Mechanical and dielectric properties of Si3N4-SiO2 ceramics prepared by digital light processing based 3D printing and oxidation sintering

Si₃N₄-SiO₂ ceramics are considered as the preferred high-performance wave-transmitting material in the aerospace field. However, traditional fabrication methods for Si₃N₄-SiO₂ ceramics have the disadvantages of high cost and complicated fabrication process. In this paper, Si₃N₄-SiO₂ ceramics with excellent mechanical and dielectric properties were fabricated by digital light processing-based 3D printing combined with oxidation sintering. Firstly, the curing thickness and viscosity of slurries with different solid loadings for vat photopolymerization-based 3D printing were studied. Then, the effects of the sintering temperature on the linear shrinkage, phase composition, microstructure, flexural strength, and dielectric properties of Si₃N₄-SiO₂ ceramics, and the influences of solid loading on them were explored. The curing thickness and viscosity of the slurry with a solid loading of 55 vol% were 30 μm and ∼1.5 Pa‧s, respectively. The open porosity and the flexural strength of Si₃N₄-SiO₂ ceramic with a solid loading of 55 vol% were 4.3 ± 0.61% and 76 ± 5.6 MPa, respectively. In the electromagnetic wave band of 8–18 GHz, the dielectric constant of Si₃N₄-SiO₂ ceramics was within the range of less than 4, and the dielectric loss remained below 0.09. The method of digital light processing-based 3D printing combined with oxidation sintering can be further extended in the preparation of Si₃N₄-based structure-function integrated ceramics.     

Y2O3 doped Ba0.9Ca0.1Ti0.9Sn0.1O3 ceramics with improved piezoelectric properties

Lead-free Ba_0.90Ca_0.10Ti_0.90Sn_0.10O₃-xY₂O₃ (BCTSY, x = 0–0.09) ceramics were prepared by traditional solid-state sintering method. All the BCTSY samples showed pure perovskite structures without detectable impurity. Orthorhombic/tetragonal phase coexisted in the sample of x = 0.03 to 0.07. Remarkable enhancement of the electric properties were achieved at x = 0.03 with d₃₃ of 650 pC/N, K_p of 59.6%, and the remnant polarization P_r of 10.2 μC/cm². The strengthened temperature stability of piezoelectricity is beneficial to the application of the piezoceramics.     

Effects of ZrSiO4 content on properties of SiO2-based ceramics prepared by digital light processing

SiO₂-based ceramic cores are widely used in the preparation of gas turbine engine hollow blades due to their excellent chemical stability and easy removal after casting. In this paper, ZrSiO₄ reinforced SiO₂-based ceramics were fabricated using digital light processing (DLP) technology. The results showed that the addition of ZrSiO₄ reduced the cure depth due to its high UV light absorptivity and refractive index. When the content of ZrSiO₄ increased to 15 wt%, the cristobalite content reached the maximum, and radial shrinkage reached the minimum of 1.4%. ZrSiO₄ grains could hinder the propagation of cracks, enhancing the room-temperature flexural strength. At 1550 °C, fracturing across SiO₂ grains in SiO₂-based ceramics led to the great improvement of high-temperature flexural strength. When the content of ZrSiO₄ reached 15 wt%, the flexural strength at room temperature and high temperature was 11.5 MPa and 36.7 MPa, respectively. Therefore, the SiO₂-based ceramics prepared using DLP technology have good room temperature and high temperature properties, and are expected to be used for hollow blade casting.     

Effect of organic fibers addition on digital light processing for Al2O3 porous ceramics

Porous ceramics based on additive manufacturing have great application potential in many industries, including filtration, catalysis, and heat insulation. In this research, we propose a method for manufacturing porous ceramics with connected channels structure through ceramic digital light processing (DLP) and organic fiber decomposition. The crossed fibers in the green body, working as a pore-forming agent, were decomposed and removed to form connected channels in ceramic. It was confirmed that ball milling changed the fiber morphology during slurry preparation, which was beneficial to promote fibers crossing. Besides, we focused on the influence of the “Sponge Compression effect” during the DLP process, which affected the fibers distribution. The existence of fibers in the green body resulted in uneven pressure distribution during the debinding process, providing a potential source of cracks. Results show that this method can produce channels with a diameter of 100 μm and high connectivity, providing great potential in fabricating high connectivity porous ceramics with complex shapes and structures.     

High-performance Al2O3 ceramics prepared by DCC-HVCI via microwave heating

A novel and rapid fabrication method for Al₂O₃ ceramics by the DCC-HVCI method via microwave heating was proposed. Effects of microwave heating temperature on coagulation time, micromorphology, as well as performance of the green body and ceramic sample were studied. As the microwave heating temperature rises, the coagulation time gradually reduced and compressive strength of green sample decreased while relative density and flexural strength of ceramics rose at the beginning and then dropped. The 50 vol.% Al₂O₃ suspension was coagulated and demolded after treating at 60°C for 800 s by microwave heating. The compressive strength of green samples reached 1.12 ± 0.13 MPa. The relative density of Al₂O₃ ceramic samples reached 99.39%. And the flexural strength of Al₂O₃ ceramics reached 334.55 ± 26.41 MPa. The Weibull modulus of Al₂O₃ ceramics reached 19. In contrast with the ceramic samples heated through water bath, the ceramic samples treated through microwave possessed uniform microstructures. Microwave heating could reduce the coagulation time by 77%. Meanwhile, it could significantly raise the compressive strength of green bodies by 65%. Additionally, it could increase the flexural strength of ceramics by 30%.     

Effect of polystyrene addition on properties of porous Si3N4 ceramics fabricated by digital light processing

Porous Si₃N₄ ceramics with high strength and high transmittance have been widely used in the field of defense and military. Additive manufacturing (AM) technology is one of the effective means to fabricate porous Si₃N₄ ceramics. Nevertheless, it is difficult to prepare porous Si₃N₄ ceramics by using digital light processing (DLP) because of the large refractive index difference between Si₃N₄ powders and photosensitive resin. In this study, the effects of the amount of polystyrene (PS) powders on the properties of Si₃N₄ ceramic slurries and sintered ceramics were systematically discussed. The addition of PS reduced the overall refractive index of powders and increased the average particle size of powders, thus improving the cure depth of Si₃N₄ ceramic slurries from 11.0 ± 2.0 μm to 55.7 ± 1.8 μm. With the increase of PS content, the shrinkage and porosity of Si₃N₄ ceramics gradually increased, and the bulk density and flexural strength showed the opposite trend. The slurry with low viscosity (2.38 Pa٠s at a shear rate of 30 s⁻¹) and high cure depth (51.2 ± 4.6 μm) was obtained when the content of PS was 15 wt%, which met the thickness requirements for printing. The total porosity of Si₃N₄ ceramics reached the maximum values at 28.21 ± 2.58%. The addition of PS solved the problem of low cure depth of slurries, and PS as a pore-forming agent could help Si₃N₄ ceramics form porous structure. This research provides valuable insights into the fabrication of non-oxide ceramics with high refractive index using DLP technology.     

Influence of TiO2 additive on sintering temperature and microwave dielectric properties of Mg0.90Ni0.1SiO3 ceramics

(1-x)Mg_0.90Ni_0.1SiO₃-xTiO₂ (x = 0, 0.01, 0.03, 0.05) ceramics were successfully formed by the conventional solid-state methods and characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS), and their microstructure and microwave dielectric properties systematically investigated. It was observed that when TiO₂ content increased from 0 to 5 wt%, the Q_uf_o of the sample decreased from 118,702 GHz to 101,307 GHz and increases the τ_f value from −10 ppm/°C to +3.14 ppm/°C accompanied by a notable lowering in the sintering temperature (125 °C). A good combination of microwave dielectric properties (ε_r ∼ 8.29, Q_uf_o ∼ 101,307 GHz and τ_f ∼ −2.98 ppm/°C) were achieved for Mg_0.90Ni_0.1SiO₃ containing 3 wt% of TiO₂ sintered at 1300 °C for 9 h which make this material of possible interest for millimeter wave applications.     

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.     

Synthesis and microwave dielectric properties of BaO-Sm2O3-5TiO2 ceramics with NdAlO3 additions

BaO-Sm₂O₃-5TiO₂ (BST5) ceramics with NdAlO₃ additions of up to 15 wt% were produced with a solid state reaction method, and their structural and microwave dielectric properties were determined. Experimental results showed that NdAlO₃ neither merged nor altered the orthorhombic tungsten bronze structure of the main phase of the produced ceramics (except for a shrinkage in the crystal lattice), but it was segregated in distinct grains in the microstructure of the produced ceramics. However, the amount of NdAlO₃ strongly influenced the densification and the microstructure (i.e. grain shape and size) of the produced ceramics. Analysis of the experimental results suggests that the microstructural features can be correlated to the dielectric properties of these ceramics. Accordingly, the dielectric constant (ε_r) and the temperature coefficient of resonant frequency (τ_f) of the produced BLT5 ceramics can be tuned with the amount of NdAlO₃ additions and the sintering process parameters. The best dielectric properties were achieved for BaO-Sm₂O₃-5TiO₂ ceramics with 7.5% NdAlO₃ (ε_r=73.22, Q×f =10,300 GHz, and τ_f=−1.05 ppm/°C).     

Significantly enhanced breakdown field with high grain boundary resistance and dielectric response in 0.1Na0.5Bi0.5TiO3-0.9BaTiO3 doped CaCu3Ti4O12 ceramics

Electrical performances are strongly associated with the electrical heterogeneity of grains and grain boundaries for CaCu₃Ti₄O₁₂ (CCTO) ceramics. In this work, the dielectric ceramics of 0.1Na_0.5Bi_0.5TiO₃-0.9BaTiO₃ (NBT-BT) doped CCTO were fabricated by a conventional solid-state reaction method, and the ceramics were sintered at 1100 °C for 6 h. Relatively homogeneous microstructures are obtained, and the average grain sizes are characterized about 0.9∼1.5 μm. Impressively, a significantly enhanced breakdown field of 13.7 kV/cm and a noteworthy nonlinear coefficient of 19.4 as well as a lower dielectric loss of 0.04 at 1 kHz are achieved in the 0.94CCTO-0.06(NBT-BT) ceramics. It is found that the improved electrical properties are attributed to the increased grain boundary resistance of 3.7 × 10⁹ Ω and the Schottky barrier height of 0.7 eV. This is originated from the NBT-BT compound doping effect. This work demonstrates an effective approach to improve electrical properties of CCTO ceramics by NBT-BT doping.     

High permittivity and low dielectric loss of the (Ca0.9Sr0.1)1-xLa2x/3Cu3Ti4O12 ceramics

In this work, we have systematically studied the effects of La³⁺/Sr²⁺ dopants on the crystal structure, microstructure, dielectric response and electrical properties of (Ca_0.9Sr_0.1)_1-xLa_2x/3Cu₃Ti₄O₁₂ (x = 0, 0.025, 0.05 and 0.075) ceramics. XRD results show that the lattice parameter increases with the increase in the La³⁺ content. SEM micrographs illustrate that a small amount added of La³⁺ can reduce the grain size of CCTO during sintering. With increasing La³⁺ content, the grains grow larger. Dielectric measurements indicated that all doped samples synthesized by the solid-state reaction exhibit giant dielectric constants ε'>10⁴ over a large frequency range (10 Hz to 1 MHz) and at any temperature below 600 K. In particular, the ceramic with x = 0.05 exhibits a colossal dielectric permittivity ～5.49 × 10⁴; which increases by about 50% compared to that of the undoped ceramic. In addition, the doped ceramic also presents a low dielectric loss ～ 0.08 at 20 °C and 0.6 kHz. The giant dielectric properties of these samples can be explained by the (IBLC) model.     

Compressive strength and impact resistance of Al2O3/Al composite structures fabricated by digital light processing

The combination of multi-materials is an alternative way to meet the diverse requirements for various applications. However, the processing difficulty especially in ceramic forming limited the structural innovations. In this paper, a combined methods of ceramic digital light processing (DLP) and metal infiltration is proposed to fabricate the Al₂O₃/Al composite structures with controllable ceramic skeletons. The interfacial, compressive and impact resistance properties were studied. The results showed that the Al₂O₃ grains were closely bonded, and no destructive defects occurred at the interface between Al₂O₃ and Al. The energy dispersive spectrometer (EDS) and X-ray diffraction (XRD) results showed that the formation of protective phases contributed to the improvement of bonding strength. The compressive tests showed that the composite structures had better capabilities to absorb and resist the applied loads compared with Al structures. Finally, the impact resistance of the structures was discussed, the finite element analysis and the experimental results showed that the composite structures had advantages in dissipating the energy of incident objects and reducing the penetration depth. Based on these results, the damage model of Al₂O₃/Al structures was established, and the roles of different materials were revealed.     

Effect of ZnO doping on structural,dielectric, ferroelectric and piezoelectric properties of BaZr0.1Ti0.9O3 ceramics

This study provides a fundamental understanding of structural, dielectric, ferroelectric and piezoelectric properties of bare and ZnO-doped BaZr_0.1Ti_0.9O₃ (BZT) solid solutions synthesized using mechanochemical activation technique. Structural investigation has been carried out using XRD patterns of the synthesized compositions by Rietveld refinement method. It confirms the formation of tetragonal phase with P4mm space group for ZnO doping up to 2.5 wt%, while in 5.0 wt% ZnO-doped sample, BZT and ZnO are distributed as individual phases of tetragonal (P4mm) and hexagonal (P6₃mc), respectively. Microstructural analysis shows that average grain size increases quite rapidly with the increase of ZnO content. Detailed analysis of dielectric constant as a function of temperature for the prepared samples shows that the frequency independent dielectric constant maximum shifts to lower temperature with increase in ZnO doping. The ferroelectric ordering is confirmed using P-E loop measurements. The piezoelectric constant of the synthesized specimens were found to decrease with increasing ZnO content.     

Improvement in dielectric,ferroelectric and ferromagnetic characteristics of Ba0.9Sr0.1Zr0.1Ti0.9O3-NiFe2O4 composites

Multiferroic composites with the composition (1-x)Ba_0.9Sr_0.1Zr_0.1Ti_0.9O₃-(x)NiFe₂O₄ (x=0.0, 0.05, 0.10, 0.20 and 0.30) were prepared by mechano-chemical activation technique. Rietveld refined X-ray diffraction pattern shows the formation of individual perovskite (BSZT) and inverse spinel (NFO) phases for all the four composite samples. Microstructural investigation reveals that the inhomogeneity in grains increases with addition of NFO as compared to bare BSZT sample. Dielectric studies show that all the samples exhibit a well-defined ferroelectric to paraelectric transition peak. Further, the diffuseness of the samples increases as NFO content increases. Ferroelectric properties were found to be superior for (1-x)BSZT-(x)NFO sample with x=0.05 and decreases on further increase in NFO content. Dielectric breakdown strength also shows the similar trend as ferroelectricity and shows a maximum for x=0.05 ferrite (NFO) fraction. Magnetic measurement of BSZT-NFO composite samples shows a gradual increase in saturation magnetization and coercive field (H_c) with increasing NFO content.     

Influence of the processing way for La3+-doping on crystal structure,microstructure, and microwave dielectric properties of Ca0.7Ti0.7La0.3Al0.3O3 ceramics

Perovskite ceramics with a formula of Ca_0.7Ti_0.7La_0.3Al_0.3O₃ (CTLA) were produced through a conventional solid-state reaction procedure following three different La³⁺-doping methods using powders of La₂O₃, or La₂O₃/Al₂O₃ powder mixture, or LaAlO₃. La³⁺ doping favored grain growth and densification, affected the grain size distribution, and improved the dielectric properties of the produced sintered CTLA ceramics. The doping methods had a strong influence on these properties. More specifically, doping with La₂O₃ and La₂O₃/Al₂O₃ resulted in formation of solid solution, while a secondary phase formed in the CTLA ceramics doped with LaAlO₃, which caused a coarsening of the microstructure and lowered the La³⁺ doping effects on the dielectric properties. The experimental results suggest that La³⁺ doping improves the dielectric properties of the sintered CTLA perovskite ceramics, which are further enhanced by doping with Al³⁺ ions in small amounts. However, further increase of Al³⁺ ions content jeopardizes them.     

Y-TZP,Ce-TZP and as-synthesized Ce-TZP/Al2O3 materials in the development of high loading rate digital light processing formulations

Y-TZP, Ce-TZP and Ce-TZP/Al₂O₃ materials are widely investigated in dentistry. Digital Light Processing (DLP) is considered as a breakthrough technology for the dental field to fine print Y-TZP green parts. High loading photocurable formulations (>45 vol%) with Y-TZP, Ce-TZP commercial powders and Ce-TZP/30 vol% Al₂O₃ as-synthesized powder suitable to DLP printing were achieved in this study. A low specific surface area (5–13 m²/g) of particles without any pores and 1 wt% to 2 wt% of steric dispersant are required to obtain high loading formulations. The as-synthesized composites provide these properties by increasing the calcination temperature from 800 °C to 1200 °C. The as-prepared ceramic formulations based on the same photocurable resin exhibit a curing behavior suitable to DLP process for Y-TZP formulations (thickness > 50 μm in few seconds with a high conversion rate) in comparison with ceria ceramic. The ceria is a strongly UV absorbing material and a specific formulation is developed to obtain 80% of conversion and a cured thickness of 75 μm in 0.5 s.     

Enhanced dielectric breakdown strength in Ni2O3 modified Al2O3-SiO2-TiO2 based dielectric ceramics

Dielectric ceramics have raised particular interest since they enable pulsed-power systems to achieve high voltage gradient and compact miniaturization. In this work, x wt%Ni₂O₃ doped Al₂O₃-SiO₂-TiO₂ based dielectric ceramics were prepared using conventional solid-state reaction and the effects of Ni₂O₃ on the crystal structure, dielectric properties and dielectric breakdown strength were investigated. It was found that with the doping of Ni₂O₃, the Al₂O₃-SiO₂-TiO₂ based dielectric ceramics became denser and the distribution of each phase was more uniform. For the composition of x?=?2.0, the dielectric breakdown strength was increased into 82.1?kV/mm, more than twice compared with that of the undoped one. In addition, the relationship between the dielectric breakdown strength and the resistance of Al₂O₃-SiO₂-TiO₂ based dielectric ceramics was discussed. The results show that the doping of Ni₂O₃ is a very feasible way to improve the dielectric breakdown strength and optimize the dielectric properties for the Al₂O₃-SiO₂-TiO₂ based dielectric ceramics.     

Thermal conductivities and mechanical properties of AlN ceramics fabricated by three dimensional printing

AlN ceramics were successfully fabricated through a joint process of digital light processing (DLP) 3D printing technology and heat treatment at 1780 °C∼1845 °C. DLP is an addictive manufacturing process, enabling the near net shape fabrication. The AlN grains in this work developed well and there were small amounts of grain-boundary phases at the three-grain junctions. The particle size of AlN became larger and the densification increased with increasing sintering temperature. The pores of AlN ceramics also decreased, which led to the increase of thermal conductivity and flexural strength. The optimal thermal conductivity and flexural strength of AlN ceramic reached 155 W/(m·K) and 265 ± 20 MPa when sintered at 1845 °C.     

Structural ordering and dielectric properties of Ba3CaNb2O9-based microwave ceramics

Ba₃CaNb₂O₉ is a 1:2 ordered perovskite which presents a trigonal cell within the D_3d³ space group. Dense ceramics of Ba₃CaNb₂O₉ were prepared by the solid-state reaction route, and their microwave dielectric features were evaluated as a function of the sintering time. From Raman spectroscopy, by using group-theory calculations, we were able to recognize the coexistence of the 1:1 and 1:2 ordering types in all samples, in which increasing the sintering time tends to reduce the 1:1 domain, leading to an enhancement of the unloaded quality factor. We concluded that this domain acts as a lattice vibration damping, consequently raising the dielectric loss at microwave frequencies. The best microwave dielectric parameters were determined in ceramics sintered at 1500 °C for 32h: ε′ ~ 43; Q_u×f_r = 15,752 GHz; τ_f ~ 278 ppm °C⁻¹.