Mica Glass-ceramics Research Articles

Crystal morphology engineering in SiO 2–Al 2O 3–MgO–K 2O–Na 2O–F − mica glass-ceramics

The morphology of mica crystals formed in extruded glass-ceramics of the system SiO 2–Al 2O 3–MgO–K 2O–Na 2O–F 2 was characterized using transmission electron microscopy. Detailed information on the crystal shape could be obtained since mica crystals in the extruded material exhibit an extraordinary degree of orientation. Highly defective mica plates start growing in coexistence with norbergite from a Mg 2+- and F −-depleted glass. Upon the dissolution of norbergite at elevated temperatures, defective plates are sandwiched between 1M polytype phlogopite slabs epitaxially formed on {001} faces of the defective mica crystals. As shown by means of several examples, depending on the schedule of heat treatment the thickness of the defective slab as well as the diameter ratio of inner and outer plates can be adjusted. Linear growth rates are given for defective and non-defective mica plates at different temperatures enabling the controlled engineering of the mica crystal morphology.

Effect of Stress Cycling on Fatigue Behavior of Mica Glass Ceramics and Alumina.

In order to investigate the effect of cyclic stress on fatigue behavior of mica glass ceramics and alumina, the specimens with an indentation induced flaw were tested under static tensile loading and cycling loading at stress ratio R=0 and -1. The frequencies in the cyclic loading tests were 5 and 20 Hz respectively. The main results were as follows ; (1) A significant decrease in fatigue lifetime by cyclic loading was observed on both materials, though there being large scatters on mica glass ceramics. (2) An appreciable decrease in fatigue lifetime due to the increase of frequency was observed on mica glass ceramics, while a little effect of frequency was observed on alumina. (3) The effect of stress ration under cyclic loading was small on both materials, though a somewhat significant decrease in fatigue lifetime being observed in the case of frequency of 20 Hz on alumina. (4) SEM fractography revealed a dominant interglanular fracture on both materials, but in the case of cyclic fatigue fracture of alumina more comminution was observed than in the case of static fatigue fracture. (5) It was considered that the decrease in fatigue lifetime due to cyclic loading in mica glass ceramics results on the degradation of grain briding effect by the pull-out of mica from glass matrix, while the effect of cyclic stress in alumina results on the degradation of grain bridging effect by comminution and fretting in the grain.

Origin of unusual fluorophlogopite morphology in mica glass-ceramics of the system SiO 2–Al 2O 3–MgO–K 2O–Na 2O–F 2

Extruded mica glasses were exposed to various heat treatments to convert them into glass-ceramics exhibiting anisotropic properties. In dependence on duration and temperature of the treatment, different morphologies of the mica crystals were observed. The TEM study proved the crystallisation of fluorophlogopite formed in the glass system used to be a two-stage process. In coexistence with an intermediate phase (norbergite, Mg 3F 2SiO 4), highly defective and magnesium depleted phlogopite plates are formed in the first step. Upon dissolution of norbergite, epitaxial growth of non-defective and nearly stoichiometric fluorophlogopite along the c *-direction occurs on both sides of the defective slab in the second step. The morphology of the resulting sandwich structure is sensitively dependent on the tempering conditions. The origin of this effect is discussed based on the analysis of the defects observed in the inner part of the sandwich.

Effect of the aspect ratio of mica crystals and crystallinity on the microhardness and machinability of mica glass-ceramics

The change in the microhardness and machinability of mica glass-ceramic is related closely to its microstructural parameters. The aspect ratio of the crystals, the volume crystallinity and the spatial arrangement of the particles must be considered in order to be able to estimate the characteristics of the material. With a high aspect ratio and crystallinity, the microhardness decreases because of high connectivity. By introducing the effective crystallinity, indicating the effectiveness of disk-like crystals in forming a connected structure of crystals, the variation in microhardness can be explainable. A steep decrease in the microhardness of mica glass-ceramics occurs due to the connection of the mica crystals, leading to good machinability.

Compatibility of Mica Glass-ceramics as Gas-sealing Materials for SOFC

Compatibility of Mica Glass-ceramics as Gas-sealing Materials for SOFC

Mechanical Properties of Mica Glass‐Ceramics

The mechanical properties of fluorophlogopite mica‐based glass‐ceramics, such as fracture toughness, flexural strength, and machinability, were investigated. It has been shown that toughening increments in the glass‐ceramics occurred by crack deflection and branching by crystals with a high aspect ratio. All the glass‐ceramics heat treated at 1000°–1150°C exhibited a higher fracture toughness of 1.2–2.2 MPa‐m1/2 as compared to 0.8 MPa‐m1/2 for the parent glass, and showed average flexural strengths of 140–160 MPa. It has been suggested that (H/KIc)2 of the mica glass‐ceramic be used to estimate the machinability, because it decreases linearly with machinability.

A comparative evaluation method of machinability for mica-based glass-ceramics

The machinability of mica glass-ceramics is evaluated using a tool dynamometer. Several samples with different chemical compositions and microstructures were tested in turning operations using TiCN cermet tools. The cutting rate dependence of specific cutting energy has been studied to find a simple method for the evaluation of machinability. The mechanical strength, the surface roughness of the machined surface and the fracture toughness were measured to support the machining behaviour. For the determination of machinability, the specific cutting energy at low cutting rate conditions, neglecting an elastic impact effect, and the slope of the log-log plot of the specific cutting energy versus cutting rate were considered as the reasonable parameters. These results are correlated with the microstructure and the hardness of the workpiece. In particular, the microhardness of the sample is shown to control the cutting characteristic.

Application of Mica Glass-ceramics as Gas-sealing Materials for SOFC

Application of Mica Glass-ceramics as Gas-sealing Materials for SOFC

Material Design of Shape-Memory Ceramics.

The authors have studied the shape-memory phenomena of ceramics, mainly of glass ceramics. They found that most polycrystalline ceramics exhibit shape-memory phenomenon. The starting temperature of shape recovery corresponded to the starting temperature of stress relaxation in every shape-memory ceramics. These experimental results showed the shape recovery process as follows. If a specimen is deformed in the high temperature range and cooled to the room temperature and released, the viscosity change of the glass matrix or boundary restrains the elastic deformation of the crystal. The specimen in this condition can recover its original shape upon reheating. The authors also found that two factors of mica glass ceramics enabled the network of mica crystals to behave like a high compliance mechanical spring system. One is the "card-house-type" microstructure ; the other is the connected form of mica crystals. This is why mica glass ceramics can recover their shape 5 to 20 times better than ordinary polycrystalline ceramics. Based on these results, this paper describes a guideline for designing the shape-memory ceramics and shape-memory composite materials.

その場観察によるマイカ・ガラスセラミックスの形状回復機構の検討

その場観察によるマイカ・ガラスセラミックスの形状回復機構の検討

High‐Strength Mica‐Containing Glass‐Ceramics

Glass‐ceramics containing barium–mica in the system Ba0.5 Mg3 (Si3AIO10)F2–2MgO · 2Al2O3· 5SiO2–Ca3 (PO4)2 are two to three times stronger than conventional mica‐containing glass‐ceramics. Moreover, the barium‐mica glass‐ceramics are easier to machine, as confirmed by a drilling test using conventional steel tools. Such mechanical properties are attributable to the microstructure of the barium–mica glass‐ceramics. Very fine, interlocking mica crystals are precipitated in the glass, and a crack‐deflection mechanism is observed by scanning electron microscopy.

The effect of fluorine source and concentration on the crystallization of machinable glass-ceramics

Abstract Transparent and clear glasses were prepared from the MAS system (based on MgO-Al 2 O 3 -SiO 2 ) with different fluorine contents. Their crystallization behaviour to giving mica glass-ceramics upon heat treatment was studied as a function of the source of fluorine and its concentration. The predominant crystalline phase in these glass-ceramics is fluoborite, norbergite, or fluorphlogopite, depending on the heattreatment temperature, fluorine source, and concentration. An interlocking microstructure of fluorphlogopite in a glassy matrix is observed above 870°C.

結晶化ガラスと焼結セラミックスの形状記憶現象

The shape recovery property of some glass-ceramics and sintered ceramics has been investigated with the high temperature range torsion test apparatus. The results are summarized as follows.(1) The shape memory phenomena occurred in both β-spodumene glass-ceramics and 2ZnO-B2O3 glass-ceramics. This shows that the shape memory phenomenon of ceramics is not caused by the peculiar characteristics of mica crystal.(2) The sintered ceramics such as mica (KMg3AlSi3O10F2), silicone nitride (Si3N4), silicone carbide (SiC), zirconia (ZrO2) and alumina (Al2O3) brought the similar phenomenon, indicating that the shape memory phenomenon of ceramics is not limited to glass-ceramics generated by the special manufacturing process, but found in the material which contains little glass phase.(3) The correspondence between the temperature range for the occurrence of shape memory phenomenon and the stress relaxation temperature range is recognized in all the materials studied in this paper.(4) The critical prestrain, within which the 100% shape recovery occurs, is about 0.1% with β-spodumene glass-ceramics, silicone nitride and zirconia. This value is 1/5 that of mica glass-ceramics.

ChemInform Abstract: Preparation and Characterization of Machinable Mica Glass‐Ceramics by the Sol‐Gel Process.

AbstractMica glass‐ceramics are prepared by the sol‐gel process using KF, MgSiF6, Al(OiPr)3, Si(OEt)4, and Mg(OMe)2 as starting materials.

マイカ・ガラスセラミックスの形状記憶現象と応力緩和挙動

Using a high temperature range torsion test apparatus, the shape recovery property of the three mica glass-ceramics are clarified. These mica glass-ceramics investigated have almost equal microstructures of mica/glass mixture but are different in chemical composition of the glass matrix. Moreover, the difference in the recovery behavior of the mica glass ceramics is investigated. The results are summarized as follows:(1) It was found that the shape memory phenomenon occurs in the mica glass-ceramics called ‘MACERITE’ or ‘PHOTOVEEL’ which are different from ‘MACOR’ in chemical composition of the glass matrix.(2) The pre-deformation temperature range and the shape recovery temperature range, in which the shape recovery property is best, changes by the chemical composition of the glass matrix.(3) The effect of the prestrain and the prestrain rate on the shape recovery property does not change so much by the chemical composition of the glass matrix.(4) The stress relaxation temperature range corresponds to the shape recovery temperature range in all cases.(5) The thermal expansion curves show that corresponding to the shape recovery temperature range, there is a break point which suggests the glass transition point of the glass matrix in all of the three mica glass-ceramics.

Preparation and characterization of fluorphlogopite-fluormuscovite mica glass-ceramics

Transparent and clear glasses were prepared from fluorphlogopite-fluormuscovite system mixtures. Mica crystals were precipitated in the glass by later heat treatment. The mica crystals were identified as fluorphlogopite by x-ray powder diffraction patterns. An interlocking microstructure of mica was observed at above 800°C. The density of crystallized glass increased with increasing heat treatment temperatures. The density attained a maximum at about 900°C.