Effect of TiO2 on the sintering behavior and microstructure of stoichiometric spinel (MgAl2O4)

In this work, magnesium aluminate spinel (MA) (MgO 28 wt%, Al2O3 72 wt%) stoichiometric compound , were synthesized via solid state reaction (SSR) Single firing stage, and the impact of sintering on the physical properties and thermal properties as well as the fine structure and morphology of the ceramic product were examined. The Spinel samples were pressed at of (14 MPa) and sintering soaking time (2h). The effect of adding oxide titania (TiO2) was studied. The obtained powders were calcined at a temperature range of 1200 and 1400 °C. The calcined samples spinel were characterized by XRD, it showed the presence of developed spinel phase end also showed that the best catalyst is titania. The SEM image showed the high sintering temperature increased the regularity of the grain growth, as well as minutes from the acquisition of the spherical shape of the minutes spinel.


Introduction
Magnesium aluminate spinel (MgAl 2 O 4 ) is one of the most famous oxide ceramic material type with special properties e.g high mechanical strength, low thermal expansion high melting point (2135 °C ), good thermal shock resistance coefficient, excellent resistance to acid and bases, and also having catalytic and optical properties [1][2][3]. Due to these desirable properties, it found widely application in optical, electric fields, metallurgical, electrochemical, radiotechnical and chemical industries [4][5][6]. Thus, the preparation of magnesium aluminate powders with high purity, chemical homogeneity, control of stoichiometry, fine particle size, narrow particle size distribution, and minimum particle agglomeration with high sinter activity has received considerable attention in order to improve the material properties [7]. A number of techniques such as conventional solid state reaction (SSR), sol-gel method, wet chemical method, self-heat-sustained (SHS) technique and spray drying (atomization) have been employed for the preparation of spinel (MgAl 2 O 4 ) [8,9]. The conventional solid state reaction (SSR) method is one of the method for preparation of spinel oxides. To reduce the sintering temperature of the spinal ceramics, sintering additives should be introduced. Effects of additives on the sintering of stoichiometric or nonstoichiometric spinel are reported for various additives [10]. Managed to obtain well-densified spinal body with a singlestage firing using calcined Al 2 O 3 , sintered MgO as starting material and MgCl 2 as sintering aid. Other studies have shown that the presence of additives such as B 2 O 3 , V 2 O 5 , Y 2 O 3 and MgCl 2 help to produce a more densified spinel. Kim et al. [11] investigated the effect of SiO 2 , CaCO 3 and TiO 2 on the sintering of spinel, and found that in case of SiO 2 and CaCO 3 , the densification was enhanced with SiO 2 and CaCO 3 additives by forming glassy phases in grain boundaries; whereas the additives of TiO 2 resulted in the formation of secondary phase at grain boundaries and inside grains a long with enhanced densification. Sarker et al. [12] found that the addition of TiO 2 enhanced densification by exsolution of alumina and dissolution of TiO 2 .
Zagrafou et al. [13] study the effect of dopants such as Al 2 O 3 , MgO and SiO 2 on the sintering behavior of spinel [10]. Their study showed that the densification of spinel was highly influenced by variation in composition. Ganesh et al. [14] prepared different grades of stoichiometric and nonstoichiometric dense spinel (MgAl 2 O 4 ) by conventional doublestage firing process. They found that TiO 2 and moisture present in the precursor oxides highly influence the spinel formation. Used the present study, used TiO 2 as sintering aid, stoichiometric spinel was prepared form magnesia and alumina. Sintered samples were characterized for densification, phase analysis and microstructure studies. The effect of TiO 2 addition on the thermal conductivity of sintered samples was also studied.

Experimental
A commercially available materials, magnesia (Espana source, 99% pure, polycrystalline materials with grain size 30-40micron) and fine powdered alumina (Switzeriand supplied, 99.9% pure) powdered titania (TiO 2 ) (India, 99% pure grain size 20-30 micron) were used as starting materials. The spinel compositions was prepared using these materials with MgO: Al 2 O 3 weight ratio (MgO 28wt%, Al 2 O 3 72 wt%) (stoichiometric spinel composition). The amount of TiO 2 added to the starting powder mixture varied from (0 to 8 wt%) at the interval of (2wt%). The powder mixtures were initially mixed for 5hr. in planetary ball mill (Model #8000 M, USA) using alumina balls. The powder mixtures were compacted by semi dry pressing into cylinder (50mm in height and 12mm in diameter) under a pressure of 14 MPa using 4% PVA solution as binder. The green compacts were dried at 110 °C (0.5 h) and then heating at 1200 and 1400 °C for 2h in an electric furnace. During sintering process, the compacts have undergone phase transition forming (MgAl 2 O 4 ) spinel. The density of the green and sintered compacts was calculated by measuring weight and volume of the compact samples. The porosity and apparent absorption of the samples was calculated by liquid absorption technique. The XRD studies were carried out by the (D2 PHASER, Bruker, USA) X-ray deffractometer (Cu Ka irradiation). The morphological studies were carried out using a scanning electron microscopy (SEM) have number (9922650, model INSPECT S50, 2013 Dutch).

Results and discussion 1. Phase analysis
XRD diffraction patterns for the produced magnesium aluminate spinel products synthesized by solid state reaction at 1200 °C firing temperature for all composition are shown in Fig.1. It was found that the strongest four peaks of the produced samples appeared at 2θ values (31.7°, 37.3°, 65.6° and 77.2°). These peaks correspond to (202), (311), (404) and (533) diffraction planes of the magnesium aluminate phase as shown in Tables 1 and 2. The peak intensities in 1200°C sintered for all compositions were found to be strong due to starting of spinel reaction, which confirms that the crystalline spinel phase formation starts below 1200 °C . At 1200 °C , spinel phase is present with the reactant Al 2 O 3 and MgO phases and with increasing TiO 2 amount and firing temperature spinel phase, peak intensity has increased with a decrease in peak intensities of the reactant phases. Complete spinel formation was observed in the 1400 °C fired compositions as shown in Fig. 2 where D is the crystallite size, K is a shape factor with a value of 0.9-1.4, λ is the wavelength of the X-rays (1.54056 Å), θ is the Bragg angle and β is the value of the full width at half maximum (FWHM).  minutes and an increase in its growth and acquisition particleboard spherical shape which helps to achieve the crystallization of those at temperatures lower than those for models spinel without adding this oxide.

3-Physical properties
Noting a significant increase in density at high additive ratios of TiO 2 and high temperatures burning. We find that the highest density is (1.91g / cm 3 ) at firing (1400 °C ) as shown Fig. 5. There is a marked increase in product density increase ripening time as a result of the grain growth of spinel completeness down to complete (spinellisation process). From the equation it was calculated Density [16]: (2)

4-Porosity
The porosity is considered one of the important properties of the materials, ceramic and when the increase index to increase the absorbance of the product and a decrease in durability of mechanical stimuli, as porosity factors: the first important affected by the impact of the burning process and the second will be channels and voids inside the body as a result of liberated gases, that the impact of these two factors will be contrasted, as it shows the influence one of the workers superiority over the other. As reduced porosity spinel after the addition of titania oxide, which confirms its positive role as an adjunct in the sintering process, which complies with many of the previous research, we also note the advantage of titania in low porosity spinel models treated thermally burn at temperatures (1400 °C ). As shown Fig. 6 at firing (1200 and 1400 °C ).
From the equation it was calculated apparent porosity: where W d = the weight of the dry sample (in air). W s = the weight of sample after saturation with kerosene (in air). W i = the weight of the sample suspended in kerosene [5].

5-Thermal conductivity
The decrease in thermal conductivity models spinel (MgAl 2 O 4 ) high temperature sintering and the proportion of added and of which lead to low porosity and complete (Spinallization Process) which helps the emergence of developed spinel. The best value for thermal conductivity

Conclusions
The results showed that increasing the degree of sintering increases the crystallization and grain growth spinel result of sintering which will reflect positively on its physical properties, as well as properties Insulating. Micrographs (SEM) show that the high temperature sintering temperatures increase the regularity of the grain growth and homogeneity in the bloc minutes as well as the acquisition of spherical shape for a few minutes with spinel advantage when adding (TiO 2 ) by adding (8 wt%) and the degree of sintering (1400 °C ). (XRD) were pattern show the existence of crystallization phase spinel. These properties can be used as a thermally and electrically insulating material