Thin-walled Ceramics Research Articles

Influence of filament patterning in structural properties of dense alumina ceramics printed by robocasting

Robocasting is an extrusion-based additive manufacturing (AM) technique, highly explored in the fabrication of porous structures or thin-walled ceramics. Comparing to other slurry-based AM technologies, robocasting presents a high potential to produce dense bulk ceramics, thanks to the low organic binder contents and high solids loading used in the feedstock. Despite these benefits, the robocasting fabrication of ceramics with similar mechanical performance as those produced by traditional methods remains a challenge. The goal of this work is to fine-tune the robocasting printing process of dense alumina ceramics by evaluating the influence of in-fill filaments orientation and the medium where the deposition process occurs (air or paraffin bath), on defects and mechanical performance of the sintered parts. Relative density, shrinkage, three-point flexural strength, Young's modulus and hardness were analysed. The specimens with the highest performance were sintered at 1550 °C or 1600 °C temperatures for 2 h holding time and the results were compared with slip casting (SC) samples sintered in the same conditions. Alumina robocasting samples, printed longitudinally in air with controlled humidity and sintered at 1600 °C, exhibited relative density ≈99% with flexural strength (≈350 MPa), Young's modulus (≈366 GPa) and hardness (≈17 GPa) values comparable to those obtained for SC samples. Numerical modelling and simulation were used to analyse the experimental values and the anisotropic behaviour of the printed specimens. Experimental Young's modulus results were compared with the Voigt-Reuss bound and the Hashin-Shtrikman upper-bound, as well as the analytical predictions of a Mooney-type exponential relation.

Dünnwandige Keramik, Keramik Mit Marmorierter Verzierung Und Gestempelte Keramik in Lágymányos

The vessel types of the Lágymányos potter’s workshop highlighted in this present paper shed new light on our knowledge of the Roman Age pottery production of the Aquincum district. Thin-walled ceramics draw attention to the significance of South-Pannonian import and the production of its imitations. Dating and design of stamped ceramics, the operation and hundreds of linked motifs of Resatus represent a significant advance. The local variant of marbled ceramics also belongs to the new ceramic types of the period, which, besides vessels of Celtic tradition, became an important product of the workshop.

Qing monochrome porcelain from the Collection of the Bohdan and Varvara Khanenko National Museum of Arts

The article analyzes Qing monochrome products of Chinese porcelain and studies the complex technology of their manufacture. This publication is also devoted to the main identification problems of Chinese monochrome thin-walled ceramics and its terminology. When dating and attributing Chinese monochrome porcelain causes the most difficult problem. Thin-walled monochrome glaze ceramic products manufactured at different times are often distinguished only by barely noticeable changes in parts and other proportions of products. Furthermore, the plagiarism was uncharacteristic of Chinese, and the imitation and copying of works of past epochs, in their opinion, only confirmed the value of original works and even gave a sacred meaning to them. The basis for this approach was the idea of ​​connection between epochs, the continuity of traditions and the respect to ancestors supported by the ruling elite, because it was a part of the official religion of the statе – Confucianism. The secret of regenerative firing has been discovered that restores the natural color of metals in their oxides in the glaze composition thanks to the carbon by giving the rich hue color intensity to the monochrome Chinese ceramics. The play of different colors achieved exclusively by ceramic techniques is very attractive, and products colored in such way are sometimes more expensive than the painted glazed ceramics. In Qing times ceramists manufactured a lot of porcelain products that mimicked samples of previous epochs. From time to time potters created almost exact copies of products, so it is sometimes difficult for present-day researchers to tell the difference between late imitations and original ancient works. That’s way the purpose of this article is to analyze works of Chinese Qing monochrome porcelain from the collection of the Bohdan and Varvara Khanenko National Museum of Arts, to demonstrate the manufacturing technology and terminological features and to specify attributions. This study has been illustrated by pieces from the Bohdan and Varvara Khanenko National Museum of Arts.

New hierarchical titania-based structures for photocatalysis

A new preparation method of bulk hierarchic titania structures for photocatalysis is proposed. A highly porous metallic Ti preform was converted into TiO2 for the first time using a combination of an oxidative construction of thin-wall ceramics (OCTWC) method, intermediate transformation into sodium titanate by hydrothermal treatment in alkaline aqueous solutions and shape preserving removal of sodium by a secondary hydrothermal treatment in acidic aqueous solution yielding a material of rutile/anatase titania with highly developed surface and high photocatalytic activity.

Research on 5 Axis Tool-Path Based on Constant Shearing Stress for Milling Thin-Wall Ceramics

Ceramics is a material of hardness and brittle, so crack arises easily when it is machined. To improve its cutting performance and enhance machining quality, a dynamic model based on constant-shearing-stress is built for milling ceramics, therefore some cutting parameters can be optimized by the model for no crack machining; moreover, an algorithm of 5 axis tool-path designed with constant-shearing-stress is put forward. According to the model and the algorithm, 5 axis tool paths of decreasing-cutting-depth and decreasing-feed-rate are realized; these are really accurate and smooth through VERICUT software simulating. Using Mikron high-speed machining center, several experiments were done to find a good method of crack control in milling thin-wall ceramics. The results show that the experiment based on the 5 axis tool paths with high pressure air cooling and regional cutting is rather successful because of no cracking in cutting thin-wall ceramics, compared to others.

Preparation of niobium nitrides via a single-step method

Oxidative construction of thin-walled ceramics (OCTC) is used to obtain compact niobium nitrides with the given phase composition in the form of finished products. The change in the sample phase composition is studied at different temperature modes.

Investigation of phase changes in massive titanium preforms in the processes of oxidative constructing

The phase composition of massive titanium preforms obtained using oxidative constructing of thin-walled ceramics (OCTC) selected from the exponential section of the kinetic oxidation curve is investigated. It is demonstrated with the help of X-ray phase analysis (XPA) that the phase composition of the preform at the oxidation temperature of 875°C represents a number of oxygen- and nitrogen-based solid solutions.

The phenomenon of bifurcation in the processes of oxidative construction of thin-wall ceramics under heating of solid titanium preforms

On the basis of experimental data, we calculate the diffusion coefficients of titanium and oxygen through rutile, which is synthesized in the process of oxidative construction of thin-wall ceramics (OCTWC), and titanium through a K-layer (a solid solution of oxygen in the metal). It is shown that, at the beginning of the exponential stage, the process rate is determined by the rate of titanium diffusion through the K-layer. Near the bifurcation point, the diffusion conductivities of the OCTWC rutile and K-layer become equal, which leads to a stepwise variation in the process rate.

Manufacturing and X-ray phase investigation into monolithic nitrides of zirconium and hafnium

The technique of synthesis of monolithic samples of zirconium and hafnium nitrides by oxidative construction of thin-walled ceramics is described; the X-ray phase investigation into the obtained material is presented. It is shown that the phase composition of the surface is inhomogeneous along the length of the sample, where regions characterized by an individual and particular set of phase states can be distinguished.

Titanium oxidation at the exponential stage of oxidative construction of thin-walled ceramics

Current views [1–4] are that oxidation of all metals,in particular, titanium, involves diffusion of metalatoms to the metal–oxidizer interface, where they reactwith the oxidizer and generate an oxide film whosethickness increases with time following a parabolic law.This law, as a rule, holds for oxide layer thicknessesfrom several tens of angstroms to several tens ofmicrons. In studies of rutile layers [5–7] (with thick-nesses ranging from hundreds of microns to severalmillimeters) formed during oxidative construction ofthin-walled ceramics (the OCTC process) [8], consid-erable deviations from the parabolic law were found forthe oxidation kinetics of bulk titanium preforms (morethan 1 mm thick). Three oxidation stages were distin-guished. At the first stage, which obeys an exponentiallaw, 20% or more of the metal is oxidized. Then, theprocess enters a “silent stage,” which lasts several hoursto several tens of days; the preform almost does notabsorb oxygen at this stage. However, after a certaintime (we referred to this period as the transient time τ

Improvement of superconductive and mechanical properties of bulk and thin-wall MT-YBCO ceramics in oxygenation

The possibility to oxygenate the YBa2Cu3O7−δ(Y123) structure to 7−δ ≈ 6.9 atoms (which ensures the highest temperature of transition into the superconductive state of this compound) at high temperature (800°C) and relatively low pressure (16 MPa) of oxygen has been first shown. This fact differs from the generally accepted notions of the equilibrium in the given system. It has been found that the MT-YBCO oxygenation at enhanced pressures and high temperatures decreases the oxygenation time and the amount of cracks and increases the twin density in the material, which positively affects the critical current density and mechanical characteristics of the ceramics. The experiments have shown that twins are largely responsible for high density of the critical currents jc and irreversibility fields in the MT-YBCO ceramics. In the case of high dislocation density (1012 cm−2) and low twin density (0–1 μm−1) in the Y123 structure, the critical current turned out to be an order of magnitude lower than in the case of high twin density (22 μm−1) and absence of dislocations and stacking faults. The density of twins and microcracks (parallel to the ab plane) in the structure of the YBa2Cu3O7−δ phase has been found to depend on the size of the Y2BaCuO5 inclusions and the pattern of their distribution, which in turn is defined by the initial materials. A process has been developed of the oxygenation of the thin-wall (cellular) MT-YBCO ceramics under the conditions of the controllable oxygen pressure from 0.5 kPa to 16 MPa and temperatures from 900 to 800°C. The process allows one to attain record high jc values and double the trapped magnetic field as compared to both the bulk MT-YBCO ceramics oxygenated under the same conditions and thin-wall MT-YBCO ceramics oxygenated at atmospheric pressure and optimal temperature.

Titanium-to-rutile oxidation kinetics in the direct-oxidation fabrication of thin-wall ceramics

The oxidation kinetics of titanium preforms for the direct-oxidation fabrication of thin-wall ceramics have been studied in the temperature range 750–1100°C using titanium plates up to 5 mm in thickness. Empirical relations for the kinetics of these processes have been obtained. The preform weight has been shown for the first time to influence the oxidation rate of titanium and the amount of forming rutile, which is inconsistent with the known patterns of metal oxide formation. One possible mechanism of rutile formation during the direct-oxidation fabrication of thin-wall ceramics is considered.

Peculiarities of High-Pressure Oxygenation of MT-YBCO

The oxygenation under the high 16 MPa oxygen pressure allows reducing the duration of oxygenation and in the case of thin-walled MT-YBCO ceramics (with drilled holes) leads to a decrease of the amount of macrocracks in the structure and to an increase in critical current density in the ab-planes. An increase in the twin density enhances the critical current density in the ab-planes of the Y123 textured structure of MT-YBCO. After oxygenation under different conditions the MT-YBCO ceramics produced from a mixture of Y123 with Y211 demonstrated a critical current density in magnetic fields above 2 T at 77 K higher in the ab-planes and lower in the perpendicular c-direction than the MT-YBCO produced from Y123 and Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . Mechanical properties (hardness, fracture toughness, Young modulus) have been estimated for various types of MT-YBCO oxygenated under different conditions. Both traditional and thin-walled ceramics have a higher microhardness after oxygenation under high 16 MPa oxygen pressure.

Oxidative Constructing of Thin-Walled Ceramics (OCTC) at Temperatures above the Melting Point of a Metal: Fabrication of Oxide Fibers from Filaments of Aluminum and Its Alloy

Oxidative Constructing of Thin-Walled Ceramics (OCTC) at Temperatures above the Melting Point of a Metal: Fabrication of Oxide Fibers from Filaments of Aluminum and Its Alloy

Oxidative Constructing of Thin-Walled Ceramics (OCTC)

Modern engineering widely uses high temperatures in an oxygen atmosphere, i.e., in an oxidative environment. This calls for highly oxygen-resistant materials. Metals and metal alloys whose mechanical strength and plasticity are suitable for designing the most sophisticated shapes often do not meet the current demands on thermal stability, especially in the presence of silicates; borates; alkalis; sulfur-, vanadium-, and lead-containing compounds, etc. Oxygen-free ceramics, e.g., metal carbides and nitrides, are more heat resistant; however, even these materials become unstable at high temperatures in oxidative environments and fail after a time to eventually produce oxides. Therefore, using oxide ceramics in high-temperature processes has a crucial advantage. Modern methods for manufacturing materials from oxides eventually involve sintering powders either of these oxides or of their chemical precursors. Sintering imposes severe constraints on the shape of resulting products, especially in fabricating thin-walled profiles. We suggest a fundamentally novel method for creating thin-walled oxide ceramics, based on the controlled oxidation of metallic preforms with retention of the preform’s shape. Metal oxidation is a chemical process addressed in innumerable publications. However, primary emphasis heretofore has been placed on protecting metals from