The Effect of Sintering Temperature on the Structure and Properties of Calcium Hexaluminate/anorthite Ceramics

The calcium hexaluminate/anorthite composite are prepared by magnesia slag, kaolin and industrial alumina at different sintering temperature. The effect of sintering temperature on the structure and properties are investigated by X-ray powder diffraction and scanning electron microscopy. The results show that the calcium hexaluminate begins to form at 1300 C and continues to form in a large amount between 1400 C and 1500 C, however, there is a phase transitions form gehlenite to anorthite between 1300 C and 1400 C. CA6 grow preferentially along the basal plane direction. When the crystals touch each other, the CA6 crystals turn to grow up along the axial direction. The apparent porosity decreases and bulk density increases and the flexural strength increases with the rising of the sintering temperature.


Introduction
Since 21 st century, the energy issue has become a problem we human have to face.Although the development of new energy has made some progress, world demand for energy continues keeps growing, energy are still in short supply.Reducing heat losses and improving the utilization rate of heat energy are effective means of alleviating the energy problems.In recent years, the reports of the heat insulation material are increasing.
Crystalline calcium hexaluminate (CaAl 12 O 19 , CA 6 ) is a good candidate to satisfy the requirements in view of fabrication of insulation material due to its low thermal conductivity and good thermal stabilities.Hallstedt [1,2] have reported that calcium hexaluminate has a theoretical density of 3.38 g cm -3 , melts at 1850 o C. It crystallized in hexagonal system, space group P63/mmc, and has magnetoplumbite structure [3][4].Anorthite (CaAl 2 Si 2 O 8 , CAS 2 ) also satisfy the requirement of ceramic substrates due to its low thermal expansion and low thermal conductivity [5][6].Pure anorthite melts congruently at 1550 o C, its theoretical density is 2.76 gcm -3 , but it is used at lower temperature.The refractory materials with the main crystal phase of calcium hexaluminate and the minor crystal phases of anorthite will not only have a very good effect on heat insulation but also have high service temperature.
In the present work, the calcium hexaluminate-anorthite composite was studied with magnesia slag, kaolin, industrial alumina.The effect of sintering temperature on the structure and properties were investigated.Tian)

Experimental
The composition of used raw materials is given in Tab.I. Magnesium slag, kaolin and alumina were weighed according to the proportion.These powders were wet mixed and milled for 40 min using alumina balls.After drying the resulting slurry at 90 o C for 24 h, adding 4 % plasticizers (PVA) to pelletize, then it was pressed at 20 Mpa into cylinder approximately 50 mm diameter, 8 mm height.Afterwards, the green compacts were sintered at 1300 o C, 1400 o C, 1450 o C, 1500 o C, respectively (3 h dwell time and 5 o C min -1 heating rate).
To better define effect of the sintering temperature, the qualitative analysis of formed phase was performed by X-ray powder diffraction, using a Rigaku D/Max-3C diffractometer with Cu K α radiation in the 2θ range 10-80°.The microstructure characterization was conducted by scanning electron microscopy (Hitachi S-4800 FE-SEM 4800) and energy dispersive X-ray.
The bulk density and apparent porosity of samples were measured by immersion method in water under vacuum using Archimedes' principle and calculated according to Eqs. ( 1) and (2): where m 1 (g) is the mass of dried sample in air, m 2 (g) is the mass of a sample with free bubbles on the surface, m 3 (g) is the mass of a sample in kerosene, and ρ 0 (g cm -3 ) is the density of water.
Mechanical properties of the samples have been determined at room temperature.They were tested using flexures testing machine in three-point configuration.The specimens which are sintered at different temperature were used to carry out the flexural strength test.Using the Eq. ( 3) for strength: where P[N]is the load at fracture, L[mm]is the length of support span, b[mm] is the width of the specimen, h[mm] is the thickness of the specimen.

XRD analyses
Fig. 1. shows the X-ray diffraction data of powders sintered at different temperature.In the samples, the CA, CA 2 precursors [7] and CA 6 are observed at 1300 o C，a small quantity of gehlenite (C 2 AS) and anorthite (CAS 2 ) are also observed.It is show that CA 6 crystal is grown.But at 1400 o C, gehlenite phase is disappeared, CA and the incompletely reacted alumina are also detected.As explained in the study of Karfa Traore [8], there is a phase transitions form gehlenite to anorthite, under the reaction: (4) At 1450 o C，the contents of the calcium hexaluminate increased, anorthite are also be detected.At 1500 o C, it is observed that the contents of alumina are increased.These seem to be that the temperature is too high lead to crystallizing alumina.Li [9] have reported the formation temperature of calcium hexaluminate which prepared by pure raw materials is 1500 o C. It is found that the formation temperature of the calcium hexaluminate is lower than mentioned above.There are two reasons to explain it.One reason, during ball-milling, the mechanical energy of balls is "stored" as surface energy; meanwhile the specific surface and the activity of the particles can be increased as the size of the particles decreasing.Another reason is that magnesium slag has pozzolanic reactivity, the system has a tendency to spontaneous reduce energy during sintering, it lead to the formation temperature is lower.MgAl 2 O 4 are always detected in different temperature, it is because of magnesium slag has unrefined magnesium; the unrefined magnesium reacted with raw materials to form MgAl 2 O 4 [10].

SEM analyses
Fig. 2. is the SEM microphotographs of the specimens sintered at different temperatures.Fig. 3. is the EDS analyses of particle A. At 1300 o C, there are alternant distribution of plate like crystal and granular crystal.Although the plate like crystal does not have good crystal structure, it is clearly between the plate-like crystals, the porosity among the plate like crystal provides growing space for CA 6 .With the temperature rising, the crystal morphology is more regular.Through the EDS analyses of particle A (plate like crystal), it is found that the plate like crystal is CA 6 , it is also verified that CA 6 grow preferentially along the basal plane direction when the growing space enough [2].At 1450 o C，the crystal touch each other and the crystal thickness increased.With the rise of temperature, the crystal grow up; when the crystal touch each other, the CA 6 crystal grow up along the basal plane direction is inhibited, turn to along the axial direction.At 1500 o C, the temperature is too high lead to the amount of liquid phase increases.

Physical properties
Fig. 4. shows the variation in bulk density and apparent porosity in relation to sintering temperatures between 1300 o C and 1500 o C.However, the apparent porosity decreases from 57% to 35% and the bulk density increases from 1.4 g cm -3 to 2.2 g cm -3 with the rising of the sintering temperature.Also it is found that the apparent porosity and bulk density change in a small scale between 1300 o C and 1500 o C, but they clearly changes between 1400 o C and 1500 o C. The reason for these changes is that the grain contract, sintering neck growth, grain boundary migration and grain growth in the range of 1400 o C to 1500 o C which is the reaction temperature of CA 6 .With the growth of sintering neck, the sizes of air hole decreased and the particles arranged more closely.On the macroscopy, it is presented as the increase of the bulk density and the decrease of the apparent porosity.
Changes of strength values related to temperature are given in Fig. 5.The strength values increased with the rising of the temperature.As seen from Fig. 5, the strength increase up to 60 MPa.The reasons of these increases are the increase of the CA 6 and the liquid phase.

Conclusion
The calcium hexaluminate/anorthite composite are prepared by magnesia slag, kaolin and industrial alumina and the effect of sintering temperature on the structure and properties are investigated.Based on the above results, the followings are concluded: [1] At 1300 o C, the CA, CA 2 precursors and CA 6 are observed, a small quantity of C 2 AS and CAS 2 are also observed; there is a phase transitions form C 2 AS to CAS 2 between 1300 o C and 1400 o C; CA 6 form in a large amount between 1400 o C and 1500 o C. The mechanical energy of balls and the pozzolanic reactivity of magnesium slag can reduce the formation temperature of CA 6 .[2] At 1300 o C, the plate like crystal and granular crystal are alternant distribution.CA 6 grow preferentially along the base direction, but when the crystal touch each other and the crystal thickness increased, the CA 6 crystal grow up along the basal plane direction is inhibited, turn to along the axial direction.[3] With the rising of the temperature, the apparent porosity decreases and bulk density increases.The apparent porosity and bulk density are changed clearly between 1400 o C and 1500 o C which is the main reaction temperature.The flexural strength also increases with the rising of the temperature.преференцијално у правцу базне равни.Када кристали дођу у међусобни контакт, кристали CA 6 крећу да расту дуж аксијалног правца.Привидна порозност опада а густина расте и отпорност према савијању расте са порастом температуре синтеровања.Кључне речи: температура синтеровања; калцијум хексаалуминат/анортит; структура; својства.

3 )Fig. 4 .
Fig. 4. Changes of apparent porosity and bulk density of samples under different temperatures.

Fig. 5 .
Fig. 5. Changes of flexural strength of samples sintered at different temperature.