Influence of Mechanical Activation on Structural and Electrical Properties of Sintered MgTiO 3 Ceramics

The aim of this work was to analyze the influence of mechanical activation on the MgCO3-TiO2 system. Mixtures of MgCO3-TiO2 were mechanically activated for 15, 30, 60 and 120 minutes in a planetary ball mill and after that sintered at 1100C for 1h. XRD analyses were performed in order to give information about the phase composition and to determine a variety of microstructure parameters using Scherrer’s method. Also, the effect of tribophysical activation and sintering process on microstructure was investigated by scanning electron microscopy. Electrical measurements were performed in order to determine electrical properties of sintered samples. Our conclusions are that the sample activated for 120 min showed the best electrical properties (εr=23.86, Q=233, ρ=0.38) and exhibited the best sinterability.


Introduction
Ceramic materials have been in use in many different areas of human wellbeing for a very long time.Materials based on magnesium titanates, MgTi0 3 and Mg 2 Ti0 4 , are finding a variety of applications in microwave engineering.Materials applied in electronics are important fields of ceramic materials [1,2].These materials differ extremely and have low dielectric loss in the microwave range and dielectric constant of 10-20 (Mg 2 Ti0 4 has a dielectric constant e=14, while MgTi0 3 has e=16).
.) Corresponding author: stevanovicsuzana@yahoo.com Based on literature data it is well known that mechanical activation is a widely used method, which' enhances the mixture homogeneity of the starting components; their reactivity, remarkably lowers the reaction temperature and hence, significantly reduces sintering temperature [7].
In this study, the authors have attempted to reduce the sintering temperature of the MgC0 3-TiOz system using a common part of the powder preparation route, the method of mechanical activation.The measured dielectric properties were discussed from the results based upon the densification, X-ray diffraction patterns along with the microstructures of sintered MgTi0 3 ceramics.

Experimental
As starting materials in this work, we used MgC0 3 (99.9%p.a.) and TiO z (99.9 p.a.) at a molar ratio 1:1.The powders were submitted to mechanical treatment in a planetary ball mill device (Fritsch Pulverisette 5), with zirconium oxide balls and vessels.The ball to powder mixture mass ratio was 40: 1.The milling process was performed in air for 15, 30, 60 and 120 minutes, and mixtures were denoted according to the applied time of activation as MT-OO, MT-15, MT-30, MT-60 and MT-120.
Samples were heated in a tube furnace (Lenton Thermal Design Typ 1600 ) with a heating rate of 10 DC/min and when the temperature of the furnace reached 1100 DC, compacts were sintered isothermally in air atmosphere for 60 minutes.Sintered samples were denoted as MTs-O, MT s-15, MT s-30, MT s-60 and MT s-120.
The morphology of obtained sintered pellets was characterized by scanning electron microscopy (JEOL JSM-6390 LV).The pellets were broken and covered by gold.
X-ray powder diffraction patterns after sintering were obtained using a Philips PW-1050 diffractometer with ACu-K a radiation and a step/time scan mode of 0.05°/sec.
The measurements of electrical properties were performed at a frequency of 5 MHz with a HIOKI 3532-50 HiTESTER device at a constant voltage mode.The "four-probe" configuration was used.The samples were prepared by painting silver electrodes on both sides following with thermal treatment at 120°C for 2h performed in order to improve the paint conductivity.

Results and discussion
In order to investigate the effect of mechanical activation on system's microstructure, scanning electron microscopy was performed.Scanning electron images, presented in Fig. 1, show a significant difference in powders sintered for 1h at 1100°C activated for 0-120 minutes.Evolution of microstructural constituents, grains and pores occurs during the sintering process, where along with sintering time increasing, adequate processes of grain growth and decreasing pore size take place.Finally, the MT s-l20 micrograph exhibits the best sinterability.The decrease in pore size is observed, which leads to intensive mass transportation process, strengthened grain boundaries and finally to better homogeneity.This is in accordance with the X-ray analysis.Fig. 2. shows the X-ray diffraction patterns ofMTs-O, MT s-15, MT s-30, MT s-60 and MT s-120 powders sintered at 1100°C for Ih.Identification of the obtained reflections was accomplished using JCPDS cards (79-0831 for MgTi0 3 , 65-5714 for TiO z anatase, 77-0443 for TiO z rutile, 72-0021 for TiO z (0), 71-1176 for MgO, 76-2373 for Mg'Ti-O, and 79-0829 for Mg zTi0 4 ) .It is clearly visible that after heating we have the MgTi0 3 magnesium titanate phase with a small amount of MgO, metastable compound Mg'Ti-O, and TiO z anatase in a non-activated sample sintered at 1100°C for 1 h (XRDP of MTs-O).That phase composition is in accordance with our previous investigations [8] and is the consequence of sintering the non-activated sample.The presence of the MgO phase was detected within sintered samples activated for 15 minutes, and for prolonged times of milling it's presence wasn't noticed.The very first appearance of the third, orthorhombic TiO z (0) modification, which represents the phase transition from anatase over TiO z (0) to the most stable form rutile is observed.Several literature data explained the phase transition from anatase over Ti0 2 (orthorhombic) to rutile modification during mechanical activation [9,10].This explains the disappearance of the anatase phase within diffraction patterns of samples activated 15-120 minutes and sintered at 1100°C for lh.Diffraction patterns of activated and sintered samples MT s-30 -MT s-120 consist of the dominant MgTi0 3 phase and a mixture of small amounts of Mg, Ti0 4 , MgTizOs, TiO z rutile and TiO z (0) phases.
Recovery of the activated material, the disappearance of defects and grain growth are processes that occur during sintering.Also, it is obvious that the reflections of sintered samples are sharp and intensive, due to recrystallization.
Microstructure parameters were revealed from an approximation method using the well known Scherrer's equation [11] of sintered samples activated from 0-120 minutes: average particle size (D hkl), density of dislocations (Po) and lattice strain (ehkl) are given in Tab.I for MgTi0 3 phase.
Tab.I Microstructure parameters revealed from approximation method for sintered samples for MgTi0 3 phase [11] Phase (hkl) D"kl' (nm) PD•I0 The results of X-ray analyses and microstructure development are in accordance with dielectric properties of the sintered samples.The values of densities obtained before (do) and after sintering (d s , given in gem"), quality factor (Q, a reciprocal value of dielectric loss), dielectric loss (tgb), dielectric permittivity (&r) and specific resistance (P, given in MOm) are given in Tab.II.Tab.II.Electrical properties (at 5 MHz frequency), starting and sintered densities of samples activated 0, 15,30,60 and 120 minutes and sintered at 1100°C for 1h.The electrical measurements pointed out that dielectric permittivity of the specimens increased with activation time reaching its highest value for the sample activated 120 minutes.Also, densification is the greatest within the sample activated 120 minutes.It is believed that density plays an important role in controlling dielectric loss, as has been often found in other microwave dielectric materials [12].The Q value is generally affected not only by the lattice vibrational modes, but also by pores, secondary phases, impurities, lattice defects, crystallizabillity and inner stress [13].According to our analyses, since a higher density resulted in a higher dielectric permittivity owing to a lower porosity for the fixed sintering temperature and since the amount of the secondary phase is not negligible, as observed from XRD patterns, the effect of the poly-phase mixture on dielectric permittivity change is equal as the density effect.The increase in activation time is beneficial to the phase structure and densification after sintering and crystallizability until the Q value reaches its highest value in all the observed samples.

Sample
This suggests that, for the activation and sintering conditions we used, a higher density, phase composition and the homogeneity of morphology are dominantly responsible for the higher values of dielectric permittivity of the samples.

Conclusions
The phase composition in MgC0 3 -TrO, solid solutions along with the microstructures and electrical properties were studied.The main conclusions are: With the increase in activation time, the evolution of microstructural constituents, grain growth, decrease of pore size along with densification are observed within SEM micrographs during the sintering process.
The XRD patterns of sintered samples showed the existence of MgTi0 3 magnesium titanates as the dominant phase with a certain amount of Mg,Ti0 4 , MgTizOs, TiO z rutile and TiO z (0) phases.
Also, microstructural parameters revealed from an approximation method confilmed the processes of particle size growth along with the decrease in crystal lattice defects.
The electrical properties are a consequence of the crystal structure caused by the milling process of starting powders.120 minutes of activation and the ratio of MgTi03 and the rest of phases are responsible for the best electrical properties of all samples obtained after sintering (the highest Q factor of 233, relative dielectric constant e; of 23.86 and low dielectric loss tgJ of 0.0043).