High Thermal Expansion Mismatch Research Articles

박막공정의 융합화를 통한 초소형 고체산화물 연료전지의 제작: I. Spray Pyrolysis법으로 증착된 Ni 기반 음극과 스퍼터링으로 증착된 YSZ 전해질의 다층구조

Physical properties of sputtered YSZ thin film electrolytes on anode thin film by spray pyrolisis has been investigated to realize the porous electrode and dense electrolyte multilayer structure for micro solid oxide fuel cells. It is shown that for better crystallinity and density, YSZ need to be deposited at an elevated temperature. However, if pure NiO anode was used for high temperature deposition, massive defects such as spalling and delamination were induced due to high thermal expansion mismatch. By changing anode to NiO-CGO composite, defects were significantly reduced even at high deposition temperature. Further research on realization of full cells by processing hybridization and cell performance characterization will be performed in near future.

Manufacture and properties of AlON–TiN particulate composites

Abstract The mechanical properties and microstructure of different content of Titanium nitride (TiN) particulates added Aluminum oxynitride (AlON) ceramic composites were studied by using DSC, SEM, TEM, XRD techniques. AlON and TiN were the only main phases in this dense material. No reaction was noticed between the AlON matrix and TiN particles. The TiN particles can delay the decompostion temperature of AlON’s. The mechanical properties increases with an increasing TiN content. The extensive microcracking commonly existing around the very large TiN grains, due to the high thermal expansion mismatch between AlON and TiN, was considered to have brought about positive effects on the strength of the composites materials and on toughening. The mechanical properties increases with an increasing sintering temperature, due to compactness of the composites, then degraded because of enormous grain growth of AlON and TiN. Finally, possible processes were discussed for the improvement of the properties of AlON–TiN composites.

Spacers Alternatives for INTEGRATION OF (3D) STACKED SBT FeCAPs

The thermal and mechanical stability of Ir and Ir\\Pt metals spacers deposited on top of Ti(Al)N\\Ir\\IrO2 patterned structures has been investigated in pseudo 3D stacked SrBi2Ta2O9 (SBT) capacitors. Their stability was compared to standard TEOS spacers. The high compressive stress at the edge of patterned electrodes, as a consequence of the high thermal expansion mismatch between the metals used in the electrode and TEOS, make the system mechanically unstable at the SBT crystallization conditions (700°C for 1 hour). The mechanical problems could be overcome if the same noble metals used in the electrode are incorporated as spacers. However, thermal stability during the SBT crystallization conditions is still an issue. For the case of Ir, surface oxidation decreases the SBT polarization values. In the case of Ir\\Pt, Ir diffuses through Pt and oxidizes, leading to unstable patterned structures and to the oxidation of the Ti(Al)N layer.

Resistance of Alumina‐Spodumene Ceramics to Thermal Shock

A processing strategy is presented for obtaining alumina‐spodumene ceramics with controlled microstructures from a new, low‐cost alumina powder and spodumene (Li2OAl2O3·4SiO2) mineral, both being produced in Western Australia. The addition of 15 wt%β‐spodumene to alumina is used to produce ceramics with (i) high thermal expansion mismatch and (ii) a glassy phase to aid in liquid‐phase sintering. Specifically, the effects of spodumene addition and grain size on tolerance to thermal shock are addressed. The thermal shock resistance of the alumina‐spodumene ceramics is evaluated by water quenching and subsequent three‐point bend testing of strength diminution. Comparisons are made with results from parallel experiments conducted using a pure monolithic alumina ceramic. The reference alumina shows the expected substantial strength losses when thermally quenched from ∼200°C above room temperature. By contrast, the alumina‐spodumene ceramics, while displaying reduced strength relative to the reference aluminas, exhibit minimal strength degradation under severe thermal shock conditions.

Microcrystalline and Nanocrystalline Diamond Film Deposition on Cobalt Chrome Alloy

AbstractThe cobalt chrome alloy Co-28Cr-6Mo is widely used in human joint replacement applications. These joints are highly sensitive to wear and are usually replaced after approximately ten years of use. In order to extend these implants' service lifetimes, a thin film of diamond may be applied to the implant wear surfaces by microwave plasma chemical vapor deposition (MPCVD) following MPCVD nitridation. Diamond films often delaminate from cobalt chrome due to a high thermal expansion mismatch. Additionally, under most conditions diamond films degrade into graphite by dissolution of nuclei into solvents like Co and Cr. By nitriding the cobalt chrome through MPCVD prior to diamond deposition, a usable diamond film may be achieved. It may be possible to merge both the nitriding and deposition steps into a single process, since both nitridation and deposition will be performed by MPCVD. In addition, controlled amounts of nitrogen in hydrogen/methane plasma under CVD conditions result in a nanostructured diamond coating. We have investigated the formation of both micro- and nanocrystalline diamond formation on cobalt chrome.

TEM study of Y-doped α-SiAlON composite with 10 vol% TiN particulates

Abstract Microstructures of the 10 vol% TiN particulates added Y-doped α -SiAlON ceramic composites were studied by using TEM, HREM and EDS techniques. α -SiAlON and TiN were the only main phases in this fully dense material. Less than 1 vol% amount of glass was detected and the majority of the glass was located at the triple grain boundary junction nodes while the boundaries remained clean. No reaction was noticed between the α ′ matrix and TiN particles, but some TiN particles, included within the matrix grains, were epitaxial with α -SiAlON. The extensive microcracking commonly existing around the very large TiN grains, due to the high thermal expansion mismatch between α ′ and TiN, was considered to have brought about negative effects on the strength of the composite materials and on toughening. The porosity and agglomeration of TiN particles resulting from local inhomogeneity of the liquid during the sintering process further decreased the mechanical properties of the composites as they acted as cracking sources. Finally, possible processes were discussed for the improvement of the properties of α -SiAlON/TiN composites.

Toughening mechanisms for a zirconia-lithium aluminosilicate glass-ceramic

The mechanical properties of a lithium aluminosilicate glass-ceramic and the same glass-ceramic containing 5 and 15 wt% zirconia were investigated. The aim of the study was to assess the contributions to toughening from various toughening mechanisms. For the zirconia-containing compositions, zirconia initially precipitated, upon heat treatment of the glass, as tetragonal zirconia (t-ZrO2), and upon further heat treatment, transformed to monoclinic zirconia (m-ZrO2). This transformation could also be induced by grinding samples containing t-ZrO2. By heat treating, the fracture toughness of all compositions increased with increasing matrix grain size until the matrix grain size exceeded ∼ 1 μm, whereupon both the fracture toughness and strength decreased sharply. The matrix phases, lithium metasilicate and β-eucryptite, have either high thermal expansion mismatch or high thermal expansion anisotropy resulting in large thermal stresses. The initial toughness increases observed in each composition were attributed to the formation of a microcrack zone around the propagating crack. At larger grain sizes, thermal stresses caused spontaneous cracking and loss of strength. Zirconia additions also contributed to the fracture toughness improvement; however, the predominant toughening mechanism was not by transformation but due to crack deflection by the stress fields around the transformed, i.e. m-ZrO2, particles.

Thermal mismatch study in a ceramic to metal joint using the finite element method

Ceramic materials are an attractive choice in many applications due to their excellent high temperature properties, light weight, and corrosion resistance. Due to the high cost of manufacturing and fabrication of ceramics, conventional metallic materials are an integral part of most typical ceramic based engineering applications. Consequently, to take advantage of both material strengths, there must be a joint between the ceramic and metal components. The high thermal expansion mismatch between ceramic and metal components and temperature dependent material properties of each component make the joining problem a complex one. In this ceramic-to-metal joint study, finite element analysis was used to identify significant parameters controlling stresses arising from thermal expansion. The simulation is, however, restricted to a brazed joint between silicon nitride and 4140 annealed steel, which has an extremely high thermal expansion mismatch. In order to reduce the effects of this thermal expansion mismatch, a number of real and fictitious materials have been studied for their potential as compliance layers. This systematic simulation of ceramic-to-metal joints has provided insight to the effects of Young's modulus (YM), thermal expansion coefficient (TEC), thickness of individual layers, and their specific arrangement between the ceramic and steel components. It was shown by simulation that a crack in the ceramic to metal joint changes the stress distribution drastically. Consequently, it is indispensable to include the simulation of cracks in a ceramic-to-metal joint in order to avoid erroneous conclusions. A simple one dimensional design formula was developed for the ceramic-to-metal joint. This formula can be used to screen/select materials for compliance layers and help in reducing the number of materials to be tested to finalize the choice of compliance layers.