The Effect of Sintering Temperature on Prepared and Properties of Calcium hexaluminate / gehlenite Composites

In this paper, the calcium-hexaluminate/gehlenite (CA6/C2AS) composites have been synthesized by traditional high temperature solid state method using magnesium slag as a source of calcium and silicon. The phases, micromorphology and physical properties of the samples were characterized and analysized. The results show that the CA6 and C2AS grains formation starts at 1300C, the main reaction has completed above 1450C. The granular particles of C2AS are distributed in CA6 grain boundaries. With the sintering temperature increased, the amount and the grains of CA6 grow up gradually, while the amount of C2AS reduces. The CA6 grains don’t growth excessively for the space hindrance function. With the sintering temperature increased the body density, linear shrinkage and flexural strength increases, while the apparent porosity decreases.


Introduction
Calcium hexaluminate (CaAl 12 O 19 or CaO•6Al 2 O 3 ), often denoted as CA 6 , occurs in nature as the mineral hibonite.This phase is the most alumina-rich intermediate compound of the CaO-Al 2 O 3 system.The micromorphology of CA 6 grains shows preferential growth along their basal plane [1].Because of its excellent anti-hydration property, slag resistance, thermal shock resistance and high refractoriness [2], CA 6 has a certain potential in application in high temperature refractory material field.The previous study on CA 6 composites are mainly focused on CA 6 and Al 2 O 3 composite [3][4][5][6][7].The study on CA 6 /MgO•Al 2 O 3 composites are started in recent years [8,9].
Gehlenite (Ca 2 Al 2 SiO 7 or 2CaO•Al 2 O 3 •SiO 2 ), has a certain reinforcing effect within the grain boundaries in the composite materials [10].C 2 AS has a potential application in high demandingness of mechanical properties and anti-hydration property field due to its good mechanical properties and anti-hydration property [11,12].To the best of our knowledge, the CA 6 /C 2 AS composites have not investaged.
With the rapid development of the magnesium smelting industry, the magnesium slag increases greatly, which can cause the pollution of the environment and the water resources.It is urgently to make full use of magnesium slag.The magnesium slag is mainly consisted of calcium oxide, silica, magnesia, aluminum oxide and ferric oxide [11].Thus magnesium slag is a potential material used as a source of calcium and silicon to prepare CA 6 /C 2 AS composites.
In this paper, the CA 6 /C 2 AS composites were sintered at temperatures in the range of 1300~1600 o C at normal pressure with magnesium slag and Al 2 O 3 as raw materials.The phases/micromorphology and physical properties of the CA 6 -C 2 AS composites were investigated.To exclude the adsorbed moisture, the raw materials were taken into constant temperature hot-blast oven of DH-101-2BS and dried at 90 o C for 12h.The raw materials were mixed by attrition milling during 40 min in water media using agate balls.The slurry was then dried and sieved until free-flowing (150 µm grid-size).The processed raw materials were uniaxially pressed at a pressure of 40 MPa to yield a cylinder green body of dimensions Φ50mm×7~9mm.The green bodies were sintered in KBF1700 box furnace under air atmosphere, for 3h at temperatures in the range of 1300~1600 o C, and then furnace-cooled.

Experimental procedure
Analysis of phases formed was performed with a Rigaku D/Max-3C X-ray diffractometer.The X-ray diffraction measurements were performed in step scan mode with a step interval of 0.02 • using a CuKα radiation (λ = 0.154060 nm) at room temperature.X-ray diffraction patterns were analyzed and refined by means of the multiphase Rietveld method [13].Multiple phases may be refined simultaneously and comparative analysis of the separate overall scale factors for the phases offers what is probably the most reliable current method for quantitative phase analysis.Analysis of micromorphology was characterized using Hitachis-4800 type field emission scanning electron microscope and Themal science noran system 7type energy dispersive spectrometer.Both apparent porosity and body density were measured by Archimedes drainage method.The linear shrinkage was calculated by measuring dimensions of specimens before and after sintering.Additionally, the fracture tests were carried out with a three-point bending method [14].

Micromorphology analyses
Fig. 3 shows the micromorphology of samples at different sintering temperatures.In Fig. 3, we can see that some irregular layered crystals is formed at 1300 o C, however the irregular layered crystals has changed into inerratic hexagonal layered crystals at 1400 o C and the crystals intertwined with each other.The hexagonal layered crystals have been almost completed at 1450 o C, and gradually thickened without excessive growth when the temperature above 1450 o C.Many granular (or octahedral) particles are distributed in hexagonal crystals grain boundaries or surface, and there are many pores in the samples (shown in Fig. 3).Considering the analysis result of XRD and EDS, the hexagonal layered crystals (A in Fig. 3(e)) are CA 6 , the octahedral particles (B in Fig. 3(b)) are MA [16] and the granular particles (C in Fig. 3(c)) are C 2 AS.

Physical properties analyses
The dynamics equation ( 1) of diffusion mass transfer at the early stage [17] is shown as: where ΔV/V is the growth rate of neck volume, ΔL/L the linear shrinkage, γ the surface tension, Ω the vacancy volume, r the radius of the powders, t the reaction time, T the Sintering temperature, k the Boltzmann constant, Q the diffusion activation energy, R the molar gas constant, D 0 the frequency factor and D * is the self-diffusion coefficient.
According to the equations (1 and 2), self-diffusion coefficient increases greatly with T increased.To improve temperature is an effencial method to attain a high sintering quality.This is consistent with the results in Fig. 3.
The physical properties of the samples are determined by the sintering temperature.Fig. 4 presents the curves of physical properties of samples sintered at different temperatures.With the sintering temperature increased, the body density, linear shrinkage and flexural strength of the samples increased gradually, while the apparent porosity decreased gradually.The analyses of physical properties indicate that with the temperature increased between 1300 o C and 1400 o C, the interfaces and aggregation degree increase, the distance between two particles decreases, grain boundaries is formed gradually and sintering necks grows up, and the reaction is mainly controlled by the surface diffusion, thus the physical properties have a subtle change.With the temperature increased between 1400 o C and 1600 o C, the bonding level is improved, the growth rate of sintering necks and grains increase gradually and grain boundaries shift faster, and the reaction is mainly controlled by the grain boundaries and crystal lattices diffusion, thus the physical properties change obviously.The pores of samples are mainly apparent pores which can be gotten from the results of physical properties and SEM.The apparent porosity is 46%~64%, while the body density is 1.3g/cm 3 ~1.9g/cm 3 , which indicated that the prepared samples of CA 6 /C 2 AS composites are porous that could be used as heat-barrier material or carrier material.To the best of our knowledge, porosity determines the flexural strength of the samples directly.The flexural strength of the CA 6 /C 2 AS composites is 4~95MPa, especially, the samples have a better flexural strength above 1450 o C. Comparing with the theory flexural strength of CA 6 or C 2 AS, the experimental flexural strength of CA 6 /C 2 AS composites is lower, which can be explained with Griffith crack strength theory [18].There are many wee cracks, defects and pores in experimental CA 6 /C 2 AS composites, which result in weakening of binding force among grains and decreasing of body density.The test result of flexural strength indicates that the CA 6 /C 2 AS composite has a high flexural strength.

Conclusion
CA 6 /C 2 AS composites can be synthesized by traditional high temperature solid state method using magnesium slag as a source of calcium and silicon.The CA 6 and C 2 AS grains formation starts at 1300 o C, the main reaction has completed above 1450 o C. The granular particles of C 2 AS are distributed in CA 6 grain boundaries.With the sintering temperature increased, the amount and the grains of CA 6 grow up gradually, while the amount of C 2 AS reduces.The CA 6 grains don't growth excessively for the space hindrance function.With the sintering temperature increased, the body density, linear shrinkage and flexural strength increases, while the apparent porosity decreases.17.Gregory C. Stangle, Ilhan A. Aksay, Chem Eng Sci, 45 (1990)

Fig. 1
Fig. 1 shows the X-ray diffraction patterns of samples at different temperatures.The major phases are CA 6 , C 2 AS and Al 2 O 3 , the trace phases are CaAl 4 O 7 (denoted as CA 2 ) and MgAl 2 O 4 (denoted as MA) at the temperature of 1300 o C and 1400 o C. The major phase and second phase are CA 6 and C 2 AS, respectively, while the trace phase is MA at temperatures in the range of 1450~1550 o C. The phase is almost CA 6 at 1600 o C. Fig. 2 presents the curves of samples phase amounts of CA 6 and C 2 AS at different temperatures.The amount of CA 6 increases with the temperature increased, while the amount of C 2 AS reduces.The high surface energy and lattice defects were generated after the raw material powers of magnesium slag and Al 2 O 3 were attrition milled, which provided a certain sintering driving force.At the same time, C 2 AS is a low-melting point phase in Al 2 O 3 -CaO-SiO 2 system.Thus the high -melting point phase CA 6 can formed at the low sintering temperature (1300 o C).With the temperature increased, C 2 AS is remelt and the CA 6 is formed.The C 2 AS does not decrease greatly above 1400 o C. Above 1450 o C, the amount of CA 6

Fig. 1 .
Fig. 1.The X-ray diffraction patterns of samples at different temperatures.

Fig. 2 .
Fig. 2. The curves of samples phase amounts of CA 6 and C 2 AS at different temperatures.

Fig. 3 (
Fig.3(a) shows that there are many crystal seeds of CA 6 formation at 1300 o C. As temperature increasing, the pores gradually closed and the densification of samples became higher.In the temperature rang from 1400 o C to 1450 o C, a great quantity of CA 6 are formed, and the formation reaction has been almost completed at 1450 o C. CA 6 crystals were gradually thickening without excessive growth at the temperature above 1450 o C.There are two reasons to explain.One is that C 2 AS or MA grains in grain boundaries inhibit crystal grains to grow up; and the other is that the formation of CA 6 accompanies with the volume dilation.

Fig. 4 .
Fig. 4. The curves of physical properties of samples sintered at different temperatures a) the curves of body density and apparent porosity b) the curves of linear shrinkage and flexural strength.