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.     

High-performance 0.9Al2O3-0.1TiO2 microwave dielectric ceramics prepared by digital light processing

In this work, the 0.9Al₂O₃-0.1TiO₂ ceramic sample with good microwave dielectric properties and complex structures can be well fabricated by digital light processing (DLP). A relationship between dispersant content and rheological behavior of 0.9Al₂O₃-0.1TiO₂ slurry was explored. When dispersant content was 3.0 wt%, 0.9Al₂O₃-0.1TiO₂ slurry with high solid loading (50 vol%) and low viscosity (2.9 Pa s) could be obtained. 0.9Al₂O₃-0.1TiO₂ ceramic parts with high accuracy were fabricated successfully by adding 3.0 wt% photoinitiator under 600 mJ/cm² exposure energy. With the increase of sintering temperature from 1400 °C to 1600 °C, relative density, dielectric constant (ε_r), and quality factor (Q × f) of 0.9Al₂O₃-0.1TiO₂ ceramic sample increased first and then decreased, and all reached the maximum value at 1550 °C due to the uniformity and densification of microstructures. The temperature coefficient of resonant frequency (τ_f) value showed an almost monotonous increase, changing from negative to positive, and near-zero τ_f value at 1550 °C. In addition, 0.9Al₂O₃-0.1TiO₂ ceramic samples sintered at 1550 °C fabricated by DLP method presented much better microwave dielectric properties: ε_r = 11.30 ± 0.02, Q × f = 35,345 ± 143 GHz (@～12 GHz), τ_f = 2.16 ± 0.21 ppm/°C than that of by dry pressing method: ε_r = 11.16 ± 0.11, Q × f = 30,195 ± 257 GHz (@～12 GHz), τ_f = 4.45 ± 0.13 ppm/°C, especially the Q × f value achieved a 17% increase. Accordingly, DLP technique, which has advantages of producing relatively high properties and complex geometry of microwave dielectric ceramics as well as without extra high-cost mold, greatly satisfies application requirements.     

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.     

Composite bioceramic scaffolds with controllable photothermal performance fabricated by digital light processing and vacuum infiltration

Photothermal scaffolds can help clear bone tumor cells after resection. In this work, hydroxyapatite-akermanite-Fe₃O₄ (HA-AK-FE) bioceramic scaffolds were fabricated by infiltrating digital light processing (DLP)-printed HA-AK scaffolds in nano-Fe₃O₄ solution. The prepared HA-AK-FE samples exhibited excellent and controllable photothermal performance under the irradiation of 808 nm near-infrared light. By controlling nano-Fe₃O₄ concentration, irradiation power and infiltration time, temperature of HA-AK-FE samples could be regulated in a wide range from room temperature to 150 °C within 15 s. Photothermal temperature remained stable after 4 times repeated irradiations. In SBF solution and under subcutaneous tissue, the heating rate and photothermal temperature decreased obviously compared with in air, but they could still meet the needs of killing tumors (41–48 °C). The Fe release concentration of wafers after immersing in SBF for 1 day was 0.037 mg/L and non-venomous. These results confirm the feasibility and controllability of fabricating photothermal scaffolds by coating nano-Fe₃O₄ with vacuum infiltration, and the prepared HA-AK-FE scaffolds are hopeful to be used in photothermal therapy of bone tumors.     

Densification and grain growth of nanocrystalline 3Y-TZP during two-step sintering

Two-step sintering (TSS) was applied on nanocrystalline yttria tetragonal stabilized zirconia (3Y-TZP) to control the grain growth during the final stage of sintering. The process involves firing at a high temperature (T1) followed by rapid cooling to a lower temperature (T2) and soaking for a prolonged time (t). It is shown that for nanocrystalline 3Y-TZP (27 nm) the optimum processing condition is T1 = 1300 °C, T2 = 1150 °C and t = 30 h. Firing at T1 for 1 min yields 0.83 fractional density and renders pores unstable, leading to further densification at the lower temperature (T2) without remarkable grain growth. Consequently, full density zirconia ceramic with an average grain size of 110 nm is obtained. XRD analysis indicated that the ceramic is fully stabilized. Single-step sintering of the ceramic compact yields grain size of 275 nm with approximately 3 wt.% monoclinic phase. This observation indicates that at a critical grain size lower than 275 nm, phase stabilization is induced by the ultrafine grain structure.     

Optimisation of two-step sintering parameters to produce bioactive and dense zirconia-hydroxyapatite composite ceramics

3Y-TZP along with Hydroxyapatite (HAp) are common bioceramics used in bone tissue engineering scaffolds to yield better patient outcomes and faster healing times. However, significant differences in thermal expansion of these ceramics result in challenges to co-sintering these materials without losing functionality and strength. In this work, a two-step sintering (TSS) process was utilised to co-sinter composites using (1-x)3Y-TZP and xHAp where x varies between 20 and 80 wt%. A peak temperature of 1300 °C, a plateau temperature of 1175 °C, a holding time of 600 min and heating rate of 10 °C/min were the optimum TSS conditions for all compositions. The TSS process produced specimens with grain sizes between 0.5 and 1.2 µm and a compression strength between 88 and 176 MPa. The similarity in compression strength of these zirconia-hydroxyapatite composites with natural bone and the retention of HAp make them suitable for bone tissue engineering applications in load bearing areas.     

Microwave dielectric properties of YAG ceramics prepared by sintering pyrolysised nano-sized powders

YAG ceramics with good dielectric properties were prepared via a modified pyrolysis method, with yttrium nitrate as the yttrium source and combined aluminium sulphate and aluminium nitrate as aluminium sources, and subsequent sintering in a muffle furnace. The effects of the different aluminium sources on the powder characteristic and the impact of sintering temperature, sintering aids (TEOS) and additive (TiO₂) on the dielectric properties of the ceramics were studied. The results show that well-dispersed pure YAG nano-powders can be obtained after calcination at 1000 °C with an aluminium sulphate and aluminium nitrate molar ratio of 1.5:2. The relative density, permittivity (ε_r) and quality factor (Q×f) of the YAG ceramics increase with sintering temperature and TEOS addition. TiO₂ can greatly promote τ_f to near-zero but decreases Q×f. The relative density, ε_r, Q×f and τ_f of the YAG–1 wt% TEOS–1 wt% TiO₂ ceramic obtained at 1520 °C are 97.6%, 9.9, 71, 738 GHz and −30 ppm/°C, respectively.     

Digital light processing of ceramic components from polysiloxanes

Additive manufacturing using photocurable polymers is one method to answer the increased demand of ceramic structures with complicated morphology by fabricating ceramic parts with high resolution and good surface quality. We introduce here a new method to fabricate SiOC ceramic structures by utilizing a simple physical blend between two different preceramic polysiloxanes, one providing photosensitive acrylate groups while the other one a high ceramic yield. Different blend ratios have been realized and respectively optimized concerning the printing additives and setting times to fabricate exact replications of highly complex polysiloxane structures by Digital Light Processing. After pyrolysis, a uniform, homogenous shrinkage was observed yielding dense, pore- as well as crack-free SiOC ceramics. By adjusting the ratio between the different polysiloxanes, parameters such as the ceramic yield, shrinkage, chemical composition and resolution after pyrolysis could be tailored in a wide range of values.     

Digital light processing of wollastonite-diopside glass-ceramic complex structures

In this work, the possibility of shaping a glass-filled photosensitive polymer resin with Digital Light Processing (DLP) into a complex 3D structure and transforming it subsequently into a bioactive glass-ceramic scaffold was investigated. The influence of the printing conditions and the heat-treatment was studied using a 41?vol% glass-filled acrylated polymer resin. Scaffolds with designed architecture were turned into a wollastonite-diopside glass-ceramic at 1100?°C. They completely maintained their shape, exhibited no viscous flow and showed a homogenous linear shrinkage of around 25%. At 83?vol% porosity structures with Kelvin cell design exhibited a compressive strength exceeding 3?MPa, demonstrating that the material is suitable for the fabrication of bioceramic scaffolds for bone tissue engineering applications.     

Silicon carbide whiskers reinforced SiOC ceramics through digital light processing 3D printing technology

Polymer derived silicon oxycarbide ceramic materials and silicon carbide whiskers reinforced ceramic composite are prepared through digital light processing (DLP) 3D printing technology in the present work. A new type of UV-curable preceramic polymer is firstly synthesized and then two types of photopolymer resins with and without SiC whiskers as reinforcement are prepared. Due to the high curing rate and good fluidity of the resins, they are applied in DLP 3D printing and various 3D objects with complicated structures and high printing resolution have been printed. The derived ceramic materials show amorphous microstructure and there is no apparent porosity and cracking throughout the whole sample surface of the ceramic materials and the SiC whiskers are uniformly embedded in the ceramic matrix and remain intact and unaffected during the pyrolysis process. The SiC whiskers reduced the shrinkage and mass loss. More importantly, it significantly improves the mechanical performance of the derived ceramic materials in which the compressive strength increases from 77.5 ± 10.2 MPa to 98.4 ± 12.3 MPa. Benefiting from the easiness of the fabrication, high printing resolution and excellent mechanical performance, the derived ceramic materials have great potential applications in various areas.     

Improvement of microstructural properties of 3Y-TZP materials by conventional and non-conventional sintering techniques

3 mol% Y₂O₃-stabilized zirconia nanopowders were fabricated using various sintering techniques; conventional sintering (CS) and non-conventional sintering such as microwave (MW) and pulsed electric current-assisted-sintering (PECS) at 1300 °C and 1400 °C. A considerable difference in the densification behaviour between conventional and non-conventional sintered specimens was observed. The MW materials attain a bulk density 99.4% theoretical density (t.d.) at 1300 °C, while the CS materials attain only 92.5% t.d. and PECS 98.7% t.d. Detailed microstructural evaluation indicated that a low temperature densification leading to finer grain sizes (135 nm) could be achieved by PECS followed by MW with an average sintered grain size of 188 nm and CS 225 nm. It is believed that the high heating rate and effective particle packing are responsible for the improvements in these properties.     

Digital light processing of lunar regolith structures with high mechanical properties

It is highly desirable to establish an extraterrestrial base on the moon due to its practicality and scientific significance in the future space explorations, which promotes aerospace scientists to propose many conceivable fabrication methods. Herein, we fabricated architectural and functional structures with lunar regolith simulants via digital light processing (DLP) technology, followed by sintering. The printing slurry was prepared by mixing CLRS-2 lunar regolith simulant powders with photocurable resins, and it exhibits excellent print-ability. The microstructures, chemical compositions, particle size distribution and thermal-gravimetric characteristic of the simulants were analyzed, respectively. The average compressive strength and flexure strength of the sintered samples are 428.1?MPa and 129.5?MPa respectively, which are higher than those reported in previous researches. These improved mechanical properties could be due to the small average diameter of pores and the chemical compositions.     

A critical review on sintering and mechanical processing of 3Y-TZP ceramics

Zirconia ceramics have been extensively applied in dental restoration due to their superior properties and excellent functionalities. Green-compact sintering and mechanical processing have become critical operations to shape these denture materials to target dimensions and desired quality. Improper sintering regulations and cutting-induced damages are crucial issues when dealing with the manufacturing of ceramic dentures as they adversely affect the performance and acceptance of eventually-machined denture products. In this paper, a critical review has been conducted to offer a scientific understanding of 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) processing, focusing on illustrating the characteristics and properties of the materials as well as the influence of the sintering process on the microstructure and machinability of the workpiece. Recent advances addressing the processing issues of zirconia ceramics for dental applications are carefully reviewed by critically analyzing the scientific findings reported in the open literature. The fundamental influences of the working conditions on the machining quality of ceramic materials are discussed. The features of emerging non-traditional machining technologies are compared and analyzed. Dentures manufacturers will benefit from this review article as they seek to achieve high-quality processing for zirconia ceramics.     

Densification,microstructure and properties of ultra-high temperature HfB2 ceramics by the spark plasma sintering without any additives

Ultra-high temperature HfB₂ ceramic with nearly full densification is achieved by using gradient sintering process of SPS without any additives. The effect of the sintering temperature on the densification behavior, relative density, microstructure, mechanical and thermionic properties is systematically investigated. The results show that the fast densification of HfB₂ ceramic occurs at the heating stage, and the highest relative density of 96.75% is obtained at T =1950 °C, P = 60 MPa and t =10min. As the temperature is increased from 1800 to 1950 °C, the grain size of HfB₂ increases from 6.12 ±1.33 to 10.99 ± 2.25 μm, and refined microstructure gives the excellently mechanical properties. The highest hardness of 26.34 ±2.1GPa, fracture toughness of 7.12 ± 1.33 MPa m^1/2 and bending strength of 501 ±10MPa belong to the HfB₂ ceramic obtained at T =1950°C. Moreover, both the Vickers hardness and fracture toughness obey the normal indentation size effect. HfB₂ ceramic also exhibits the thermionic emission characterization with the highest current density of 6.12 A/cm² and the lowest work function of 2.92 eV.     

Sintering and grain growth control of high dense YIG

The densification and grain growth of yttrium iron garnet (YIG) were systematically studied to produce highly densified YIG via conventional solid-state route (CSSR). The percentage of purity and structure of YIG was confirmed by XRD characterization. SEM micrographs revealed that with increasing sintering temperature and time, the grain size and the average pores radius (R_p) increased, while the number of pores per volumes (N_v) decreased. The maximum material density obtained using Archimedes principle was 97.9% of that of theoretical density (ρ_theory). It required approximately 132.55 kJ/mol of energy to produce dense YIG sintered for 6 h at 1420 °C. However, beyond this temperature, a new phase that confirmed the presence of YFe₂O_4-δ phase was found through EDX analysis along the grain boundaries. This occurrence lowered the grain boundary mobility thereby resulting in slight change in density. Therefore, the results suggested that a highly densified YIG (ρ_theory of ≈98%) could be successfully obtained when YIG is sintered at 1420 °C for 6 h.     

Densification,microstructure evolution,and microwave dielectric properties of Mg1-xCaxZrTa2O8 ceramics

In this study, the effects of the Mg²⁺ ions replaced by Ca²⁺ ions on the microwave dielectric properties of newly developed MgZrTa₂O₈ were investigated. Mg_1-xCa_xZrTa₂O₈ (x = 0–1.0) ceramics were prepared via a solid-state reaction method. Calcination of the mixed powders was performed at 1200 °C and sintering of the powder compacts was accomplished at temperatures from 1200 to 1550 °C. The substitution of Ca²⁺ significantly inhibited the densification of Mg_1-xCa_xZrTa₂O₈, led to the expansion of the unit cells, and triggered the formation of a second phase, CaTa₂O₆. The porosity-corrected relative permittivity increased almost linearly with the x value because of the replacement of the less polarizable Mg²⁺ ions by the more polarizable Ca²⁺ ions. The variation in the Q × f values followed a similar trend as that of the sintered density, and the change trend in the τ_f values was in accordance with that of relative permittivity. The best composition appeared to be Mg_0.9Ca_0.1ZrTa₂O₈, which showed excellent microwave dielectric properties of ε_r = 22.5, Q × f = 231,951 GHz, and τ_f = −32.9 ppm/°C. The Q × f value obtained is the highest among the wolframite dielectric ceramics reported in literature.     

Toughening effect of multi-walled boron nitride nanotubes and their influence on the sintering behaviour of 3Y-TZP zirconia ceramics

Different amounts (0.5, 1, 2.5 and 5 wt%) of hollow “cylindrical” and “bamboo-like” boron nitride nanotubes (BNNTs) have been used to reinforce 3Y-TZP zirconia ceramics via spark plasma sintering. No significant influence of different morphologies of BNNTs on the mechanical properties at the macro-scale (elastic modulus, hardness, and fracture toughness) has been observed. The fracture toughness increased continuously with the increasing amount of the BN nanotubes up to 2.5%, resulted in the improvement of ∼100% compared to the reference ZrO₂. A direct influence of BNNTs on the toughening of ZrO₂ has been recognized. The BNNTs strengthen the zirconia grain boundaries resulting in the alteration in fracture mode from inter- to trans-granular. The BNNTs also promoted the transformation toughening of zirconia. Their influence on the bridging and pull out has been confirmed by the investigation of the composites with the amorphous borosilicate matrix.