Abstract

This paper presents a study of the most important factors influencing the sintering process spinel (MgAl₂O₄) Minutes from the oxides of aluminum (Al₂O₄) and Magnesium (MgO) percentages by weight (MgO 28wt%, Al2O3 72wt%). The traditional method called solid-state reaction (SSR), single firing stage and the impact of these factors on the physical properties, and thermal as well as the fine structure and morphology of the ceramic product. It formed Spinel models through a steel mold and shed pressure (14 Mpa) using hydraulic piston at temperatures (1200, 1400°C) and soaking time (2 h) with the study of the effect of adding oxide calcium (CaO) by weight (0,2,4,6,8 wt%) spinel on the properties and behavior as sintered the research study of the most important physical characteristics (density, porosity) and (Thermal Conductivity). The results showed that the density increased and porosity decreased with higher temperatures due to the process of sintering, as it reached the highest density at the temperature of burning 1400°C which (1.88 g/cm³), which is porosity (47.48 %). Thermal conductivity was the best value (5.071 K w/cm. °C). As well as the study of (XRD, SEM). The calcined specimens spinel were characterized by XRD, It showed the presence of developed spinel also showed that the best catalyst is CaO, and (SEM). It showed that high temperature sintering temperatures increase the regularity of the Grain Growth and cohesion in the bloc, as well as minutes from the acquisition of the spherical shape of the minutes spinel.

1. Introduction

Magnesium Aluminate spinel (MgAl₂O₄) is the only stable phase in the MgO – Al₂O₃ (MA) system. It is an excellent material of immense technological importance as a structural Ceramic. MgAl₂O₄ possesses useful physics, chemical and thermal properties, both at normal and elevated temperature. It melts congruently at 2135°C, shows high resistance to attack by most of the acids and alkalis and has low electrical losses ^{1, 12}. Due to these desirable properties it has a wide range of applications in structural, chemical, optical and electric fields. 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 (MgAl₂O₄) ^{2, 13}. The conventional (SSR) method is one of the method for preparation of spinel oxides. To reduce the sintering temperature of the spinel (MA) ceramics, sintering additives should be introduced. Effects of additives on the sintering of stoichiometric or nonstoichiometric spinel are reported for various additives. Teoreanu and Ciocea ³. Managed to obtain well-densified spinal body with a single – stage firing using calcined Al₂O₃, sintered MgO as starting material and MgCl₂ as sintering aid. Other studies have shown that presence of additives such as B₂O₃, V₂O₅, Y₂O₃ and MgCl₂ help to produce a more densified spinel. Kim et al, ⁴ investigated the effect of Sio₂, CaCO₃ and Tio₂ on the sintering of spinel, and found that in case of Sio₂ and CaCO₃ the additives, the densification was enhanced with the additives by forming glassy phases in grain boundaries; whereas the additives of TiO₂ resulted in the formation of secondary phase at grain boundaries and inside grains a long with enhanced densification. Sarker et al, ⁵ found that the addition of TiO₂ was founded to enhance densification by exsolution of alumina and dissolution of TiO₂. Zagrafou et al ^{6, 7} studied the effect of dopants such as Al₂O₃, MgO and SiO₂ on the sintering behavior of spinel. Their study showed that the densification of spinel was highly influenced by variation in composition. Ganesh et al, ⁸ prepared different grades of stoichiometric and nonstoichiometric dense spinel (MgAl₂O₄) by conventional double- stage firing process. They found that CaO and moisture presented in the precursor oxides highly influences the spinel formation. In the present study, by using CaO as sintering aid, stoichiometric spinel was prepared from magnesia and alumina. Sintered products were characterized by densification, phase analysis and microstructure studies. The effect of CaO addition on the thermal conductivity of spinel specimens sintered is also studied.

2. Experimental

Commercially available magnesia (Espana source, 99% pure, polycrystalline materials with grain size 30-40micron) and fine powdered alumina (Switzeriand supplied, 99.9%pure) were used as starting materials. The spinel compositions were prepared using these materials with (MA) MgO: Al₂O₃ weight ratio 28.3, 71.7 (stoichiometric spinel composition). The amount of CaO 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 and then heating at 1200, 1400°C for 2h in an electric furnace. The process of sintering is the process of hardening the ceramic materials produced from the powders and then burn them at high temperatures without the emergence of a liquid phase and can be sintering with the presence of liquid. Sintering is an automatic phenomenon whose direction is determined by a decrease in free energy coupled with a decrease in surface area. During sintering process, the compacts have undergone phase transition forming (MgAl₂O₄) spinel. The density of the green and sintered compacts was calculated by measuring weight and volume of the compact sintered specimens. The porosity and apparent absorption of the samples were calculated by liquid absorption technique. The X-ray diffraction studies were carried out by the (D2 PHASER, Bruker, USA) X-ray deffractometer (Cu Ka irradiation) in order to characterize the phase transformation of specimens. Also, morphological studies were carried out using a scanning electron microscopy (SEM) have number (9922650, model INSPECT S50, 2013Dutch).

3. Results and Discussion

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 Figure 1. It was founded that the strongest three 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. The peak intensities in 1200°C sintered for all compositions were founded 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 presented with the reactant Al₂O₃ and MgO phases by increasing CaO 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 Figure 2. Complete spinellisation of MA spinel specimens in the 1400°C fired compositions occurred due to increase in surface area leading to an increase in reactivity of the reactants.

From Table 1, Table 2.

Grain size (D) of the Crystallite spinel specimens can be estimated from the X-ray spectrum by means of full width at half-maximum (FWHM) method according to Scherrer formula ¹¹:

(1)

Where k=0.94, is the wavelength of incident X-ray radiation, is the intrinsic full width at half-maximum of the peak, and is the Bragg's diffraction angle.

Figure 3a and Figure 3b. shows the microstructure of spinel specimens with (8wt% CaO) sintered at 1200°C and 1400°C respectively.

The microstructure observations revealed that the fine-grained microstructure develops at low sintering temperature, while transforms rapidly to the exaggerated –grained microstructure above critical temperatures depending on CaO content ⁹. Give examination of the results of models scorched degree burn (1400°C) and the amount of pressure (14 Mpa) the emergence of phase spinel. The addition of titanium dioxide gives minutes spinel advantage in terms of homogeneity conglomerate minutes and an increase in its growth and acquisition particleboard spherical shape which helps to achieve the crystallization of those minutes at temperatures lower than those for models spinel without adding this oxide.

As noticed sintered density increases from 1.52 g/cm³ to 1.88 g/cm³ and apparent porosity decreases gradually form 57.87 % to 47.48 % with increases of CaO addition from 0 to 8 wt% due to Figure 4. And Figure 5.

From the equationit was calculatedapparent porosity:

(2)

Where Wd= the weight of the dry sample (in air).Ws = the weight of sample after saturation with kerosene (in air). Wi = the weight of the sample suspended in kerosene ¹⁰.

The thermal conductivity (W / m.°C) of the specimen samples was measured using the Lee's disc method as shown in the figure below. Samples were prepared with 50mm internal diameter, 60mm external diameter and 80mm height and 14 Mpa pressure to study the effect of both additive factor and for burning grades (1200, 1400°C). The thermal conductivity was calculated according to the following relationship:

(3)

As:

K = (W / m.C °)

A = r² (mm)

Q: The amount of energy

T_B-T_A: difference in disk temperature (°C)

X: Thicknes (mm).

Thermal conductivity decreases gradually form 9.921 (w/cm.°C) to 5.071(w/cm.°C) with increases CaO addition from 0 to 8 wt% at 1200 °C and 1400°C due to Figure 6.

The figure shows that the best value of the ceramic material at the addition ratio and the temperature of 1400 °C Which (5.071 K(w/cm.°C).

4. Conclusions

This study reaches the following conclusions:

• The possibility of preparing Spinel by the method of solid-state reaction (SSR) with one burning stage, despite of the disadvantages of this method, which produces a reasonable sintering characteristics compared with the results of previous researches.

• Increasing of sintered density is noticed due to high firing and with decreasing of the porosity of spinel specimens with (0-8wt% CaO).

• It is noted from the results that the increase of the burning degrees helps to increase the crystallization and growth of particle of Spinel which will positively reflect on its physical properties, as well as properties of insulation.

• Through the X-ray diffraction package (XRD) we were able to prove the existence of amorphous phase Spinel of burning degrees with (1200, 1400°C).

• By investigating (SEM) it is observed that the high temperature sintering temperatures increases the regularity of the Grain Growth and homogeneity in the bloc minutes as well as the acquisition of spherical shape for a few minutes.

References