Influence of Mechanical Activation on Synthesis of Zinc

Investigations of a ZnO-TiO2 binary oxide mixture during mechanical treatment were mainly focused on obtaining orthotitanate Zn2TiO4 with a spinel structure. Due to the specific way of energy transfer during mechanical treatment using a high-energy ball mill, the system passes through low temperature ZnTiO3 metatitanate phase formation. Mechanical activation was performed on an equimolar ratio mixture of ZnO and TiO2. The anatase phase was previously submitted to heat treatment for achieving а starting mixture rich in a rutile phase. Milling conditions were preset for observing the formation of a low temperature ZnTiO3 phase with a perovskite structure. The powder microstructure was characterized using scanning electron microscopy. A nitrogen gas sorption analyzer with the BET method was used to determine the specific surface area and porosity, indicating changes of powder sample properties during mechanical activation. Also, X ray powder diffractometry was applied to obtain the phase composition. Powders were then pressed into pellets and their compressibility was observed through density changes. According to microstructures obtained by scanning electron microscopy analysis, the system underwent a primary and secondary agglomeration process. Specific surface area measurements supported that conclusion. Compressibility investigations established the difference between compressibility of the non-activated mixture and activated powders. X-ray diffraction analysis revealed that a perovskite structure forms simultaneously with a spinel phase during the process of mechanical activation.


Introduction
First investigations of a binary oxide system consisting of ZnO and TiO 2 introducing a phase diagram were conducted by Dulin and Race 1960 [1].The applications for zinc titanates were at that time mostly paint pigments and fusion cast thermistors.Zinc-titanate's first application was as a sorbent for removing sulfur from coal gasification products [2][3][4].Nowadays, due to the development of microwave dielectrics, zinc-titanates can be used as dielectric resonators and filters in microwave devices [5,6].Orthotitanate with a spinel structure and cubic lattice is considered the most stabile form of three compounds that coexist in the phase diagram of the ZnO-TiO 2 system and as a high temperature titanate form, it was widely investigated [4].Metatitanate ZnTiO 3 with a perovskite structure and with a hexagonal unit lattice is also a good candidate for microwave resonator materials [7], and also as a catalyst [8].A third most commonly mentioned form of titanate is Zn 2 Ti 3 O 8 .It is declared as a low temperature cubic form of ZnTiO 3 [9].
The most common way for obtaining metatitanate ZnTiO 3 is the sol-gel method [9,10].It provides fine and homogeneous ZnTiO 3 particles with a high specific surface area.In such a way, because the diffusion path is reduced, the temperature needed for synthesis in the subsequent thermal treatment is lowered.Low temperature is required for the formation of ZnTiO 3 because of the decomposition of ZnTiO 3 to Zn 2 TiO 4 and rutile at 945 o C.Recently zinc-titanate was obtained by a solid state reaction [11], which was avoided since a very narrow temperature interval is reserved for obtaining pure ZnTiO 3 as it slowly passes from the Zn 2 Ti 3 O 8 cubic form to pure hexagonal ZnTiO 3 , and decomposes at about 950 o C to Zn 2 TiO 4 and rutile [9].The solid-state route is simple to operate but needs high temperatures and produces powders with a large particle size.Very recently, high energy ball milling was conducted on the ZnO-TiO 2 system in order to obtain the most stable phase of orthotitanate Zn 2 TiO 4 [12,13], where intensive milling conditions enabled low tempearture titanate forms.The purpose of this work is to investigate events in the early stage of mechanical activation within the ZnO-TiO 2 system.

Experimental procedures
The starting materials were commercially available ZnO (99.9% Kemika-Zagreb) and TiO 2 (99.9%Alfa product-Ventron).Thermal treatment was conducted on TiO 2 powder to induce the phase transition from anatase to rutile.Heat treatment consisted of 3 times 8 hours at 1000 o C. Appropriate amounts of ZnO and TiO 2 were weighed in the ratio of 1:1.The powder mixture was afterwards placed in 500 cm 3 volume zirconium oxide vessels together with zirconium balls 10 mm in diameter and the ball to powder ratio was estimated as 20:1.The powders were submitted to mechanochemical treatment in a planetary ball mill device (Fritsch 'pulverisette ' 5) with the angular speed of the supporting disk set on 400 rpm.The overall time of milling was 160 minutes and the changes were observed in the following intervals: 5, 10, 20, 40, 80, 160 minutes.Powders were then sieved through a 0.2 mm sieve.
X-ray diffraction patterns of powder mixtures after milling were obtained using a Norelico-Philips PW 1050 diffractometer with λCuK α radiation and a step/time scan mode of 0.033º/1s.The morphology of obtained powders was characterized using a JSM 5300-JEOL, 30 kV scanning electron microscope.The specific surface area was determined with a Micromeritics InC.ASAP 2000 V1.03 sorbtomate with gas nitrogen analysis at -196 o K using the BET method.
The binder-free powders of non-activated and activated mixtures were compacted using the uniaxial double action pressing process in an 8 mm diameter tool (Hydraulic press RING, P-14, VEB THURINGER).The compaction pressure was varied from 49 MPa to 883 MPa to investigate powder behaviour during compaction.Specimen density was calculated from precise measurements of the specimen's diameter, thickness and mass.During the first 20 minutes of activation SSA increased from approximately 12 to 24 m 2 /g.The second period of mechanical treatment until 40 minutes, regarding the specific surface area, is intensive decreasing from 24 to 19 m 2 /g.The milling process consisted of two consecutive processes, namely breaking particles and secondary agglomeration originating from plastic flow.The rise and fall of SSA is subscribed to these processes in that particular order of appearance.A third period from 40 to 180 minutes of mechanical treatment, where SSA slowly increased from 19 to 23 m 2 /g, is assigned to the mechanochemical reaction and formation of new phases.Since we are following evolvement of phases that exist in a low temperature region, the chemical reaction passes through the formation of three different phases: Zn 2 TiO 4 orthotitanate, ZnTiO 3 metatitanate, and TiO 2 rutile.The specific surface area increased at the end of the observed period because the reaction is not finished and gravitates to the most stabile orthotitanate Zn 2 TiO 4 form.

Results and discussion
SEM characterization on fig. 2. reveals events during mechanical treatment.The non-activated mixture on the scanning electron micrograph (a) has large edge shape particles originating from the rutile phase obtained from anatase in a phase transition during thermal pretreatment.After 5 minutes of mechanical activation (b) we can see that due to the process of fracture, particles are smaller compared with the non-activated mixture and also different in size.The sample activated 10 minutes (c) has particles without any geometrical form together with particles approximately 0.3 micrometers in size that are agglomerated.The powder that was submitted to 20 (d) minutes of activation has particles that are different in size in the range from 0.2 to 0.5 micrometers, respectively, but not agglomerated.Particles activated 80 minutes (e) are most uniform in size, which can be subscribed to the presence of a mechanochemical reaction.Powders activated the maximal time observed (f) are edge shaped large particles in size of 1 micrometer and small particles of 0.2 micrometers indicating evolving of different phases.Phases that are present in the diffractogram pattern for 80 minutes of activation are zinc orthotitanate Zn 2 TiO 4 and a rutile phase.On the diffractogram pattern for the milling time of 160 minutes, together with zinc-orthotitante and rutile reflections, a reflection characteristic for metatitanate ZnTiO 3 occurred.The rutile reflection intensity has increased on this pattern.Diffractograms with this number of phases are very difficult to denote due to overlapping of reflections, especially for the step-scan ratio used here.However, the presence of three different phases is clearly visible.It is also important here to emphasize that a third form of titanate, namely Zn 2 Ti 3 O 8 is not considered, since it can be regarded as a low temperature cubic form of hexagonal ZnTiO 3 , consisting of identical TiO 6 octahedrons, with the same angle position of reflections, but different intensity proportion [9].The purpose was obtaining zinc-metatitanate.Besides using the stichiometric ratio of 1:1, zinc-metatitanate was aimed for with thermal pretreatment of TiO 2 where the anatase phase transformed to rutile.TiO 2 in a rutile modification is, according to literature [14], more convenient for obtaining a pervoskite structure.Also, the ball to powder ratio was estimated for energy transfer that is more appropriate for obtaining perovskite with mechanical activation in a high-energy planetary ball milling device [15].Yet, the ZnTiO 3 phase was obtained simultaneously with Zn 2 TiO 4 .All ZnO-TiO 2 phase diagrams presented until now, for the stoichiometric ratio 1:1, predict coexistence of these three phases in a low temperature region bellow 945 o C [1,4,6].A direct parallel between the phase diagram and mechanical activation is not easy to construct.The mechanical treatment route does not enable an physical analysis of the elementary reactive act -impact event.The unique way of energy transfer attached with the milling process does not enable such a comparison.Only preliminary investigations were conducted of the process of compaction, fig. 5. Powders of the non-activated mixture and the mixture activated 160 minutes show a similar response in density during compaction, while the powder activated 5 minutes achieves the highest densities of all three observed samples.In the non-activated mixture in the range from 200 to approximately 600 MPa, the supposed densification mechanism changes from particle rearrangment to particle deformation mechanism.The value of density then stagnates at around 3.0 g/cm 3 .The same course of densification is present for the powder activated 160 minutes, as product phases consisted of particles different in shape and size.The powder activated 5 minutes, is more convenient for compaction since particles have broken and as they are smaller their density constantly increases in the whole observed region. ZnO-TiO

Conclusion
For the milling conditions investigated, it is determined that the reaction simultaneously leads to formation of the phase mixture consisting of zinc-orthotitanate Zn 2 TiO 4 and metatitanate ZnTiO 3 .After the period of particle breaking until 20 minutes, and the succeeding period of secondary agglomeration, the reaction started after 40 minutes of milling.During compaction the powder activated for 5 minutes showed better compressibility, than the non-activated powder and the powder activated 160 minutes.

Fig. 1
Fig.1Specific surface area determined using the BET method as a function of the milling time.

2 θ
CuKα (deg) X ray diffraction patterns on figure 3 are sorted in the order of descending times of mechanical activation and labeled with times of milling as ZT 0, for the mechanically non activated mixture and ZT 5, ZT 10, ZT 20, ZT 40, ZT 80 and ZT 160.Diffracotogram patterns of powders activated 5, 10, 20 and 40 minutes show sharp reflections of the starting ZnO and TiO 2 oxides.During the activation time intensities of peaks lower significantly.First detectable traces of product phases are visible for times of milling denoted as ZT 80 and ZT 160.Those diffractograms are presented in the enlarged picture on fig. 4.

Fig. 5
Fig.5 Density as a function of pressure in a process of compacting ZnO-TiO 2 powders activated for different times: t0-non-activated mixture (0 minutes), t5-activated 5 minutes and t160-activated 160 minutes.