Mullite needles

Since mullite melts incongruently at 1810 10°C, its composition lies outside its primary phase field. For compositions in the field of mullite with a liquidus temperature below about 1400°C, Schairer and Bowen [4] encountered considerable difficulty in crystallizing this compound. For compositions with liquidus temperatures below around 1200°C, many months were required to form small mullite needles. 

Dear [33] described the mechanism of attack as the solution of first the glass, cristobalite, and other forms of silica in brick, followed by the solution of the finely divided mullite and clay body. The presence of well-crystallized mullite needles at the slag-interface indicated that the liquid at the contact surface was enriched with alumina. 

Another diagnostic test for silicate ash is to treat the particles with hydrofluoric (HF) acid solution (18-20). The acid will dissolve the glassy phase revealing skeletons of crystalline species which may be in the form of mullite needles (Figure 6e) or quartz crystalloids (Figure 6f). The ratio of the glassy phase to... 

Figure 6. Diagnostic features of flame heated ash. a, unfused quartz particle b, elongated silicate particles c, microids on ash d, acid cleaned ash e, mullite needles in ash and f, quartz crystalloids.

Mullite also exhibits a different morphology compared with other crystals. The double einer-chain silicate demonstrates preferred crystallization in a needlelike habit. If crystallization is uncontrolled, undesirable large mullite needles may occur in the glass-ceramic as a secondary reaction in the ceramic-forming process. This type of crystallization can be initiated by surface nucleation. The properties of such a material, its strength in particular, are negatively influenced. 

The clay phase initially shrinks, and fissures frequently appear. Fine mullite needles appear at about 1000°C but cannot be resolved with an optical microscope until temperatures of at least 1250°C are reached. With further increases of temperature, the mullite crystals continue to grow. After firing at temperatures above 1400°C, mullite is present as prismatic crystals up to about 0.01 mm in length. 

Mullite needles growing into a feldspar relict (etched 10 s, 0°C, 40% HF). (a) at lower magnification, (b) at higher magnification. 

The whole operation takes place over three to five days because cooling also has to take place. The microstructure is constituted by Mullite needles in a matrix of glass. If any free silica is present, it may be in the form of quartz, tridymite, or cristobalite. 

The conditions under which a high mullite content is created are obvious from the phase diagram. The first necessary condition is a suitable composition of the system. At low temperatures (up to 1300 °C), ihe system cannot be expected to approach the state of equilibrium. When formed from kaolinite, mullite in this case frequently keeps the pseudomorphous shapes of kaolinite. The typical mullitic forms, usually needle-shaped crystals, arise only at higher temperatures. The rate of formation as we]] as the final shape of its crysta]s may be considerab]y aflected by the impurities present or by the non-clay components of the raw materia] mixture. These components also take part in the formation of the melt which is then produced in higher amounts and at lower temperatures than would correspond to the pure system A1203 —Si02. 

Flame Vitrification and Spheridization of Silicate Particles. The alumino-silicate particles vitrify and take a spherical shape in the flame and are partially recrystallized on cooling. Micro-needles of mullite up to 10 pm long and crystalloids of quartz are the principal devitrification products enveloped in a glassy material matrix. 

Figure 4. SEM of blistered nickel over fired molybdenum via— opened with needle, showing mullite and anorthite.

Phase morphology Exposures to high temperature water vapour had clearly different effects on the materials. In as-received condition typical features of the microstructure of both materials weresmooth, amorphous areas and needle shaped mullite grains on the pore side surface of the binder. On polished cross-sections the pore shape was smooth and spherical, and in many areas the side surface of pore was amorphous. Another common feature was the cristobalite layer on the SiC surfaces next to the amorphous binder. The binder itself contained several typical areas of microstructure in both materials but the completely amorphous areas and mullite in silica rich matrix were common for both. 

The Nextel 720 fiber has both a seeondary phase and elongated grains incorporated into its microstructure. The (—55 volume percent) mullite is present primarily as needles surrounding the AI2O3 grains ( 45 volume percent). Thus, the creep resistance of the Nextel 720 is better than for other oxide fibers. However, the use temperature under a 100 MPa (15 ksi) load (1 percent strain in 1,000 hrs) is lower than 1,200°C (2,192°F). 

Ft. 2 suggests formation of a mullite phase surrounding the grain, since mullite (3Al203.2Si02) has an atom ratio Si Al 0 = 9.5% 28.6% 61.9%. The formation of the mullite phase was confirmed by XRD. The needles indicated by pt. 1 are most probably Si02. 

After 1200 °C oxidation (see Z-1200), there were large numbers ofwhite needlelike oxides of less than 1 pm in length on the surface of Zl-1200. Combining the EDS analysis and XRD result, we are convinced that the needle-like oxides should be mullite crystals which grew in amorphous Si02. Clearly, no oxide crystals were seen on the surface of Z2-1200, except for large numbers of white spots which should be mullite oxide as same as the needle-like crystals in Zl-1200. However, their size was smaller than that in Zl-1200. For Z3-1200, we can see that the surface of the specimen was oxidized considerably. Combining the element analysis with the result of XRD, we can conclude that the oxide is mullite. 

When the oxidation temperature was increased to 1400 ° C (Z-1400), the needlelike mullite crystals in Zl-1400 grew up to more than about 10 pm in length. In Z2-1400, the mullite showed two kinds of shapes, needle-like crystals and agglomerates. The needle-like crystals were same as those in Zl-1400, and the agglomerates were same as those in Z3-1400. The whole surface of Z3-1400 is covered with the scale consisting of agglomerates of mullite. 

Above 1100 °C the rearrangement of the defect spinel into amorphous silica and mullite accelerates with the mullite becoming progressively richer in aluminium up to 1500 C. On cooling the mullite crystallises into needles, which are embedded in an amorphous aluminosilicate glass. It is a very hard, abrasive mineral, which is virtually inert to all chemicals and environments. 

Figure 3.31 Scanning electron microscopy images of porcelain-like stoneware, (a) Vitreous China ware from Sawankhalok, Thailand (15th century C.E.), showing primary platy mullite (Mu I), secondary needle-shaped mullite (Mu II), and a partially dissolved residual quartz grain (Qz)...

Needle-like mullite particles are more attractive for applications like reinforcement in high temperature structural components. Needle-like mullite samples prepared from... 

Strength is due to the interlocking of long, needle-like crystals. Some authorities have stated that to get long crystals there mnst be present impurities which will promote then-growth, but a mullite body with short, interlocking crystals is more stable to load-deformation at high temperatures. 

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