is usually difficult to fabricate into flexible filaments and print due to high viscosity.     

Effect of particle grading on the properties of photosensitive slurry and BaTiO3 piezoelectric ceramic via digital light processing 3D printing

To improve the density and piezoelectric constant of BaTiO₃ ceramics prepared by Digital Light Processing 3D printing, the properties of photosensitive slurry were investigated from the perspective of particle grading, and the nitrogen-air two-step debinding and sintering process on the relative density and electrical properties were explored. It was found that as the mass ratio of coarse particles increased, the viscosity, shear stress and cure depth of the slurry decreased. When the mass ratio of fine and coarse particles was 2:8 and sintered at 1350 °C, the ceramic had better performance, with relative density reaching 95.39 ± 0.63 %. The piezoelectric constant d₃₃ was 215 ± 13 pC/N, 29.52 % higher than the single-peak powder. The relative permittivity (ε_r) and polarization (P_r) were 978 and 16.656 μC/cm². Finally, BaTiO₃ ceramics with Triply Periodic Minimal Surface structures were prepared as piezoelectric sensors, which had the highest output voltage at the same displacement when the mass ratio was 2:8.     

Sintering Behavior,Properties, and Applications of Co‐Fired Piezoelectric/Low Temperature Co‐Fired Ceramic (PZT‐SKN/LTCC) Multilayer Ceramics

Materials and processing conditions have been developed allowing co‐firing of fluxed PZT‐SKN materials with commercial low temperature co‐fired ceramic (LTCC) tapes. Previously, Pb(Zr_0.53, Ti_0.47)O₃–Sr(K_0.25, Nb_0.75)O₃ (PZT‐SKN) ceramics fluxed with 1 wt% LiBiO₂ and 1 wt% CuO addition were shown to sinter to high density at 900°C for 1 h, with a large d₃₃ piezoelectric coefficient of ~415 pm/V. Currently, the master sintering curve (MSC) approach has been used to study the densification behaviors of fluxed PZT‐SKN and LTCC tapes. Different sintering mechanisms for fluxed PZT‐SKN ceramics and LTCC materials are confirmed by analyzing the apparent activation energy (Q_a). Using knowledge gained from MSC results, an optimized sintering profile was developed. Multilayer PZT‐SKN/HL2000 (HeraLock^? Tape, Heraeus) stacks co‐fired at 900°C for 0.5 h maintain large piezoelectric coefficient (high field d₃₃ > 340 pm/V). EDS analysis reveal limited interdiffusion of Pb from PZT‐SKN layers in LTCC and the appearance of Al, Ca, and Si in the PZT‐SKN near the PZT‐SKN/LTCC interface. Further, elemental interdiffusion was not detected at the center of piezoelectric layer in PZT‐SKN/LTCC multilayer ceramics and no subsequent reduction in piezoelectric coefficient d₃₃ was observed. Finally, a piezoelectric microbalance with mass sensitivity of 150 kHz/mg was fabricated using the materials and methods developed.