Evaluation of thermal shock resistance of cordierite honeycombs

A comparative study on thermal shock resistance (TSR) of extruded cordierite honeycombs is presented. TSR is an important property that predicts the life of these products in thermal environments used for automobile pollution control as catalytic converter or as diesel particulate filter. TSR was experimentally studied by quenching (descending test) the heated samples to water or by heating (ascending test) with an oxyhydrogen flame along with crack detection by acoustic emission (AE) method. TSR was also calculated by using coefficient of thermal expansion (CTE), modulus of elasticity (MOE) and modulus of rupture (MOR) of the honeycomb samples. Cordierite honeycombs of 200 and 400 cpsi were used for the above study. It was observed that the trends of TSR were same for both the experimental methods as well as by calculation. The ascending test method showed lower TSR values compared to water quench method due to early detection of cracks by AE. Finite element method (FEM) was also used to evaluate the thermal stress distribution in solid cordierite using thermal shock test data. It was observed that the maximum thermal stress calculated by FEM was lower than the strength of the material; therefore, the solid cordierite did not fail during such tests.     

Optical properties of polycrystalline and amorphous cordierite

Cathodoluminescence and infrared absorption studies were made on polycrystalline as well as amorphous cordierite, either in the as-received state or after different thermal treatments. Emission bands centred at about 400 nm were observed in all the samples studied. Red emission bands around 650 nm were also found in samples irradiated with ionizing radiation or annealed in different atmospheres. Infrared absorption measurements were performed to estimate the glass phase and to monitor the presence of OH^– ions.     

K2O杂质对铝型材厂工业废渣合成的堇青石材料晶相结构的影响

主要探讨K2O杂质对堇青石形成及晶相结构的影响.采用XRD和SEM确定各试样的晶相结构和显微结构.实验结果表明:K2O杂质含量从0.23~1.1wt%,形成的堇青石晶相量较多,其中K2O杂质含量为1.1wt%时,堇青石含量高达90%,当K2O含量继续增加时,形成的堇青石量开始减少,确定试样中K2O杂质最高允许含量为1.1wt%.plus软件分析结果:K2O杂质对堇青石的晶胞结构影响不大,晶胞参数变化很小.     

Thermal expansion behavior of plasma sprayed cordierite

Bulk cordierite has unusually low thermal expansivity. We here report on a study of the thermal expansion behavior of cordierite formed by plasma spraying. It is found that the thermal expansivity is strongly influenced by crystal structure and that the thermal expansion coefficient of the high-cordierite structure is very low (1.84 × 10⁻⁶/°C). Furthermore, the as-sprayed cordierite expands 27% in volume when it is placed next to zirconia in a high-temperature furnace. This expanded structure results in even lower thermal expansivity in the later thermal cycling.     

堇青石的二氧化硅涂敷实验研究

本文采用不同的酸对蜂窝式堇青石进行处理,研究酸蚀处理对其组成和孔结构的影响。并以自制的二氧化硅溶胶为第二载体对酸处理后的堇青石进行涂敷。采用XRD、N2等温吸附-脱附、SEM等手段对二氧化硅进行表征。结果表明:酸蚀处理并不改变堇青石的晶相结构;经酸处理后的堇青石其比表面积有不同程度的增加;而经二氧化硅涂敷后的堇青石比表面积可达93.35m2/g。     

Microstructure and mechanical properties of cordierite ceramics toughened by monoclinic ZrO2

The effects of m-ZrO₂ addition on the mechanical behaviour of the cordierite ceramics were studied. Below 10 vol % of m-ZrO₂ content, zircon (ZrSiO₄) was formed as a second phase at the expense of all m-ZrO₂ due to the reaction between ZrO₂ and silica in the cordierite. The sintered density was improved with ZrO₂ addition through glass phase formation along grain boundaries, and maximum sinterability was obtained at 4.6 vol % m-ZrO₂. When sintered at 1300 °C for 4 h, the flexural strength at 4.6 vol % ZrO₂ was 190 MPa, compared with 55 MPa for the pure cordierite. Fracture toughness was gradually enhanced from 1.75 MPa m^1/2 to 2.4 MPa m^1/2 with m-ZrO₂ addition up to 10 vol %, which could be explained partly by thermal expansion mismatch between cordierite and the second-phase ZrSiO₄ and partly by crack deflection at the cordierite-ZrSiO₄ interfaces.     

Investigation the effect of sintering temperature on Young’s modulus evaluation and thermal shock behavior of a cordierite–mullite based composite

A marked mismatch between CTE of cordierite and mullite in composition develops internal stress, which causes significant growth of cracks and subsequent damages which confer them a very good ability to thermal shock resistance. To investigate the effect of sintering temperature on the thermal shock resistance, the samples sintered at four different temperatures and evaluation of Young’s modulus monitored during 25 shock cycles. The results showed that densification behavior, thermomechanical properties and thermal shock resistance of this refractory was closely related to sintering temperature. Furthermore, during specific (5th to 10th) thermal-shock cycles a notable increase occurs in the Young’s modulus of the samples, this attributed to the formation of viscose bridges, which shield the tip of growing cracks.     

High-temperature deformation of cordierite glass-ceramic/SiC platelet composites

Composites based on barium-containing cordierite glass-ceramic matrices reinforced with 20 and 30 vol% SiC platelets were fabricated by hot-pressing. The ceramed specimens were tested in compression at 1000, 1200 and 1300 °C in air, parallel and perpendicular to the hot-pressing direction. Compressive fracture stresses up to 460 MPa were recorded at 1000 °C. Lower stresses were observed at 1200 and 1300 °C. The compressive stress decreased with increase in the SiC platelet content and with the compressive direction being parallel to the hot-pressing axis. The mechanical behaviour at high temperatures was related to the presence of a residual glassy phase within the cordierite matrix, as well as to the mode of the crack nucleation within the composites.     

Synthesis of cordierite monolithic honeycomb by solid state reaction of precursor oxides

The synthesis of cordierite monolithic honeycombs by solid state reaction at high temperature of alumina, kaolin, talc and silica has been studied. The synthesized honeycombs have been characterized by DTA and XRD. The crystal composition of the samples have been observed to be very dependent on temperature and time of reaction. Thermal treatment of the monolithic precursors at 1400°C for 30 minutes allows formation of high purity cordierite with high crystallinity, maintaining the desired monolithic structure of the honeycombs.     

Effect of aluminum nitride on the properties of cordierite

The low dielectric constant and thermal expansion coefficient of cordierite are two attractive properties for the material to be used in electronic applications. However, compared to some other materials such as alumina, aluminum nitride (AlN), and beryllium oxide, it has a much lower mechanical strength and thermal conductivity. In the present work, AlN-cordierite composite systems are fabricated using different AlN contents, and the effect of AlN content on the mechanical, thermal, and dielectric properties of the AlN-cordierite system are investigated. It was observed that there exists an optimum AlN content for the mechanical and thermal properties while the dielectric property decreases with an increase of AlN content. An explanation for the observed data trend is offered based on an effective void concept.     

ESR study of CuO-doped cordierite

The reactivity of cordierite precursor with CuO powder was studied by means of X-ray diffraction measurements and electron spin resonance spectroscopy. Two concentrations of CuO were used: 10 and 20 mol %. An ESR signal was detected after heating the mixture of cordierite precursor-CuO at 500°C showing that CuO begins to dissolve with the cordierite precursor. The reactivity strongly increased between 700 and 800°C, in relation to the transition amorphous -cordierite. However, the crystallization of -cordierite prevented the dissolution. Cu²⁺ was located in the amorphous phase and characterized byg=2.35 andg=2.08.     

Thermal expansion behaviour of unidirectional carbon-fibre-reinforced copper-matrix composites

Continuous carbon-fibre-reinforced copper-matrix composites prepared by diffusion bonding technology have been used for investigation of the coefficient of thermal expansion. For reasons of economy and ease of availability, continuous Torayca T300 fibres have been used for sample preparation. They were coated continuously with copper (galvanically and then chemically) and unidirectional composites were prepared by diffusion bonding in vacuum at 873 K for 30 min. The linear coefficient of thermal expansion (CTE) of samples with different volume fractions of carbon fibres was measured in directions parallel and perpendicular to the fibre direction. The samples were heat-treated for one temperature cycle in the range 293–573 K or cycled three times in a temperature range from 253 to 573 K. Measured CTE values are compared with those predicted by the well-known Schapery model and the model derived by Kural and Min. Better agreement was achieved with the predictions of the longitudinal CTE of the composite. Prediction of the transverse CTE was more difficult because of a lack of knowledge of the transverse CTE of carbon fibres. Models including the transverse CTE of carbon fibres (Kural–Min) gave better results for prediction of the transverse CTE of the unidirectional composite.     

Study on properties of forsterite/cordierite ceramic composites

The forsterite/cordierite ceramic composites are prepared by standard ceramic method, which properties and microstructures are characterized by X-ray diffraction and Scanning electron microscopy. It is found that the values of volume resistivities rapidly decline from 1 × 10¹³ to 10³ Ω cm as the testing temperatures increase from 20 to 600 °C and the resistivity transition temperatures of forsterite, cordierite and their composites are at about 300, 200 and 250 °C, respectively. The values of ε _r and tan δ are somewhat independent of the temperatures between 20 and 200 °C, but increase rapidly between 200 and 600 °C. The thermal expansion coefficient of ceramic composites decline with the cordierite content increasing and could change from about 2.5 × 10^?6–10.5 × 10^?6 °C^?1, in which the major phases are Mg₂SiO₄ and Mg₂Al₄Si₅O₁₈.     

Effect of cerium addition on phase transformation and microstructure of cordierite ceramics prepared by sol-gel method

Low-temperature sintering of cordierite ceramic depends on the phase transformation into cordierite and the properties depend on its microstructure. In the present work, the effect of cerium on the phase transformation and microstructure of cordierite ceramics prepared by sol-gel method is studied by X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM) in order to lower the sintering temperature and improve the properties of cordierite ceramic with the addition of cerium. It is observed that the cerium addition obviously lowers the crystallization temperature of -cordierite while slightly raises that of -cordierite. The lowest temperature for cordierite transformation, which approaches the crystallization temperature of -cordierite, is achieved in the sample containing 4 wt% of cerium, implying a possibility to lower the sintering temperature of cordierite ceramics. The Ce-contained ceramics show a biphasic microstructure that is dependent on sintering temperature. Sintered below 1300°C, a cordierite-CeO₂ microstructure is present; while sintered at the temperature above 1300°C, appears a cordierite-glass microstructure, of which the amount of glass phase is limited to a small extent. Since the addition of 4 wt% cerium to this MgO-Al₂O₃-SiO₂ system substantially enhances the densification of cordierite ceramics and lowers the sintering temperature to the level of around 1000°C, it makes the ceramics suitable for such applications, where the low-temperature sintering is required, as the substrates for electronic circuit and the catalytic supports (with oxygen storage capacity) for cleaning of automotive exhaust emissions.     

Structural study of a K-substituted synthetic cordierite

The crystal structure of a K-substituted hexagonal cordierite with composition K_0.17Mg₂Al_4.17Si_4.83O₁₈ has been solved by single crystal X-ray techniques. The K atoms have been clearly located inside the channels of a typical cordierite framework. They are statistically distributed among the four positions of a 4e ($\text{0, 0,}\text{z}\text{;}\text{z}\text{? 0.15}$) site. This original location was confirmed by X-ray analysis of a powder sample with composition K_0.5Mg₂Al_4.5Si_4.5O₁₈. A very convenient semi-quantitative test to determine the presence and content of K in cordierite is proposed.     

The role of TiO2 and composition in the devitrification of near-stoichiometric cordierite

Devitrification behavior and thermal expansion of glasses and glass-ceramics, doped with TiO₂, near the stoichiometric cordierite composition were investigated. The activation energy for growth of surface nucleated crystals was shown to be approximately 433 kJ/mol. and was independent of TiO₂ content in the glass. Volume nucleation was achieved by the addition of approximately 8 wt% TiO₂, but the mechanism of volume nucleation was different depending upon cordierite composition. Regions in the MgO-Al₂O₃-SiO₂ phase diagram near the stochiometric cordierite compound were investigated using thermal analysis, X-ray powder diffraction and dilatometry. Glass in glass phase separation was postulated for MgO-lean glasses, whereas precipitation of mullite and Al₂TiO₅ preceded devitrification in other compositions. In each case, the formation of a silica-rich glass is believed to initiate the devitrification. Coefficients of thermal expansion for the glass-ceramics increased with increasing TiO₂ content resulting from increasing levels of uncrystallized glass and the formation of mullite and rutile during crystallization.     

Controlled crystallization and characterization of cordierite glass-ceramics for magnetic memory disk substrate

Glass-ceramics containing cordierite (2MgO-2Al₂O₃-5SiO₂) as a crystal phase based on the glasses in the system MgO-Al₂O₃-SiO₂ were investigated for the application to magnetic disk substrate for higher storage capacity. Parent glasses were prepared with CeO₂ addition as a flux and were crystallized by a controlled 2-step heat treatment The maximum nucleation and crystal growth rates were 2.4 × 10⁹/mm³ · hr at 800 °C and 0.3 m/hr at 915°C respectively. Only - cordierite was precipitated after heat-treatment. After nucleation at 800 °C for 5 hours prior to crystallization at 915 °C for 1 hour, the resulting crystal volume fraction and crystal size were 17.6% and 0.3 m, respectively. Heat-treated specimens through the above condition showed the optimum properties for magnetic memory disk substrates as follows.; Bending strength of 192 MPa, Vickers hardness of 642.1 kg/mm², thermal expansion coefficient of 39 × 10^–7/K and surface roughness of 27 Å.     

Effect of mixing ratio on bactericidal action of MgO–CaO powders

MgO–CaO powders were prepared with different molar ratios (MgO/CaO) at 1400 °C for 2 h in air. By using the powder samples obtained, the change in bactericidal effect as a function of MgO–CaO composition was studied by colony count method. From the XRD measurements, it was found that CaO solid solution was formed by the replacement of Mg²⁺ ion with larger Ca²⁺ ion. However, no formation of MgO solid solution was observed. The average particle size and the specific surface area of the samples used in this study were about 0.2 m and 10.5m²g^–1, respectively. The pH values of physiological saline containing powder samples increased with the increase of CaO content, and the value reached 12.1 in sample with the molar ratio (MgO/CaO) of 0.25. From the results of bactericidal tests for Staphylococcus aureus, it was found that the bactericidal effect increased with the increase of CaO content in the samples.     

The crystallisation of cordierite glass

A study has been made of the crystallisation of a glass near the cordierite composition (2MgO.2Al₂O₃.5SiO₂) using the techniques of differential thermal analysis and multiple attenuated total reflectance infra-red spectrophotometry. An extensive review of the theoretical aspects of glass crystallisation, and in particular the crystallisation of cordierite-type glasses, was undertaken and is presented as part 1 of this work.     

Effect of cycled combustion ageing on a cordierite burner plate

A combination of ⁵⁷Fe-Mössbauer spectroscopy and X-ray Powder Diffraction analysis has been employed to study modifications in chemical and mechanical stability occurring in a cordierite burner aged under combustion conditions which simulate the working of domestic boilers. Mössbauer study shows that Fe is distributed into the structural sites of the cordierite lattice as Fe²⁺ and Fe³⁺ ions located mostly at octahedral sites. Ferric oxide impurities, mainly hematite, are also present in the starting cordierite material accounting for ?40% of the total iron phases. From Mössbauer and X-ray diffraction data it can be deduced that, under the combustion conditions used, new crystalline phases were formed, some of the substitutional Fe³⁺ ions existing in the cordierite lattice were reduced to Fe²⁺, and ferric oxides underwent a sintering process which results in hematite with higher particle size. All these findings were detected in the burner zone located in the proximity of the flame and were related to possible chemical reactions which might explain the observed deterioration of the burner material.