Structural-Phase Transformation Kinetics During Sintering of Alumina Ceramics using Metastable

The processes taking place during pressureless sintering of nanometastable Al2O3, compacted up to high densities (0.7 of the theoretical density) using the magnetic pulsed method were studied. The influence of MgO, TiO2 and ZrO2 additives on the kinetics of Al2O3 polymorphous transition, shrinkage and microstructure evolution during annealing at temperatures up to 1450°C has been studied. We have found that the process of annealing is two-staged starting with a polymorphous transition. Doping changes the starting temperature as well as the shrinkage depth at both stages. TiO2 and ZrO2 additives decrease the temperature of the onset of shrinkage, whereas MgO increases it. The best composition contained MgO in the series of examined types of ceramics with an α-Al2O3 matrix. The positive role of Mg addition in the production of dense and hard Al2O3 ceramics is related to the nature of Mg influence on the activation of diffusion processes in Al2O3, as well as to the way of uniform distribution of MgO dopant in the material. All these factors provide effective damping of diffusion processes and limit α-Al2O3 crystal growth. Highly dense MgO, ZrO2 and TiO2 doped Al2O3 ceramics with a grain size of 190, 220, and 250 nm and microhardness of 22, 17 and 17 GPa, correspondingly have been obtained.


Introduction
Due to accessibility and the unique combination of thermomechanical properties corundum seems to be a perspective material for a wide range of construction applications.The transition to a nanoscale structure promises essential improvement of the properties of corundum ceramics and extension of the application sphere.Though sintering of corundum ceramics is complicated with polymorphous transitions and a high speed of recrystallization this leads to a large grain size and thus to high non-uniformity of the material and low mechanical properties [1,2]._____________________________ *) Corresponding author: Khrustov@iep.uran.ruA technique, which includes the use of nanosized powders and their compaction up to high densities, enabling a temperature decrease and shortening of the duration of ceramics sintering, possesses obvious advantages.Though poor compressibility of nanosized powders requires the use of high-energy methods of consolidation [3].The magnetic-pulsed compaction (MPC) method, used in the present work, provides high (~0.7)relative density of nanopowder compacts and also mechanically activates the material and increases the stable modification content [4].Besides, the technique of limiting corundum crystal growth by insertion of small additives into the material, which form the second phase during sintering, is well known [5].Due to its different solubility in starting metastable modifications of Al 2 O 3 and in the formed corundum, MgO [6] is of especial interest.Also, the influence of additives of titania and zirconia appears to be interesting [5,7].
In the present study the research of alumina structural-phase transitions in the controlled processes of thermal annealing of nanopowder samples of pure alumina as well as alumina with additions of magnesia (2,6 mass%), titania (1 mass %) and zirconia (10 mass%), compacted up to high green density was made.High density of green bodies, achieved by MPC, as well as doping, allowed sintering of ceramics with a fine-grained structure.

Experimental methods
The research included the MPC of nanopowders, annealing of the ceramic samples, investigation of the structure and the phase content of the ceramics, measuring of microhardness.The characteristics of nanopowders, used in the investigation for ceramics sintering are presented in table.I (Pulsed processes laboratory, Institute of Electrophysics, UD RAS).The powders were produced by the method of electrical explosion of wire (EEW) followed by sedimentation in isopropanol, providing removal of large particle fractions with sizes greater than 200 nm [9].AT1 and AZ10 powders are the mechanical mixtures, prepared by cosedimentation of Al 2 O 3 and TiO 2 or ZrO 2 nanopowders correspondingly.The AM1-1 powder was produced by EEW of the alloy (Al+1.3mass% Mg) with the following sedimentation.Phases containing Mg were not detected in its composition.Hypothetically, Mg atoms take places of Al in the γ-Al 2 O 3 crystalline lattice.Due to the production method, the distribution of the alloy additive in the AM1-1 powder is more uniform compared to AT1 and AZ10 powders.Considerable contents of metastable phases are characteristic for all used nanopowder oxides: specifically, Al 2 O 3 consisted of metastable γ− and δ− modifications, TiO 2 contained a large amount of anatase and ZrO 2 -a tetragonal modification.All the powders are characterized mainly by spherical shaped particles, a wide spectrum of size distributions and weak agglomeration [8,9].
The powders were compacted by the MPC method with pressure pulses with amplitudes up to 1.2 GPa.The compacts were disk-shaped with the diameter of 30 mm and 3.5 mm thick and relative density of ~0.7 [10].The X-ray diffraction investigation indicates intensive mechanical activation of the material in terms of additional band broadening of Xray diffraction lines as well as the increase of the more stable modifications content.
Annealing of the ceramic items was realized in air at temperatures of up to 1450°C in an electroresistive furnace with controlled heating and cooling.
The phase content, microdistortions and crystallite size, d x , were investigated by the X-ray diffraction method (DRON-4).The crystalline size d x was determined by X-ray diffraction band broadening using the Sherer-Selyakov technique.The ceramics fracture microstructure was investigated by atomic-force microscopy (AFM) (Solver 47).The microhardness of the ceramics in the terms of Vickers, H v , the specific surface area, S BET , were determined using standard methods.The sample's density was measured by hydrostatic weighting.Note: γ and δ -polymorphs of Al 2 O 3 , A and R -polymorphs of TiO 2 anatase and rutile, М and Тmonoclinic and tetragonal modifications of ZrO 2 , d x -crystallite size, evaluated using the X-ray data, S ВЕТ -specific surface area using BET.

Results and discussion
A polymorphic transformation (γ+δ)→α, accompanied by volume change occurs during sintering of the precursor powder.Therefore shrinkage occurs in two stages (fig.1), and the first stage in temperature range Т А <T<Т В , corresponds to the polymorphic transformation, as detected by X-ray analysis [11].
The relative change of density during the polymorphic transformation is determined from the difference between theoretic densities of γand α-polymorphs: (2) The shrinkage value (∆L R ) = 1-(L TB /L TA ) of all powders investigated exceeds the calculated value (∆L R ) PT (fig. 2 b).Several investigators [6,12,13] noted a similar phenomenon of "hyper shrinkage" for polymorphous systems (Al 2 O 3 , TiO 2 ).The origin of such behavior is explained well by a model described in [6], where the change of crystalline volume during a polymorphous transformation leads to the appearance of nonsymmetrical forces, which move particles.The result of such movements is particle rearrangement resulting in closer packing.Thus, the observed shrinkage (∆L R ) in the temperature range Т А < T < Т В is the result of two processes: polymorphous transformation and particle rearrangement and its value exceeds the calculated one -(∆L R ) PT .It is worth mentioning, that the starting temperatures of shrinkage Т А correlated with (∆L R ) values (fig.2a): addition of MgO increases ТА, compared with pure Al 2 O 3 , and the value of (∆L R ) is the highest (fig.2b, AM1-1).On the other hand, additions of TiO 2 and ZrO 2 decrease Т А and the value (∆L R ) of considered powders is small (fig.2b, AT1  and AZ10).Data, presented in fig.3, show, that the evolution of crystalline size is governed by doping.Anomalous changes of corundum crystalline sizes were found in the TiO 2 doped system.Primary α-crystals have dimensions that significantly exceed the size of crystals of the initial phases.However, in the temperature range of bulk α-formation, a large number of crystals appeared to be comparable in the size with the crystals of the initial phases (fig.3b).In the case of MgO doping first α-crystals have sizes close to the size of crystals of the initial alumina phases (fig.3a).Structural evolution during the final stage of sintering has been analyzed using AFM images of cleavage surfaces of ceramic items made in "height" (relief) and "Mac*Cos" (phase contrast) modes (figs.4 and 5).In all cases the ceramic material consisted from polycrystalline blocks, and their shape and dimensions depended on doping.AFM data of the averaged block size, d, (see table II), demonstrates the restrictive effect of MgО and ZrO 2 additives on recrystallization of the Al 2 O 3 matrix.Uniformly distributed, well contoured, light, 10-20 nm sized areas are well distinguished in AFM "Mac*Cos" images (fig.4a).Probably the above-mentioned areas correspond to the second phase of MgAl 2 O 4 , previously determined by X-ray analysis.It is confirmed by an integrated analysis of the structure in "height" and "Mac*Cos" modes (fig.5).Sharp peaks on the "Mac*Cos" profile curve (fig.5c) and outlines of the relief corresponding to them testify of block non-homogenity and hence, the non-single phase nature of blocks.The above-mentioned peculiarity remains valid with increased sintering duration (fig.4b).Thus, the applied doping method enables sintering of fine-grained Al 2 O 3 ceramics with a homogeneous structure and uniformly distributed nanophase MgAl 2 O 4 .
The limiting effect of the ZrO 2 additive on recrystallization in Al 2 O 3 is entirely different.Despite of the closeness of microstructure scales of AM1-1 and AZ10 (tab.II), the structure of AZ10 is extremely non-uniform.AFM images display blocks of essentially different dimensions and a non-homogeneous admixture distribution (light areas in fig.4c).The observable areas of the second phase are polycrystalline due to a large discrepancy between crystalline size values measured by X-ray and defined from AFM images.It is important to note, that ZrO 2 in the alumina matrix mainly exists in a tetragonal modification (75 %) with a mean crystalline size of 70 nm.A minor part of ZrO 2 is found in the monoclinic state with a mean crystalline size of 30 nm.TiO 2 doped alumina ceramics is characterized with the largest block size (fig.4 d).Their surface, compared to the ceramic types analyzed above, is smooth, implying the proximity of block and crystal dimensions.Clusters of the admixture phase are nonhomogenously dispersed and located as layers 100 nm thick between alumina blocks (light areas in fig.4d).Some isolated grains of similar dimensions also exist.
The distinctive feature of AM1-1 and AT1 ceramics is that the ceramic structure forms very fast.Prolongation of sintering has a weak effect on it.On the contrary, a long duration of sintering is necessary for the formation of a dense structure in the case of ZrO The microhardness of all ceramic groups investigated was in the range 17 -22 GPa.The most hard ceramics was obtained in the case of MgO doping in a "short" regime.
All facts mentioned above testify the preference of MgO doped alumina ceramics among the α-alumina matrix ceramics investigated.In our opinion, the beneficial role of magnesium in the formation of dense and hard alumina ceramics is related to both the nature of the dopant influence on the activation of diffusion, and the way of dopant introduction.Magnesium in the initial powder is solved in the alumina crystalline lattice and does not form an individual phase.It is only during sintering that the magnesium-containing admixture (MgAl 2 O 4 ) is isolated at the surface of α-alumina crystals.Uniformity of admixture distributing provides effective retarding of diffusion and limits α-Al 2 O 3 grain growth.

Fig. 1
Fig. 1 Typical shrinkage curve of nano-Al 2 O 3 at constant heat rating.The range of polymorphic transformation is bordered with a dotted line.

Fig. 2
Fig. 2 Features of shrinkage in the first stage of nano-Al 2 O 3 , doped with MgO, TiO 2 , ZrO 2 : (a) -initial temperature of shrinkage (T A ); (b) -relative shrinkage (∆L R ) value in the first stage (T A < T < T B )

Table I
Characteristics of the precursor powders.

Table II
Characteristics of annealing conditions, structure and properties of the ceramics samples.