Ceramic Struts Research Articles

Improving strength and microstructure of SiC reticulated porous ceramic through in-situ generation of SiC whiskers within hollow voids

SiC reticulated porous ceramic with excellent strength and high-density ceramic struts was successfully prepared using the polymer replica method, followed by pressureless sintering under a buried charcoal atmosphere. First, a polyurethane (PU) template was coated with a Si slurry and then a SiC-containing slurry, and subsequently heated under the buried charcoal atmosphere. To ensure excellent coating ability of the slurries, the viscosity, thixotropy, and yield stresses of the Si slurry were optimized by adjusting the content of the thickening agent. During heating, Si in the coating layer reacted with the residual C and CO gas from the PU template and buried charcoal, forming SiC whiskers that filled hollow voids within the SiC struts. Additionally, catalyst ferric nitrate was added to the Si slurry to promote the generation and growth of SiC whiskers. As a result, when compared to the untreated SiC reticulated porous ceramic, the SiC reticulated porous ceramic pre-coated with Si layers exhibited significant improvements in mechanical strength and thermal shock resistance, despite minor differences in porosity. Furthermore, an industrial test conducted in the copper smelting industry showed that the structure of SiC reticulated porous ceramic, prepared in this study and used as filters, remained intact even after 7 days of continuous use. Meanwhile, a significant number of inclusions was adhered to the surfaces of the filters. Therefore, the processes combined with in-situ generation of SiC whiskers is an ideal and low-cost method for fabricating SiC filters with excellent properties.

Fabrication of SiC reticulated porous ceramics with dense struts by in-situ generation of SiC

SiC reticulated porous ceramics have been widely utilized as filters in the smelting industry. To address the problem of hollow ceramic struts and improve the strength and thermal shock resistance of SiC reticulated porous ceramics, a one-step coating process was carried out using slurries with SiC, metallic Si and various carbon sources (carbon spheres, carbon black, and flake graphite). The effect of the carbon sources on the rheological behavior of the coating slurries was investigated, as well as the relationship between the rheological behavior of the slurries and the structure of the green bodies. The hollow struts and flaws resulting from the combustion of polymer templates were then filled with SiC whiskers formed by the reaction between Si and carbon sources during heating. Among the carbon sources, the carbon spheres significantly stimulated the growth of the SiC whiskers, which filled the voids and flaws. After heating at 1550 °C, the SiC reticulated porous ceramics prepared with Si and carbon spheres reached a porosity of 80%, compressive strength of 1.85 MPa, and residual compressive strength rate of 81%. In industrial tests, the fabricated SiC reticulated porous ceramics were used as filters to effectively remove inclusions in molten copper. Importantly, the structure of the SiC filters remained intact after use.

Direct ink writing of boron carbide monoliths

Direct ink writing – an extrusion-based additive manufacturing process – followed by pressureless sintering was investigated to produce boron carbide monoliths. The effects of ceramic powder loading and Pluronic binder concentration on the rheology of boron carbide pastes were studied and linked to both processing behaviour and final outcome in terms of sintered density and hardness. The effects of printing parameters, in particular orifice diameter and printing speed, were also investigated. Reducing the size of the extrusion nozzle from 584 μm to 406 μm led to significantly better shape retention, lower surface roughness, as well as higher density and hardness. A 203 μm printing orifice was also trialled but was unsuccessful due to faster drying kinetics that occurred with smaller ceramic struts resulting in rapid warping and nozzle clogging. Carbon-black – 8 wt% relative to B4C – acted as an effective sintering aid to increase both density and hardness. After optimisation of feedstock and printing parameters, few-layer samples (3–5 layers) had a density as high as ∼ 97 % TD and a hardness of ∼ 30 GPa. On the other hand, 18-layer specimens had a sintered density of ∼ 87 % TD, despite a fully dense microstructure, due to the formation of a 3D array of inter-strut pores. Nevertheless, several issues that arose during manufacturing and post-processing were detrimental to the density and structural integrity of printed specimens; these issues were identified, discussed, and suggestions for future work are provided.

Reinforcement of thermally-conductive SiC/Al composite with 3D-interpenetrated network structure by various SiC foam ceramic skeletons

In this paper, 3D-SiC network skeleton with different structures was successfully constructed by regulating the microstructure of SiC foam ceramic struts via polyurethane replication technology combined with reactive infiltration of Si and solid-state sintering. Then, SiC/Al composites with 3D-interpenetrated network structure (3D-SiC/Al) were fabricated via vacuum pressure infiltration of Al alloy into the 3D-SiC skeleton, for application as electronic packaging. The effects of different skeleton structures on the thermal conductivity (TC), coefficient of thermal expansion (CTE), and flexural strength of the composites were investigated in detail. Furthermore, representative volume element (RVE) models of the 3D-SiC/Al composites were developed and their heat transfer properties and dimensional stability were also comparatively analyzed by finite element (FE) simulations. The results showed that the 3D-SiC/Al composites exhibited high TC of 196 W/m·K, low CTE of 8.03 × 10−6 K−1, and excellent flexural strength of 272 MPa. Low thermal deformation parameter (TDP) and Ashby map demonstrated that the synergistic effect of a complete 3D network skeleton and a clean well-bonded interface in the 3D-SiC/Al composites facilitated the integration of low CTE and high TC even at a low SiC content of 30 vol%. The FE results revealed a 63.5 % reduction in thermal stress and nearly an order of magnitude increase in heat flux for the RVE model of 3D-SiC (30 vol%)/Al composite compared to the RVE model of SiCP (60 vol%)/Al composite. This work provides a novel and scalable method for designing and synthesizing various composites for electronic packaging.

A development to determine spectral radiative properties of semitransparent struts of open-cell ceramic foams: From macro-scale measurement to pore-scale simulation

The radiative properties of a ceramic strut cannot be strictly the same as that of a dense sintered ceramic bulk composed of the same material. This contribution develops an approach to determine the radiative properties of semitransparent ceramic struts of open-cell foams. First, a macro-scale measurement was conducted to obtain the normal-hemispherical transmittance and reflectance spectra of alumina foam slices. Second, a pore-scale forward model was established to predict the radiative behaviors of 3D digital porous structures obtained from micro-computed tomography technique. Last, an identification method was proposed to retrieve the radiative properties of semitransparent ceramic struts from the measured data. The results indicate that it is unreasonable to simply assume the radiative properties of alumina struts as that of corresponding dense bulks. Within the investigated wavebands 0.4–2.2 μm, the average extinction coefficient of alumina struts is 33.5% lower than that of dense bulks. The scattering albedo shows a slight difference between alumina struts and dense bulks. Besides, the retrieved extinction coefficients and scattering albedos of alumina struts are proposed as a function of wavelengths, which can be used in potential applications.

Processing and thermal characterization of polymer derived SiCN(O) and SiOC reticulated foams

Highly porous polymer-derived SiCN(O) and SiOC ceramics with low thermal conductivity were developed by replicating polyurethane (PU) foams. The PU templates were impregnated with polysilazane or polysiloxane precursor, followed by pyrolysis at different temperatures (1200 °C - 1500 °C) yielding SiCN(O) or SiOC ceramic foams, respectively. The swelling and cross-linking behavior of the used precursors had a significant impact on the morphology of the prepared foams. The samples had bulk densities ranging from 0.03 g.cm-3 to 0.56 g.cm-3 and a total porosity in the range from 75 to 98 vol%. Fourier transform infrared (FT-IR), Raman spectroscopy, X-ray diffraction (XRD) were employed to follow the structural evolution together with morphological characterization by scanning electron microscopy (SEM). The obtained ceramics were thermally stable up to 1400 °C, and the linear thermal expansion coefficient values of the porous SiCN(O) and SiOC components in the temperature range from 30 to 850 °C were found to be ~1.72 x 10-6.K-1 and ~1.93 x 10-6.K-1, respectively. Thermal conductivity (λ) as low as 0.03 W.m-1 K-1 was measured for the SiCN(O) and SiOC foams at room temperature (RT). The λ of the ceramic struts were also assessed by using the Gibson-Ashby model and estimated to be 2.1 W.m-1 K-1 for SiCN(O), and 1.8 W.m-1 K-1 for SiOC.

Preparation of novel reticulated porous ceramics with hierarchical pore structures

Al2O3-ZrO2 reticulated porous ceramics (RPCs) with hierarchical pore structures were prepared by vacuum infiltration. Al(OH)3 was applied to provide the pore structure. Vacuum infiltration using an alumina/aluminum hydroxide slurry via a pre-sintering cycle was performed to fill up the hollow struts generated due to burnout of the polyurethane foam template and the coating on the Al2O3-ZrO2 ceramic struts. The results showed that as the Al(OH)3 content in the infiltration slurry was increased from 5% to 13%, the pore size distribution of the ceramic struts widened and the major pore size decreased. In addition, a compressive residual stress developed within the multilayered struts because of the difference in thermal expansion of the Al(OH)3-Al2O3 coating layer and Al2O3-ZrO2 ceramic struts. Moreover, with increasing Al(OH)3 content in the infiltration slurry, the porosity of the ceramic struts gradually increased, resulting in a decrease in Young's modulus and thermal conductivity, which influenced the compressive residual stress and thermal stress within the ceramic struts. Furthermore, the potential of the fabricated RPCs as catalyst supports was demonstrated.

Mechanical and physical characterization of Al2O3–C foam filters produced by distinct processing routes: The importance of the ceramic strut morphology

In this work the influence of the production route and filter dimensions on the mechanical properties of carbon-bonded alumina foam filters was investigated. Features like cold crushing strength and fracture behavior were analyzed and used to compare them. Microfocus computer tomography (μCT) was applied to characterize the geometry of the filters, providing a deep correlation with their fracture behavior. Furthermore, computer-generated filter geometries were used as reference. The results indicated that the centrifugation process is better suited for the production of carbon-bonded alumina filters, and is even more effective when the foam dimensions are increased in the range of those analyzed in this research. Finite element simulations showed the influence of the relative density and strut tapering on the cold crushing strength of filters, providing a correlation between filter structure and its failure mechanism. The attained results provided further insights towards the production of inclusions-free metal parts.

A novel design of Al2O3-ZrO2 reticulated porous ceramics with hierarchical pore structures and excellent properties

The use of reticulated porous ceramics(RPCs) was of great interest in high-temperature catalytic application owing to their high surface area. In order to further optimize the pore structure and mechanical properties of RPCs, vacuum infiltration with CaCO3-Al2O3 slurries process was applied to fabricate Al2O3-ZrO2 RPCs with hierarchical pore structures. The pores within ceramic struts were prepared by processes of CaCO3 decomposition and calcium hexaluminate grains growth. And the compressive residual stress was formed within multi-layered struts owing to the difference in the thermal expansion of coating layer and ceramic struts, which was established as a key factor in improving the mechanical properties and thermal shock resistance of RPCs. Furthermore, the size of pores within struts ranging from 5 μm to 14 μm affected the thermal shock resistance of RPCs significantly based on grey incidence analysis. And the potential of this materials as high-temperature catalyst supports was demonstrated.

Tomography-based radiative transfer analysis of an open-cell foam made of semitransparent alumina ceramics

This study aims to fully investigate the radiative characterization of ceramic foams in terms of local scale (single strut) and macro scale (foam sheet). A radiative transfer model was established in the limit of geometric optics for a porous structure obtained from real alumina ceramic foams by computed tomography technique. The model considers the reflection and refraction at solid-void interface and the volumetric transmission, absorption and scattering process inside semitransparent ceramic solids. It is found that at local scale, neglecting the real hollowness may bring considerable errors to the local radiative behavior of ceramic struts (maximum errors up to 20.4–39.3% for the cases investigated). The cross-sectional thickness of the strut increases from the middle to the extremities, which causes a significant variation of the local radiative behavior. At macro scale, a peak in transmittivity of foam sheet can be observed at the wavelength around 4.4 µm. The scattering albedos obtained from the predictive model indicate that alumina ceramic foams behave strongly scattering within the wavelength 4 µm but very absorbing over 6 µm. The asymmetry factors obtained show a turning point at the wavelength around 1.8 µm, suggesting a transition of the scattered radiation from backward predominance to forward predominance.

Fabrication and Thermal Structural Characteristics of Ultra-high Temperature Ceramic Struts in Scramjets

ZrB2-SiC based ultra-high temperature ceramic (UHTC) struts were firstly proposed and fabricated with the potential application in the combustor of scramjets for fuel injection and flame-holding for their machinability and excellent oxidation/ablation resistance in the extreme harsh environment. The struts were machined with electrospark wire-electrode cutting techniques to form UHTC into the desired shape, and with laser drilling to drill tiny holes providing the channels for fuel injection. The integrated thermal-structural characteristic of the struts was evaluated in high-temperature combustion environment by the propane-oxygen free jet facility, subject to the heat flux of 1.5 MW/m2 lasting for 300 seconds, and the struts maintained integrity during and after the first experiment. The experiments were repeated for verifying the reusability of the struts. Fracture occurred during the second repeated experiment with the crack propagating through the hole. Finite element analysis (FEA) was carried out to study the thermal stress distribution in the UHTC strut. The simulation results show a high thermal stress concentration occurs at the hole which is the crack initiation position. The phenomenon is in good agreement with the experimental results. The study shows that the thermal stress concentration is a practical key issue in the applications of the reusable UHTC strut for fuel injection structure in scramjets.

Preparation of interpenetrating alumina–copper composites

To prepare interpenetrating alumina–copper composites, alumina foams were activated with titanium coating by chemical vapor deposition and then were infiltrated with molten copper by expendable casting process. The microstructure and phase composition of the composites were analyzed, and bending strength, electrical conductivity, friction and wear properties were tested. The results showed that the bonding between ceramic and metal was fine in the composites while no reactions took place between them because of the undissolved titanium coating. With increase of ceramic fraction, the electrical conductivity of the composite decreased, whereas the bending strength increased. The composite failure occurred by ductile fracture of the metal followed by fracture of the ceramic. The wear rate of the composites decreased with increase of ceramic fraction. And the wear of the composites was featured with ceramic struts peeling compared with ploughing and adhering wear for pure copper.

Bioactive glass-reinforced bioceramic ink writing scaffolds: sintering, microstructure and mechanical behavior

The densification of pore struts in bioceramic scaffolds is important for structure stability and strength reliability. An advantage of ceramic ink writing is the precise control over the microstructure and macroarchitecture. However, the use of organic binder in such ink writing process would heavily affect the densification of ceramic struts and sacrifice the mechanical strength of porous scaffolds after sintering. This study presents a low-melt-point bioactive glass (BG)-assisted sintering strategy to overcome the main limitations of direct ink writing (extrusion-based three-dimensional printing) and to produce high-strength calcium silicate (CSi) bioceramic scaffolds. The 1% BG-added CSi (CSi-BG1) scaffolds with rectangular pore morphology sintered at 1080 °C have a very small BG content, readily induce apatite formation, and show appreciable linear shrinkage (∼21%), which is consistent with the composite scaffolds with less or more BG contents sintered at either the same or a higher temperature. These CSi-BG1 scaffolds also possess a high elastic modulus (∼350 MPa) and appreciable compressive strength (∼48 MPa), and show significant strength enhancement after exposure to simulated body fluid—a performance markedly superior to those of pure CSi scaffolds. Particularly, the honeycomb-pore CSi-BG1 scaffolds show markedly higher compressive strength (∼88 MPa) than the scaffolds with rectangular, parallelogram, and Archimedean chord pore structures. It is suggested that this approach can potentially facilitate the translation of ceramic ink writing and BG-assisted sintering of bioceramic scaffold technologies to the in situ bone repair.

Influence of sintering temperature on the mechanical properties of ϵ-PCL-impregnated 45S5 bioglass-derived scaffolds fabricated by robocasting

The effect of sintering temperature on the mechanical performance of 45S5 Bioglass® scaffolds fabricated by robocasting and infiltrated by ϵ-polycaprolactone (ϵ-PCL) has been investigated in this work. Infiltration was made by immersion of the robocast scaffolds in a polymer melt to produce fully impregnated, dense co-continuous composites, or in a polymeric solution to produce a polymer-coated bioceramic structure. Polymer infiltration produced a significant increase in the strength and toughness of the scaffolds, both under compression and bending. The strengthening and toughening provided by the polymer was larger in fully impregnated structures than in coated scaffolds, and the toughening was especially dramatic (up to more than two orders of magnitude) under flexural stresses. The strengthening and toughening produced by polymer infiltration increased monotonically with the level of microporosity in the ceramic struts, i.e., they decreased with the sintering temperature. In spite of that, denser ceramic struts yield stronger and tougher hybrid structures after polymer infiltration.

Collapse criteria for high temperature ceramic lattice truss materials

Based on the temperature dependent behaviors of the Young's modulus and the strength of high temperature ceramics, temperature dependent failure characteristics of high temperature ceramic lattice truss materials were analyzed to build temperature-dependent failure theory. In compression, compression fracture and buckling are the competing failure modes. In shearing, tensile fracture and buckling are the competing failure modes. High temperature softening of the ceramic strut induces fatal buckling collapse for the lightweight lattice truss structure at ultra-high temperature. Collapse criteria under different planar stress states were built and are supported by high temperature experiments. Instructed by the criteria, collapse surfaces consisting of fracture surface and buckling surface were plotted and give helpful suggestions to design high temperature resistant as well as lightweight lattice truss ceramic composites.

Computational Modeling of Porous Ceramics with Bioactive Layer

The paper deals with a creation of computational model of a high porous ceramic material. This type of material has a large-scale industrial utilization. The computational model was created based on micro-CT data in the ANSYS 14.0 software using Finite Element Method. A creation of a porous ceramic struts model which respect a micro architecture is quite difficult (computer demanding and micro-CT data). The micro-CT slices are converted into a 3D model using image processing (used software STL Model Creator). The local first principle stress was analyzed because, ceramic is the brittle material. Furthermore, the influence of the thick layer around the individual struts was analyzed.

Decoration of Ceramic Foams by Ceramic Nanowires via Catalyst-Assisted Pyrolysis of Preceramic Polymers

Silicon oxynitride ( Si 2 N 2 O ) nanowires (NWs) were grown in situ on the cell walls of silicon carbide and alumina foams by coating preceramic polymers containing a Co-based catalyst, followed by pyrolysis in nitrogen at temperatures ranging from 1250°C to 1450°C. The relationship between different processing factors (type of substrate and preceramic polymer, pyrolysis temperature) and the growth of NWs was examined. Varying processing factors were found to be effective for controlling the growth of NWs. The use of a preceramic polymer containing a high amount of carbon as well as silica impurities present in the SiC foams led to a larger amount of longer NWs, homogeneously covering the ceramic struts of the porous substrates. The overall morphology and growing mechanism of the NWs on ceramic foams were investigated.

Modeling the mechanical properties of optimally processed cordierite–mullite–alumina ceramic foams by X-ray computed tomography and finite element analysis

Bulk and cellular cordierite ceramics were prepared from a non-stoichiometric powder consisting of corundum, talc (triclinic), α-quartz, K-feldspar, kaolinite, mullite and a small amount of a glass phase. The optimal sintering processing route was evaluated to obtain good mechanical properties. A high flexural strength of 120MPa and a Young’s modulus of 99GPa were achieved. The ceramic foams were fabricated by impregnation of polymer preforms with the optimized stock suspension. The mechanical properties of ceramic foams were studied by impulse excitation and compression tests. The Gibson–Ashby model predicted the ceramic foam’s effective modulus and its elastic limit strength well, as measured experimentally. In addition, the actual three-dimensional (3-D) structure obtained from X-ray computed tomography (CT) coupled with the finite element method (FEM) was used to calculate the Young’s modulus and the elasticity limit of the ceramic foam; however, this did not produce aby better agreement between the calculated values and the experimental results. The discrepancy between the Gibson–Ashby model and FEM could probably be attributed to the accuracy and small volume of representative reconstructed 3-D cellular structure. Taking account of the effect of the internal hollow structure on the stress localization in the ceramic struts, the CT–FE modeling provides a good measure of the adaptability and predictability of actual ceramic foam structures for realistic damage modeling.

Advancement of Cellular Ceramics Made of Silicon Carbide for Burner Applications

Lower emissions of CO and NOx as well as a higher power density were observed in combustion processes performed in porous media like ceramic foams. Only a few materials are applicable for porous burners. Open-celled ceramic foams made of silicon carbide are of particular interest because of their outstanding properties. Two different SiC materials have been investigated, silicon-infiltrated silicon carbide (SiSiC) and pressureless sintered silicon carbide (SSiC). The oxidation behaviour of both has been characterized by furnace oxidation and burner tests up to 500 h operating time. Up to a temperature of 1200 °C SiSiC exhibited a good oxidation resistance in combustion gases by forming a protective layer of silica. High inner porosity up to 30% in the ceramic struts was found in the SSiC material. Caused by inner oxidation processes the pure material SSiC allows only short time applications with a temperature limit of 1550 °C in combustion gases. An increase of the lifetime of the SSiC foams was obtained by development of a new SSiC with an inner porosity of less than 12%. The result was a considerable reduction of the inner oxidation processes in the SSiC struts.

Plasma electrolytic oxidation of aluminium networks to form a metal-cored ceramic composite hybrid material

A commercially-available low density aluminium network material (Duocel™) has been processed by plasma electrolytic oxidation to produce a ceramic hybrid material comprising an assembly of ceramic struts with metallic cores. The architecture and microstructure of this material were studied using X-ray tomography, scanning electron microscopy and densitometry. Conversion fractions were determined from mass gains and by image analysis of cross-sections, and the ceramic density was evaluated by hydrostatic weighing. Tensile and compressive testing of the hybrid material was used to study the toughness, as a function of the conversion fraction. Such material retains some of the beneficial mechanical properties of a metal (ductility and toughness), while also exhibiting a low overall density and a high specific surface area of ceramic. It can thus be considered as a highly permeable ceramic scaffold, with a relatively high toughness.