CC BY
17
UDC 621.762+546.831+541.4
V. V. Prisedsky, V. M. Pogibko, I. V. Mnuskina
COMPACTION AND SINTERING KINETICS OF PZT NANOPOWDER OBTAINED FROM OXALATE PRECURSOR
Key words: sintering, consolidation, nanocrystalline powder, piezoceramics, lead zirconate-titanate.
Compact ceramic samples of lead zirconate-titanate (PZT) have been fabricated by sintering nanocrystalline Pb(Zr0.52Ti0.48)O3 powder synthesized by thermal decomposition of oxalate precursor. Conditions of nanopowder cold compaction and kinetics of its sintering have been studied. PZT ceramic specimens consolidated from nanopowders are sintered at lower (by 300-350 °С) temperatures and have higher (by 25-45 %) dielectric and piezoelectric properties as compared to samples fabricated by conventional solid-state technology.
Ключевые слова: спекание, консолидация, нанокристаллический порошок, пьезокерамика, цирконат-титанат свинца.
Компактные керамические образцы цирконата-титаната свинца (ЦТС) получены спеканием нанокристалли-ческих порошков Pb(Zr0.52Ti0.48)O3, синтезированных термическим разложением оксалатного прекурсора. Изучены условия холодного прессования и кинетика спекания нанопорошков. Керамические образцы ЦТС, консолидированные из нанопорошков, спекаются при более низких температурах (на 300-350С) и обладают более высокими (на 25-45 %) диэлектрическими и пьезоэлектрическими свойствами по сравнению с образцами, изготовленными по традиционной твердофазной технологии.
Due to their superior electrophysical properties, lead zirconate-titanate (PZT) solid solutions [1] remain for decades the most widely used piezoelectric ceramic materials. PZT exhibits noncentrosymmetric perovskite structure and many its properties reach maximum at the morphotropic phase boundary (MPB), near the composition Zr:Ti = 52:48, where a tetragonal distortion of the perovskite cell changes for a rhombohedral one as the Zr:Ti compositional ratio increases. Among other factors influencing the properties of PZT are dopant composition and concentration, nonstoichiometry, density and porosity of sintered ceramic specimens, grain and crystallite sizes, method of production and type of raw materials etc.
During the last decades, the potential of influencing the properties by the synthesis of nanocrystalline PZT powders and consolidated nanostructured ceramics attracts increasing interest of researchers. Many techniques have been developed to produce nanocrystalline oxide perovskites: high-energy ball milling [2], laser deposition [3], various alternative wet-chemistry-based procedures such as hydrothermal methods, sol-gel processes, co-precipitation [4-8], thermal decomposition of organometallic, in particular oxalate, precursors [9-12]. The latter seems to have the best potential of adjusting the exact stoichiometry of a material.
In the present paper, we report the synthesis and properties of bulk nanostructured Pb(Zr0.52Ti0.48)O3 ceramics consolidated by compaction and sintering of nanocrystalline PZT powder (dav = 25 nm) obtained from an oxalate precursor.
Experimental
Nanocrystalline PZT powder was synthesized as described earlier [13].
Morphology and dimensions of grains in powdered and ceramic samples were studied by transmission (JEM 200A, JEOL) and scanning (TSM T30, JEOL) electron microscopy. Surface area was measured by Brunauer-Emmet-Teller (BET) method (SoftSorbi-
II). Shrinkage curves at sintering were recorded with DIL 402 PC dilatometer.
Capacity and dielectric loss were measured at 1 kHz. Piezoelectric properties were measured by reso-nance-antiresonance method on disk 10*1 mm specimens at 1 kHz. The specimens were polarized for 30 min in polyethylsiloxan at 120-150 °C in the field of 4 kV/mm.
Results and their discussion
Electron microphotographs (TEM) of synthesized PZT powder are shown in Fig. 1. The particles are uniform in sizes with average size dav 23 nm determined by the linear-intersept method and their form is
Fig. 1 - TEM microphotographs of PZT powder synthesized from oxalate precursor
close to polyhedral. The photomicrograph in Fig. lb is taken from greater amount of powder and reveals a considerable interparticle adhesion of nanocrystals.
0 150 300 450 P, Mra
Fig. 2 - Pressing curves for nano- (1, 2) and micro-(3,4) crystalline PZT powders. 1,3 - without binder; 2,4 - with DBC + PVB complex binder
Complete PZT solid solution formation after calcination oxalate precursor at maximum temperature of 750 °C is confirmed by X-ray diffraction analysis. According to XRD, synthesized powder is a single-phased perovskite. The average crystallite size (CSR) calculated from the observed broadenings of <ll l> and <200> reflexes equals 25 nm in practical agreement with the TEM measurements.
Ceramic specimens in the form of disks 10 mm in in diameter and l.0-l.5 mm thick were pressed and then sintered from the synthesized PZT powder. A high degree of dispersion, the uniformity in sizes and an extensive interparticle adhesion create substantial difficulties in forming bulk compacts from nanocrystalline powders.
As seen from pressing curves (Fig. 2), the total porosity of a compact pressed without binder at 300 MPa from the nanocrystalline PZT powder exceeds 45 % - twice as much as that of microcrystalline powder prepared by conventional solid-state synthesis. Such pressed pellets are easily broken. To obtain satisfactory results at the forming step, we added a liquid SAS binder to enhance sliding and rotation of nanocrystallites in a pressed pellet, and also the maximum pressure of forming was increased to 600 MPa. The best results were obtained using a complex binder composed of dibutylsebacinate (DBC) and polyvinylbutiral (PVB) dissolved in acetone.
Fig. 3 shows the results of the dilatometric analysis. The sintering behavior of nanocrystalline PZT powder is compared with that of conventionally synthesized microcrystalline PZT. In both cases, a phase of small volumetric expansion is observed before a pronounced densification starts. In the polythermal regime of heating the nanocrystalline sample at the rate of l0°C/min, the shrinkage starts at 600 °C and is over at 950°C. This is lower by more than 300°C than those temperatures for conventionally prepared PZT powder.
(AL'l)x103 80 ■
0
-80 "
0 200 600 1000 V, °C
Fig. 3 - Dilatometric curves for sintering PZT pressed powders: 1 - conventional solid-state synthesis; 2 - nanocrystalline powder obtained from oxalate precursor. Heating rate: 10 °C/min
The isothermal kinetics of densification in the course of free sintering of pressed pellets was investigated in the temperature interval 700 to 850°C. For this purpose, the volume densities of specimens were calculated both from diametral and thickness shrinkage measurements and also by the Archimedes method on pellets quenched to room temperature after a predetermined isothermal sintering time. The total porosity 0 was calculated from comparison between experimentally determined and theoretical (calculated from from XRD) densities.
The isothermal kinetics of densification in the course of free sintering of pressed pellets was investigated in the temperature interval 700 to 850°C. For this purpose, the volume densities of specimens were calculated both from diametral and thickness shrinkage measurements and also by the Archimedes method on pellets quenched to room temperature after a predetermined isothermal sintering time. The total porosity 0 was calculated from comparison between experimentally determined and theoretical (calculated from XRD) densities.
One and half an hour of isothermal sintering at 850 °C is sufficient to decrease the total residual porosity of the specimens below 3 % (Fig. 4a). This value is characteristic for high-quality PZT ceramics obtained by free sintering. The sintering kinetics shows a semilogarithmic behavior: the decrease in porosity (00 -0) changes linearly with logarithm time (Fig. 4b). The semilogarithmic behavior is valid both for intermediate and final stages of sintering and corresponds to the equation
„ _ r,.r. N-a-D-á3 .
60-e=BX) nr= AkT ■ Inr (1)
proposed by Coble [14] for the densification model of vacancy lattice diffusion from pores to their sinks at the grain boundaries with concurrent grain growth according to the cubic law:
<^-^0 )=AT)r (2)
Fig. 4 - Densification kinetics in the course of sintering nanocrystalline PZT in direct (a) and semilogarithmic (b) coordinates. Temperature, °C: 1 - 700; 2 - 750; 3 - 800; 4 - 850
In these equations N is a numerical constant, N = 10 for the intermediate stage (mainly continuous pore phase) and N = 3n for final stage (isolated pores) of sintering; c is the surface energy; D - the diffusion coefficient, m2/s; а - the lattice spacing, m; dg u dg(0) - current and initial grain sizes correspondingly, m; k = 1.38-10-23 J/K -Boltzmann's constant; T - temperature, K.
The kinetic parameters for densification in the course of sintering pressed nanosized PZT powders were calculated from given experimental results (Tabl. 1).
The activation energy EA for densification was found from the temperature dependence of В(Т) parameter. The diffusion coefficients D of particles which limit the rate of mass transfer in the process of sintering were calculated from found В(Т) values using Eq. (1) (the value of surface tension for PZT was taken as c = 1.6 J/m2). Calculated activation energy for densification ЕА = 75 ± 4 kJ/mol may be compared to 140 kJ/mol found for sintering pressed parts from PZT powder obtained by conventional solid-state synthesis [15].
The orders of magnitude of estimated D and ЕА are close to the selfdiffusion parameters of Pb cations and are much different from the diffusion parameters for oxygen or smaller cations Zr and Ti in PZT [15].
Electrophysical properties of Pb(Zr0/52Ti0.48)03, piezoceramics consolidated by sintering nanocrystalline powder (nc) are substantially higher than those of samples sintered by conventional technology (ct). The values of the piezomoduli d3i = 120 ± 5 pC/N and d33 = 270 ±10 pC/N are higher by 25-30 %, the dielectric
permettivity z33/z0 = 1100 ± 60 - by 45 %. The values of the electromechanical coupling factor Rp, the mechanical quality factor Qm, the dielectric losses tg 5 are also improved for nanostructured samples.
Table 1 - Kinetic parameters of densification in sintering nanocrystalline PZT
Parameter Temperature, °C
700 750 800 850 950
Coefficient В in eq.(1) B102 2.42± 0.19 3.84± 0.22 5.02± 0.16 7.50± 0.12 -
Activation energy of densification, kJ/mol 75 ± 4
Diffusion coefficient D-1017, m2/s 0.74± 0.06 1.22± 0.08 3.4±0 .2 8.9±0 .3 -
Activation energy of diffusion, Ea, kJ/mol 188±15
Parameter Temperature, °С
700 750 800 850 950
Coefficiet В in eq.(1) B102 2.42± 0.19 3.84± 0.22 5.02± 0.16 7.50± 0.12 -
Activation energy of densifica-tion, kJ/mol 75 ± 4
Diffusion coefficient D-1017, m2/s 0.74± 0.06 1.22± 0.08 3.4± 0.2 8.9± 0.3 -
Activation energy of diffusion, Ea, kJ/mol 188±15
In this respect consolidated ceramics is distinguished from free standing particles in a nanopowder, in which a decrease in size produces a reduction in the Curie temperature and suppression of ferroelectric properties.
Conclusions
Compact ceramic samples of lead zirconate-titanate (PZT) are sintered from nanocrystalline Pb(Zr0.52Ti0.48)03 powder (dav = 25 nm) at temperatures lower by 300-350 °C than conventional ceramic samples. To make nanopowders, liquid binders based on surfactants were selected to promote sliding and rotation of nanocrystallites in compacts. The sintering and grain growth kinetics corresponds to the model with bulk dif-
fusion in the transient and final sintering periods accompanied by simultaneous grain growth by cubic law.
Consolidation of the nanocrystalline powder during sintering leads to much coarser (micron) grains. The latter do not result from normal diffusion-controlled growth of the starting nanocrystalline particles but from the association of many (103-104) crystallites under the action of high surface energy, which changes their orientation in a correlated manner through slipping and rotation. The resultant ceramics have two-level grain structure: nanosized crystallites divided by low-angle boundaries and descending from nanocrystalline powder particles and microcrystalline grains divided by highangle boundaries. The use of nanocrystalline powder to make PZT ceramics allows the nanosize of crystallites to be controlled, thus promoting nanostructured consolidated material.
The differences in the size ratio of crystallites (CSDs) and grains divided by high-angle boundaries lead to better dielectric and piezoelectric properties of the PZT ceramics compared to conventionally synthesized samples.
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© V. V. Prisedsky, D. SC., Professor, head of the Department of General chemistry, Donetsk national technical University, faculty of ecology and chemical technology, Donetsk; V. M. Pogibko, Ph. D., Deputy Director STC «Reaktivelektron", Donetsk; I. V. Mnuskina, Ph. D., associate Professor of the Department of commodity science and food technologies, Kazan cooperative Institute of Russian University of cooperation, the Dean of engineering and technology faculty, Kazan, irina.valerevna.71@mail.ru.
© В. В.Приседский - д.х.н., проф., зав. каф. общей химии Донецкого национального технического университета; В. М. По-гибко - к.х.н., заместитель директора НТЦ «Реактивэлектрон», г. Донецк; И. В. Мнускина - к.х.н., доц. каф. товароведения и технологии общественного питания Казанского кооперативного института Российского университета кооперации, т-na.valerevna.71 @таП.гц.
