3D printing ceramic cores for investment casting of turbine blades,using LCD screen printers: The mixture design and characterisation

The primary objective of this study is to demonstrate the possibility of developing silica, alumina, and zircon-based photocurable ceramic suspensions that can be used for visible light photopolymerization (> 450 nm) and to optimise the binder formulations for the purpose of LCD-based ceramic 3D printing applications. Reference ceramic components for this work are ceramic cores employed in the investment casting of high-pressure turbine blades and vanes. Arguably, one of the most critical steps in photoinduced ceramic 3D printing is developing suitable ceramic suspensions, having high ceramic loading, low viscosity, and short curing times. Ceramic suspensions with four different novel binder formulations and commercial ceramic powders used in core manufacturing (SiO₂, Al₂O₃ and ZrSiO₄) were investigated to achieve the best trade-off between: (1) their curing performance (cure depth and curing speed), (2) rheological properties of the binder mixtures at the solid loadings of 60 vol.% for SiO₂, 55 vol.% for ZrSiO₄, and 45 vol.% for Al₂O₃; and (3) the green body mechanical properties of the mixtures after printing. The effect of ceramic particles on the selected binders was examined individually, and the correlation between cure depth (C_d), volumetric loading, and curing speed are evaluated. The results show all binders designed in this study provide an adequate cure depth, even at high ceramic loadings. When the curing behaviour of all unloaded binder mixtures from the previous study [1] compared with the 10 vol.% SiO₂ loaded mixtures, the cure depth of all formulated binder mixtures increased 50–55 % and the curing thickness of 60 vol.% SiO₂ loaded suspensions were still slightly higher than their unloaded counterparts. The rheology outcomes indicate that lower viscosity binders always result in lower viscosity of the ceramic loaded inks, even without taking the effect of dispersants into account. Besides, the addition of N-Vinyl-2-Pyrrolidone (NVP) monofunctional monomer to the binder mixtures significantly reduces the viscosity and changes the normally linear relationship of the mix viscosity and its silica loading content. Among the binder formulations loaded with 60 vol.% of SiO₂, the formulation providing the lowest viscosity and highest mechanical property consists of 5 wt.% of NVP, 45 wt.% of HDDA and 50 wt.% of Photocentric 34 resin. Although this binder mixture showed the highest green flexural strength when loaded by 55 vol.% ZrSiO₄, all other mixtures loaded with zircon flour also demonstrated a near-fluid behaviour, below 200 s^?1. In Al₂O₃ loaded mixtures, the HDDA di-functional binder formulations present lowest viscosity and the di- and multifunctional monomer blends (HDDA-Photocentric27) showed the highest mechanical properties when used in a 50/50 ratio. This work summarises the best binder choices for silica, alumina and zircon based ceramic suspensions used in core printing for investment casting applications through LCD screen printing.     

A comparative study on the effect of carbon-based and ceramic additives on the properties of fiber reinforced polymer matrix composites for high temperature applications

Ablative composites have been in use for thermal protection of space vehicles for decades. Carbon-phenolic composites have proven to perform exceptionally well in these applications. However with development in aerospace industry their performance needs improvement. In this field, different carbon-based and ceramic additives have been introduced into ablative composite systems. This review article gives a comparative analysis of researches done in this field in the recent past. Density, ablative, thermal and mechanical properties of ablative composites with different ultra-high temperature ceramic particles i.e. ZrSi₂, Cenosphere, nano-SiO₂, BN etc. and carbon-based nanoparticles i.e. CNTs, nano-Diamonds, Graphene oxide etc. used as additives, have been compared and discussed. Emphasis is put on carbon-phenolic composite systems although some epoxy matrix systems have also been discussed for comparison.     

Stress Effects on Stabilizing Antiferroelectric Phase in Multilayer Ceramics

Controllable phase transformation between antiferroelectric (AFE) and ferroelectric (FE) states suggests multifunctional properties valuable for many device applications. Compared to AFE bulk ceramics with large voltage required for driving electric field‐induced phase transition, implementation of structures comprising multiple thin AFE ceramic layers can realize applications by reducing the switching operation voltage in the feasible range. Here, it is found that a compressive residual stress is developed in multilayer (Pb_0.97,La_0.02)(Zr_0.66,Sn_x,Ti_0.34?x)O₃ (PLZST) ceramic co‐fired with multiple Pd/Ag electrode layers, and the compressive residual stress can stabilize AFE phase. AFE phase forms in the PLZST multilayer ceramic with composition corresponding to FE in the bulk materials. Thermodynamic analysis based on free energy of FE and AFE phases well explains the FE to AFE phase transformation observed in the multilayer ceramic under the compressive stress. The findings exhibit a new strategy to tune structure and functional properties of multilayer ceramics through stress engineering for achieving device applications.     

New KNbTeO6 transparent tellurate ceramics

New transparent defect pyrochlore KNbTeO₆ ceramics were successfully prepared by Spark Plasma Sintering (SPS) of same composition polycrystalline powders elaborated by classic solid-state reaction from oxide precursors (K₂CO₃, Nb₂O₅, TeO₂) and followed by high energy milling powders. As such precursors are not available as commercial nanopowders, a suitable process has been developed by combining solid-state reactions and high energy milling. The determination of appropriate consolidation conditions and sintering parameters of the green body such prepared, are described in this paper. The resulting ceramic is transparent in both the visible and near infrared range (up to 5.5 μm). The maximum of transmittance is reached in the near infrared region around 2500 nm with a value of 78 % (1 mm thick sample), close to the maximum theoretical value of transmittance. This transparent KNbTeO₆ ceramic demonstrates a homogeneous and dense microstructure with an average grain size less than 500 nm. A small content of secondary phase has been detected by nanoscale observations without drastic effects on transparency. This ceramic exhibits very good mechanical properties similar to the Y₂O₃ transparent ceramic, as well as interesting dielectric properties in the microwave range. This innovative method should drive the development of new transparent materials with technologically relevant applications.     

Effect of Seed Addition on SnO2‐Based Varistors for Low Voltage Application

The effect of seed addition on the microstructure and non‐ohmic properties of the SnO₂ + 1%CoO + 0.05%Nb₂O₅ ceramic‐based system was analyzed. Two classes of seeds were prepared: 99% SnO₂ + 1%CuO and 99% SnO₂ + 1%CoO (mol%); both classes were added to the ceramic‐based system in the amount of 1%, 5%, and 10%. The two systems containing 1% of seeds resulted in a larger grain size and a lower breakdown voltage. The addition of 1% copper seeds produces a breakdown voltage (V_b) of ~ 37 V and a leakage current (f_ic) of 29 μA. On the other hand, the addition of 1% cobalt seeds produced a breakdown voltage of 57 V and a leakage current of 70 μA. Both systems are of great technological interest for low voltage varistor applications, by means of appropriate strategies to reduce the leakage current. Using larger amounts of seeds was not effective since the values of breakdown voltage in both cases are close to a system without seeds. To our knowledge, there are no reports in the literature regarding the use of seeds in the SnO₂ system for low voltage applications. A potential barrier model which illustrates the formation of oxygen species (O′_2(ads), O′_ads, and O″_ads) at the expense of clusters near the interface between grains is proposed.     

Effects of hafnium content on microstructures and properties of newly developed (Ti,Hf)(C,N) ceramics

In this study, novel (Ti,Hf)(C,N) ceramics with varying hafnium contents were fabricated via carbothermal reduction–nitridation and subsequent spark plasma sintering. The influence of Hf addition on the mechanical properties, wear properties, and corrosion resistance of the (Ti,Hf)(C,N) ceramics was systematically studied. The introduction of Hf promoted the sintering densification of the ceramics in the sintering process. The prepared (Ti,Hf)(C,N) ceramics exhibited excellent mechanical and wear properties owing to refinement and solution-strengthening mechanisms. The (Ti_0.9,Hf_0.1)(C_0.5,N_0.5) ceramic demonstrated higher Vickers hardness and fracture toughness, measuring 1997 HV5 and 4.28 MPa m^1/2, respectively, compared to the pure Ti(C_0.5,N_0.5) ceramic which exhibited values of 1635 HV5 and 3.94 MPa MPa m^1/2. The wear scar depth of the (Ti_0.9,Hf_0.1)(C_0.5,N_0.5) ceramic sample was 57.36% to that of the Ti(C_0.5,N_0.5) ceramic. Additionally, the addition of Hf improved the corrosion resistance of (Ti,Hf)(C,N) ceramics in a 0.5 M NaOH solution. The potential applications of (Ti,Hf)(C,N) ceramics include machining tools and wear-resistant parts.     

Phonon densities of states and related thermodynamic properties of high temperature ceramics

Structural components and semiconductor devices based on silicon nitride, aluminum nitride and gallium nitride are expected to function more reliably at elevated temperatures and at higher levels of performance because of the strong atomic bonding in these materials. The degree of covalency, lattice specific heat, and thermal conductivity are important design factors for the realization of advanced applications. We have determined the phonon densities of states of these ceramics by the method of neutron scattering. The results provide a microscopic interpretation of the mechanical and thermal properties. Moreover, experimental data of the static structures and dynamic excitations of atoms are essential to the validation of interparticle potentials employed for molecular-dynamics simulations of high-temperature properties of multi-component ceramic systems. We present an overview of neutron-scattering investigations of the atomic organization, phonon excitations, as well as calculations of related thermodynamic properties of Si₃N₄, β-sialon, AlN and GaN. The results are compared with those of the oxide analogs such as SiO₂ and Al₂O₃.     

Radiative properties of ‘Eu’in Li–Al–Si–O ceramics: Effect of ‘Si’ to ‘Li’ ratio

Four Li–Al–Si–O-based ceramic compounds with varying Si to Li ratios were synthesized via a high-temperature reaction route and characterized by X-Ray diffraction, scanning electron microscopy, and photoluminescence techniques. Eu was used as an activator in these four compositions namely, LiAlSiO₄, LiAlSi₂O₆, LiAlSi₃O₈, and LiAlSi₄O₁₀, and the radiative properties were evaluated. The activator concentration was optimized in all these samples to 2 mol%. The photoluminescence emission intensity and decay time values increased with the Si to Li ratio. The relative intensities of the emission spectra and the lifetime values indicated that the rare earth ion stabilizes itself at interstitial positions in the lattice having approximately C_2v symmetry. The chromaticity indices for the four ceramic samples doped with Eu were calculated that showed near red emission for all. To get an idea about their commercial utility, color purity, quantum efficiencies for the phosphors were evaluated. The emission profile of the prepared samples was compared with a commercial sample. The Judd-Ofelt parameters, transition probabilities, branching ratios, and quantum efficiencies were also calculated for these systems. It was observed that the metal-ligand bond covalency decreased with an increase in the Si to Li ratio which was responsible for the increase in the emission intensities and luminescence decay time values.     

Microstructure Development of PZT Ceramics by Doping with Small Amounts of Al2O3, SiO2, and Fe2O3

Lead zirconate titanate (PZT) ceramics are used in a wide range of applications as sensors and actuators. Typically, they are formulated by the mixed oxide route, using several mixing and milling steps. Due to wear, these processes introduce impurities into the ceramic mass, which on their part can strongly influence densification behaviour and final properties of the PZT. In this study, the effect of such impurities, such as iron oxide, alumina, and silica, on the sintering behaviour and microstructure development of PZT ceramic is evaluated. A commercial Nb‐doped PZT powder was used and doped by adding Al₂O₃, SiO₂, and Fe₂O₃ with an amount of up to 0.001 mol%. Bulk samples were prepared and sintered in air. The mass loss, density, and grain size were correlated by regression analysis using the doping elements and levels, respectively. Due to the complex interactions between the oxides and the ceramic properties, the experiments were performed with the design‐of‐experiment method (DoE). The results showed a significant influence of these low amounts of doping levels on the microstructure development. Moreover, it was shown that doping after calcination affects the microstructure in a similar way to doping before calcination. Thus, a possibility to compensate concentration variations in the calcined ceramic mass is demonstrated, to homogenize the chemical composition and the final microstructure of the sintered PZT ceramic in the manufacturing processes.     

Rare earth titano-silicates for high k gate dielectric applications

The present paper explores the properties of rare earth titano-silicates compounds for high k applications. RE₂Ti₂SiO₉ (RE=La, Pr and Nd) ceramic compounds were prepared using conventional solid state reaction route. Its structural, dielectric, optical and thermal properties were investigated. It was found that these materials possess a high dielectric constant in the range of 20–34 at room temperature, which is greater than that of HfO₂. The UV Visible absorption spectra indicates reasonably large band gap of above 5 eV. Further these compounds also have low coefficient of thermal expansion of less than 9 ppm/°C.     

Tape casting of non-aqueous silicon nitride slips

Tape casting is a powerful method for the manufacturing of flat, large area ceramic components. Silicon nitride is a reference material for high temperature structural applications. Between them, thick film/coating technologies and ceramic–ceramic joining are receiving an increased attention. In this work, the rheological behavior on non-aqueous silicon nitride slips for tape casting was investigated considering different solvent and binding systems, by controlling the total binder and plasticizer content and the binder to plasticizer ratio. A phosphate ester was used as dispersant. The characteristics of the green tapes obtained in different conditions were studied in terms of density, thickness and microstructure and related with the rheological properties of the slips. Once the slip properties were adjusted, manufacturing parameters, such as the casting speed and the gap between the blades and the carrier substrate, were also analyzed in order to improve the green tape properties and the process reliability. In order to obtain sinterable compacts, Al₂O₃ and Y₂O₃ were used as sintering aids. The effect of the sintering aids in both the rheological behavior and the green characteristics was also studied.     

Preparation and characterization of some multicomponent silicate glasses and their glass–ceramics derivatives for dental applications

The chemical corrosion and UV–vis absorption and infrared absorption spectra of binary and multicomponent lithium silicate glasses and corresponding glass–ceramics were investigated. The chemical durability of the glasses and derived glass–ceramics was found to be excellent to all leaching media. The IR absorption spectra of the glass and glass–ceramic samples reveal absorption bands of characteristic groups mainly due to major silicate network besides the possible sharing of network units due to some involving oxide constituents. X-ray analysis of glass–ceramics indicates the separation of lithium disilicate phase as the main constituent beside other phases according to the specimen chemical constituents. The obvious promising investigated chemical and physical properties are correlated with the presence of multioxides such as Al₂O₃, TiO₂, MgO and ZrO₂. Transmission and reflectivity properties reveal acceptable data. The prepared glass–ceramics are recommended for dental applications.     

EXTRUSION OF REFRACTORY OXIDE INSULATORS FOR VACWM TUBES1

A method is described whereby nonplastic materials such as refractory oxides are given a temporary plasticity, sufficient for extrusion. The organic plastic gel used consists of flour or starch paste with ammonia as an electrolyte. A short detailed account is given of the manufacture of twin-hole insulators made of MgO 98% and talc 2% for radio 227 vacuum tubes, with supplemental experimental results on other materials. The refractory oxides Al₂O₃, BeO, ZrO₂, MgO, and ThO₂ are compared from the standpoint of their physical properties and their ability to act as insulators in contact with hot tungsten filaments. Examples are given of the use of such extruded oxide insulators for hot filament supports.     

Fabricating porous ceramic materials via phase separations in blends of cellulose acetate and ceramic nanoparticles

As applications of porous ceramic materials have gradually expanded, the novel technologies for the fabrication of porous ceramic materials with a delicate and controllable structure are still attractive. In this work, three types of porous monolithic ceramic materials, including Al₂O₃–SiO₂, TiO₂, and SiC, have been fabricated by thermally impacted and non-solvent-induced phase separations in blends of cellulose acetate and ceramic nanoparticles. These materials possessed three-dimensional interconnected porous structures with low densities, high porosities, and hierarchical pores ranging from 5 nm to 6 μm. The relationships between microstructures and phase separations were systematically investigated. Furthermore, electromagnetic shielding effectiveness of 20 dB from 5 to 18 GHz in porous SiC materials has been achieved, revealing that those materials have potential applications in the electromagnetic shielding. This work provides a powerful and general approach to fabricate porous monolithic ceramic materials with a wide range of various ceramic nanoparticles.     

Thermal stability and performance of optimized ZrCx diffusion barriers in ceramic coating systems for ATF applications

Surface-modified Zr alloy claddings with advanced ceramic coatings are promising materials for accident-tolerant fuel (ATF) systems to meet stringent safety regulations concerning light water reactors. The applications of ceramic coatings are, however, limited as a result of inferior thermal stability when used in conjunction with Zircaloy-4 (Zry-4) substrates. Herein, the thermal stability of sub-stoichiometric zirconium carbide barrier layers as a function of composition was studied. Integrated ceramic coatings comprising ZrC_0.55 diffusion barriers and a Cr₂AlC top layer were synthesized via a magnetron sputtering method. After rapid thermal annealing, the ZrC_0.55 barrier layer having a thickness of 0.5 μm effectively prevented the inter-diffusion between Cr₂AlC and the Zry-4 substrate, thereby ensuring retention of the structural integrity of the integrated ceramic coating system for ATF applications.     

Pseudo Phase Diagram and Microwave Dielectric Properties of Li2O–MgO–TiO2 Ternary System

Li₂O–MgO–TiO₂ ternary system is an important microwave dielectric ceramic material with excellent properties and prospect in both scientific research and application. A phase diagram of the Li₂O–MgO–TiO₂ ternary system was established in this article, based on earlier research results and our present work. Microwave dielectric properties with compositions in different regions of the phase diagram have been analyzed. We found that the 0.33 Li₂MgTi₃O₈–0.67 Li₂TiO₃ ceramics sintered at 1200°C exhibited excellent dielectric properties: Q × f value = 80 476 GHz (at 7.681 GHz), ε_r = 24.7, τ_f = +3.2 ppm/°C. We also designed two ceramic systems in the Li‐rich region of the Li₂O–MgO–TiO₂ ternary system, which received little attention in the past decades, because many excellent single‐phase ceramics, such as Li₂MgTiO₄, Li₂MgTi₃O₈ and MgTiO₃, have been found in the Ti‐rich region. The ceramic systems have low sintering temperatures but also relatively poor dielectric properties.     

Engineering nanostructured thermal spray coatings: process–property–performance relationships of ceramic based materials

Abstract

Nanostructured powders were deposited using thermal spraying to produce coatings having internal features of nanosized dimensions. Several ceramic based materials were studied, including WC–12 wt-%Co, TiO₂, hydroxyapatite, Al₂O₃–13 wt-%TiO₂ and yttria stabilised zirconia. The effect of the thermal spray conditions on the microstructure, phase composition, properties and performance was investigated. Key nanostructural features of the coatings were identified and their potential benefit in contributing to enhanced behaviour explored. Issues relating to design strategies and process control for engineering these types of coatings with performance characteristics tailored for targeted applications are discussed.     

Cold sintering process: A new era for ceramic packaging and microwave device development

Cold sintering process (CSP) is an extremely low‐temperature sintering process (room temperature to ~200°C) that uses aqueous‐based solutions as transient solvents to aid densification by a nonequilibrium dissolution‐precipitation process. In this work, CSP is introduced to fabricate microwave and packaging dielectric substrates, including ceramics (bulk monolithic substrates and multilayers) and ceramic‐polymer composites. Some dielectric materials, namely Li₂MoO₄, Na₂Mo₂O₇, K₂Mo₂O₇, and (LiBi)_0.5MoO₄ ceramics, and also (1?x)Li₂MoO₄?xPTFE and (1?x)(LiBi)_0.5MoO₄?xPTFE composites, are selected to demonstrate the feasibility of CSP in microwave and packaging substrate applications. Selected dielectric ceramics and composites with high densities (88%‐95%) and good microwave dielectric properties (permittivity, 5.6‐37.1; Q × f, 1700‐30 500 GHz) were obtained by CSP at 120°C. CSP can be also used to potentially develop a new co‐fired ceramic technology, namely CSCC. Li₂MoO₄?Ag multilayer co‐fired ceramic structures were successfully fabricated without obvious delamination, warping, or interdiffusion. Numerous materials with different dielectric properties can be densified by CSP, indicating that CSP provides a simple, effective, and energy‐saving strategy for the ceramic packaging and microwave device development.     

Review: Effect on physical,mechanical, and wear performance of ZrB2-based composites processed with or without additives

Because of unique combination of properties, ultra high temperature ceramics (UHTCs) are considered the most suitable material for applications in extreme environments as in hypersonic flights, atmospheric reentry, and rocket propulsion system. Processing of UHTCs especially ZrB₂-based ceramic composites with additives offer advantages in terms of simple processing methodology and excellent properties. Processing route highly controls the ceramic properties. Present review share out systematically and explain the processing strategies of ZrB₂-based ceramic composites––conventional, hot press or spark plasma sintering and their effect on microstructure features, physical, and mechanical properties and tribological performance. Present review suggests that it is possible to process full dense ZrB₂–SiC ceramic composite with ultrafine or nano size particles via fast sintering technique like spark plasma sintering and gives better mechanical and wear resistant properties.     

Preparation and properties of fully esterified erythritol

Pure tetraesters of erythritol with C₁₀, C₁₂, C₁₄, C₁₆, C₁₈ saturated, and C_18:1 unsaturated (oleoyl) fatty acyl chains have been prepared for the first time and characterized using the acylating systems fatty acid/N,N′‐dicyclohexylcarbodiimide/4‐dimethylaminopyridine (DMAP), fatty acid anhydride/DMAP, fatty acyl chloride/pyridine, and fatty acyl chloride/boron trifluoride etherate. For the first three systems the yields were in the range of 80–90% while the fatty acyl chloride/pyridine system has the advantage of lower cost. The fatty acyl chloride/boron trifluoride etherate system gave lower (ca 70%) yields of the tetraesters. The tetraesters of erythritol may have applications analogues to those of triglycerides. In addition, new applications can be envisaged for these compounds, as a result of their differences in physical, chemical, and biochemical properties compared to triglycerides. Practical applications: The tetraesters of erythritol with saturated fatty acyl chains may have applications analogous to those of saturated triglycerides. However, tetraesters with unsaturated fatty acid chains may have greater prospects of having industrial uses after doing chemistry on the carbon–carbon double bonds.