The effects of silica addition on the structural , electrical and mechanical characteristics of MgAl 2 O 4 spinel ceramic phase

The ceramic compound Mg1-xSixAl2O4 (x= 0, 0.1, 0.2, 0.3, 0.4) was prepared from nano powder of Al2O3 and MgO doped with Nano powder of SiO2 at different molar ratios. The specimens were prepared by standard chemical solid reaction technique and sintered at 1450 oC. Structure of the specimens was analyzed by using X-ray diffraction (XRD). The X-ray patterns of the specimens showed the formation of pure simple cubic spinel structure MgAl2O4 phase with space group of ̅ . The average grain size and surface topology were studied by atomic force microscopy. The results showed that the average grain size was about 73-90 nm. The DC electrical properties of the specimen were measured. The apparent density was found to increase and the porosity and water absorption were found to decrease with increase of Si addition. Mechanical properties were characterized by Vicker’s micro hardness and Brazilian fracture test to measure the fracture toughness of the samples. The micro hardness decreased with Si addition increase. Fracture toughness increased with the increase in Si addition.

In this study, a series of magnesium aluminate (MgAl 2 O 4 ) doped with silica at different molar ratios was obtained.In this context, the substitution of Si at Mg site in MgAl 2 O 4 and its possibility of tailoring the structure of the resultant compound and therefore improving the mechanical and electrical properties which could further widen its range of applications were investigated.

Experimental
Single phase specimen of pure and SiO 2 doped MgAl 2 O 4 having general formula as Mg  1.The precise amounts of the reagents were mixed in the proper molar ratios.The powders were mixed for about 5min by using a speed mixer with the maximum speed of 2000 rpm.The powder were pressed by using hydraulic press with pressure of 10 ton to produce pellets with diameter of 15 mm the specimen were sintered at temperature of 1450 °C and soaking time of 2h.The porosity, apparent density and water absorption were calculation for all sintered specimens using Archimedes' principle according to the standard specifications [ASTM C373 test methods] [6,7].As follows:-1-The specimens were weighed using a digital balance of accuracy 10 -4 g after it were dried at 120 ºC for two hours using an oven and taking the weight of dry specimen (D).2-Specimens submerged in boiling water for five hours and left in the water for 24 hours and then taking the immersed weight (I).
3-Specimens were taken out of the water and wiped with a wet piece of tissue and was weighed once again to find the soaked weight S. Apparent porosity, water absorption and apparent density were calculated using the following equations (S) Apparent porosity *100% (1) Water absorption % Apparent solid density X-ray diffraction (XRD) analysis was performed using automated Philips type PW 1840 diffractometer equipment with Cu Kα radiation source (20 mA/30 kV).The specimens were scanned in the 2θ range between 20-80 o (at scan rate 2θ = 3º /min).AFM was performed by using AA3000 scanning probe Microscopy by Angstrom Advanced Inc. (USA).The electrical resistivity was measured as a function of temperature over the range 290-430 K.The measurements were done using sensitive digital electrometer type Keithley 616 and electrical oven.Hall Effect measurements were done by Van der pauw (Ecopia HMS-3000) which were carried out at room temperature using the four probe technique.The principle of Hall effect refers to potential difference (Hall voltage) on opposite sides of a thin sheet of conducting or semi-conducting material through which an electric current is flowing, created by a magnetic field B=0.55 Tesla were determined using LCR meter bridge.For this purpose silver paste was applied on both sides of the specimen to make sure of good Ohmic contacts.The Hall-effect measurements involved measuring the Hall coefficient (R H ), Hall mobility (μ H ) and sheet carrier concentration (n H ). The measurement of ACelectrical conductivity (σ a.c ), and its variation with frequency was carried out using LRC meter IA (impedance analyzer) (4274 A HEWLETT PACKARD) over the frequency range (10 kHz-1 MHz).The mechanical properties were characterized by micro hardness (H V ) and disk fracture test (Brazilian disk fracture) (σ f ).

Results and discussion
Fig.  Most of the spinel compounds have the space group Fd3m.The spinel structure AB 2 O 4 consists of almost closely packed oxygen anions, in which the tetrahedral and octahedral sites are occupied by cations.The unit cell may be obtained by doubling this fcc oxygen sub-lattice, along each of three directions.If the cations A and B are occupied at the '8' tetrahedral 'A' sites, and 16 octahedral 'B' sites, out of the 64 and 32 available sites, the arrangement is called the stoichiometric or normal spinel.In the normal spinel, Mg 2+ ions occupy the tetrahedral positions, and Al 3+ ions occupy octahedral positions.There are some spinel's systems which are occupied by both trivalent the tetrahedral positions and divalent ions known as the inverse spinel structure [8].It may be due to the arrangement of metal ions in different A and B sites while the composition changes.X-ray diffraction shows that all the specimens are of MgAl 2 O 4 single phase.The lattice parameter (a) was determined experimentally from the d hkl spacing using the following equation for cubic structure Eq. ( 4) [9].The peak with the Miller indices (311) was used in the X-ray measurement shown in Table 2.The ionic radii of tetrahedral A-sites in the spinel structure (r A ) and B-octahedral (r B ) can be determined from the Eqs.(5,6), where r A is the tetrahedral A-sites radii, r B is octahedral B-sites radii and u is the oxygen parameter.The ideal u value is close to 0.375 (Å) in spinel, and r (O -2 ) is the ionic radii of oxygen.a =d hkl (h 2 +k 2 +l 2 ) 1/2 (4) r A = (u-1/4) a (3) 1/2 -r (O 2-) (5) r B = (5/8u) ar (O 2-) (6) where the R B and R A is ionic destines bond, and (h k l) are the miller indices [10].The qualitative results showed that the phases in calcined spinel specimen at temperatures 1450 o C consisting of only MgAl 2 O 4 spinel phase having crystallite size below 25 nm.
Fig. 2 shows the AFM images for specimens MgAl 2 O 4 doped with Si where x=0, x=0.1 and x=0.4.AFM images showed that all specimens have granular nature.The average grain size and average roughness as determined from AFM are given in Table 3.Both grain size and surface roughness increased with the increase of substitution which refers to the increases in the strength of bond of linking atoms, where the average grain size of x=0.1 and x=0.4 were 94.89 nm and 90.22 nm, respectively, as shown in Table 3.On the other hand the average roughness decreases from 6.65 nm to 4.33 nm for x=0.1 and 0.4, respectively.Figs. 3, 4 and 5 show the results of apparent density, apparent porosity and water absorption as a function of SiO 2 content respectively.Apparent density was found to decrease with SiO 2 content.This decrease corresponds to the mixing rule; the reason behind the decrease in apparent density is the increase of the apparent porosity exists inside the structure.The temperature dependence of electrical resistivity arises because of the drift mobility of electrical charge carriers rather than by the localized charge carriers [11].The low values of resistivity may be attributed to the small crystalline size, as shown in Table 3.In general, the conduction mechanism in semiconductors is greatly influenced by inter-crystalline grain boundaries and strain fields associated with dislocation network.This behavior may be explained from the temperature dependent electrical resistivity of the samples surface.There is an electron across the energy transition from the filled valence-band level to the unfilled conduction -band states.The electrical resistivity of samples increased as the content of Si. increased [12].Fig. 7 shows that the microhardness decreased with Si addition due to the high porosity.In general the hardness of Mg 1-x Si x Al 2 O 4 is still high and within the acceptable limits.Results of the diametrical compression strength obtained from the Brazilian test ( ) is shown in Fig. 8.The diametrical compression increased with Si addition.This could be attributed to the increase in density since the compression strength varies proportionally with the density.

Conclusions
X-ray spectra for specimens indicated the formation of crystalline cubic spinel structure phase ceramic with the appearance of small amount of secondary phases.The average grain size for pure specimen is less than the average grain size of doped specimen.The result of Hall coefficient showed an n-type semiconductor behavior.The conduction mechanism in this ceramic is due to hopping of electron between Mg +2 and Al 3+ .The mechanical properties showed good results for Si doping.

Fig. 6
Fig. 6 shows the electrical resistivity against temperature of MgAl 2 O 4 sample doped with different ratios of Si ions.The decrease in electrical resistivity with temperature indicates the semiconducting nature of the MgAl 2 O 4 compound.The temperature dependence of electrical resistivity arises because of the drift mobility of electrical charge carriers rather than by the localized charge carriers[11].The low values of resistivity may be attributed to the small crystalline size, as shown in