Coarse-grained Ceramics Research Articles

Relaxor-like behavior of the dielectric response of dense ferroelectric composites

We study experimentally and theoretically the influence of size effects and structural factors on the dielectric and ferroelectric properties of the dense ferroelectric composites. The composites have the form of the tape-casted films with 28 vol% of nanosized or submicron BaTiO3 particles dispersed in the polyvinyl butyral. We measured and analyzed the temperature dependences of the capacitance and dielectric losses in the temperature range (−200 – +200)oC and frequency range (102–105) Hz. The nonmonotonic temperature dependences of dielectric permittivity of the studied composite films have a wide plateau-like maxima located between 80oС and 160oС, in contrast to the relatively sharp maximum near the Curie temperature (∼124oC) observed in the fine-grained and coarse-grained ceramics, sintered from the same nanosized or submicron BaTiO3 particles at 1250oC. The shift of the dielectric permittivity and losses maxima (in more than 40oC) and their strong frequency dispersion do not agree with the behavior expected for the systems with a weak interaction between ferroelectric particles and a polymeric matrix. In contrast, we reveal the relaxor-like behavior of the composite dielectric response. To explain the observed results, we propose the analytical model, which considers the strong dipole-dipole cross-interaction of the BaTiO3 particles in the dense nanocomposite. The analytical model is corroborated by the structural studies of the BaTiO3 nanosized and submicron particles by X-ray phase analysis and static 137Ba NMR spectra, as well as by the finite element modelling of the composite polar properties, which confirms the strong cross-interactions between the dipole moments of single-domain nanoparticles and/or between the domain walls of different submicron polydomain particles.The obtained results may be useful for development of cheap and flexible lead-free dense ferroelectric nanocomposites with relaxor-like behavior of the dielectric response, which are promising for non-volatile memory and energy-saving elements, modulators, electrical converters and piezoresistive elements.

High depolarization temperature and low dielectric loss in high-temperature fine-grain composite piezoceramics

High depolarization temperature (Td) and low dielectric loss (tanδ) were obtained in Sc2O3/BiScO3-PbTiO3-Bi(Zn1/2Hf1/2)O3 (Sc2O3/BS-PT-BZH) high-temperature fine-grained composite piezoceramics prepared by high-energy ball milling and one-step pressureless sintering. The heterogeneous interfacial polarization effect and thermal stress between the Sc2O3 semiconductor secondary phase and the BS-PT-BZH perovskite piezoelectric matrix phase contribute to the polarization orientation of the ferroelectric domain in the electric field and the maintenance of the poled ferroelectric domain state under temperature excitation, respectively. Benefiting from the above two heterogeneous interfacial effects, the Sc2O3/BS-PT-BZH fine-grained (∼0.4 μm) ceramic exhibits a high Td of 349 °C, a low tanδ of 5.1 % and a moderate piezoelectric constant d33 of 495 pC/N, which are comparable to those of many BS-PT-based coarse-grained (>2 μm) ceramics. The advent of Sc2O3/BS-PT-BZH fine-grained piezoceramic is helpful to promote the development of high-performance miniaturized high-temperature piezoelectric devices.

Transient Ruddlesden-Popper-Type Defects and Their Influence on Grain Growth and Properties of Lithium Lanthanum Titanate Solid Electrolyte.

Lithium lanthanum titanate (LLTO) perovskite is one of the most promising electrolytes for all-solid-state batteries, but its performance is limited by the presence of grain boundaries (GBs). The fraction of GBs can be significantly reduced by the preparation of coarse-grained LLTO ceramics. In this work, we describe an alternative approach to the fabrication of ceramics with large LLTO grains based on self-seeded grain growth. In compositions with the starting stoichiometry for the Li0.20La0.60TiO3 phase and with a high excess addition of Li (Li:La:Ti = 11:15:25), microstructure development starts with the formation of the layered RP-type Li2La2Ti3O10 phase. Grains with many RP-type defects initially develop into large platelets with thicknesses of up to 10 μm and lengths over 100 μm. Microstructure development continues with the crystallization of LLTO perovskite, epitaxially on the platelets and as smaller grains with thinner in-grain RP-lamellae. Theoretical calculations confirmed that the formation of RP-type sequences is energetically favored and precedes the formation of the LLTO perovskite phase. At around 1250 °C, the RP-type sequences become thermally unstable and gradually recrystallize to LLTO via the ionic exchange between the Li-rich RP-layers and the neighboring Ti and La layers as shown by quantitative HAADF-STEM. At higher sintering temperatures, LLTO grains become free of RP-type defects and the small grains recrystallize onto the large platelike seed grains via Ostwald ripening. The final microstructure is coarse-grained LLTO with total ionic conductivity in the range of 1 × 10-4 S/cm.

Enhanced piezoelectric properties in coarse-grained 0.7Bi(Fe0.9985Mn0.0015)O3-0.3BaTiO3 ceramics

The lead-free BiFeO3-BaTiO3 (BF-BT) based ceramics with large piezoelectric coefficient (d33) have been desired for promising applications in high-temperature fields. Herein, the 0.7Bi(Fe0.9985Mn0.0015)O3-0.3BaTiO3 ceramics with different grain sizes are intentionally prepared by solid-state method, and a large d33 (232 pC/N) and high Curie temperature Tc (501 °C) are simultaneously achieved in the case of large grain size (>9 µm). X-ray diffraction and Rietveld refinement analyses indicate that these coarse-grained samples possess stronger rhombohedral distortion and enlarged lattice parameter compared to fine-grained samples, which make domain switching easier and give rise to the increase in strain. As a result, the piezoelectric and ferroelectric properties of such samples are significantly improved with increasing grain size from 3.05 µm to 9.07 µm. In addition, electric domain configuration and field-induced strain further prove that the strong piezoelectric response in coarse-grained samples is attributed mainly to enhanced non-180° domain mobility. This study reveals the origin of large piezoelectric response in coarse-grained BF-BT ceramics, and proposes an effective approach to improve the piezoelectric properties of BF-BT ceramics.

Exaggerated grain growth and the development of coarse-grained microstructures in lithium lanthanum titanate perovskite ceramics

Li3xLa2/3−x⎕1/3–2xTiO3 (LLTO) ceramics is recognized as one of the most promising solid electrolytes for applications in all-solid-state batteries. The total ionic conductivity is usually diminished by the contribution of grain boundaries, therefore microstructures with larger grain sizes are desirable. In this work we prepared compositions with slightly decreased TiO2 additions, i.e. increased La:Ti ratio, and triggered the formation of Ruddlesden-Popper type sequences with structural elements of the Li2La2Ti3O10 phase within the matrix perovskite LLTO grains as revealed by HAADF-STEM. These grains exhibit exaggerated grain growth (EGG), which initially results in the development of bimodal microstructures. At higher sintering temperatures, grain growth continues by enhanced coarsening via Ostwald ripening and the RP-type sequences recrystallize to perovskite LLTO. This results in the development of dense microstructures with LLTO grains measuring up to 100 µm in diameter. The findings represent a novel and straightforward approach for the preparation of coarse-grained LLTO ceramics.

Grain Size Effects in Mn-Modified 0.67BiFeO3-0.33BaTiO3 Ceramics.

Grain size can have significant effects on the properties of electroceramics for dielectric, piezoelectric, and ferroelectric applications. Here, we systematically investigate the effect of grain size on the structure and properties of Mn-modified 0.67BiFeO3-0.33BaTiO3 ceramics, an important lead-free piezoelectric ceramic that exhibits both a high piezoelectric coefficient and a high Curie point. Ceramics with average grain sizes ranging from 0.46 to 6.85 μm were prepared using conventional and spark plasma sintering. It was found that the morphotropic phase boundary compositions are composed of two polar structures, rhombohedral and tetragonal, with DC poling inducing an increase in the fraction of the rhombohedral phase. All ceramics show relaxor behavior and their freezing temperature moves to higher temperatures with increasing grain size, although their Burns temperature is independent of grain size. In fine-grained ceramics, which show pronounced relaxor behavior, significant grain size dependency is seen in dielectric, piezoelectric, and ferroelectric properties, which is attributed to the presence of single ferroelectric domains and high concentrations of polar nanoregions. In coarse-grained ceramics, a critical grain size of 2.83 μm yields the highest dielectric permittivity at room temperature, with the piezoelectric coefficient plateauing at this grain size, which can be attributed to the contribution of both polar nanoregions and high domain wall density.

Columbite-rich multiphase TiO2 nanoceramic with superior mechanical and dielectric properties

Columbite-rich multiphase TiO2 nanoceramics with outstanding mechanical and dielectric properties were successfully prepared through high-pressure sintering by using anatase-type TiO2 as precursor. High-pressure sintering combined with phase transformation assisted consolidation can effectively refine the grain size of the recovered samples. This process is conducive to obtain nanocrystalline columbite-rich TiO2 ceramics with excellent performance. The highest hardness is approximately 12.76 GPa, which is 2.5 times higher than that of ordinary coarse-grained ceramics. The effect of columbite phase on the hardness of multiphase ceramics is discussed. The columbite-rich TiO2 ceramic shows a colossal permittivity (∼8 × 103) and low loss (∼0.2) at 1 kHz and room temperature, which are superior to that of undoped rutile polycrystalline ceramics. This ceramic shows a steadier frequency-dependent dielectric permittivity and loss than rutile TiO2 crystal. These results enrich the fundamental knowledge of columbite-rich TiO2, thereby enabling the exploitation of new applications.

Fine-grained relaxor ferroelectric PMN-PT ceramics prepared using hot-press sintering method

Relaxor ferroelectric PbMg1/3Nb2/3O3-PbTiO3 (PMN-PT) ceramics are high-performance piezoelectric materials that are widely used in many electronic devices. However, it is extremely difficult to manufacture thin piezoelectric components dozens of micrometers in size using commercial coarse-grained PMN-PT ceramics. This limits the application of PMN-PT ceramics to high-frequency ultrasonic devices. In this work, we prepared a dense, fine-grained PMN-PT ceramic at low temperature using a hot-press sintering (HPS) method. The HPS ceramic had small grains with an average size of 1 μm and maintained high performance with a large permittivity (εr = 4500), piezoelectricity (d33 = 650 pC/N), and electrostrain (SE = 0.14% at 20 kV/cm). The large volume fraction of grain boundaries led to stronger relaxor behavior of the fine-grained PMN-PT ceramic than of the normal coarse-grained one. Owing to its small grains and high piezoelectric performance, this fine-grained PMN-PT ceramic meets the strict requirements of manufacturing thin piezoelectric components. Our results demonstrate that PMN-PT ceramics have great potential for developing low-cost, high-frequency ultrasonic devices by replacing expensive piezoelectric single crystals.

Microstructure effects on the energy storage density in BiFeO3-based ferroelectric ceramics

Lead-free BiFeO3–BaTiO3–Bi(Mg2/3Nb1/3)O3 (BFBT-BMN) ceramics with different grain sizes were fabricated by using a conventional solid-state reaction. The high-energy ball milling technique was utilized to obtain the fine-grained ceramics. The grain size reduction lowers the remanent polarization (Pr) and enhances the dielectric breakdown strength (Eb) leading to a drastic increase of the recoverable energy storage density (Wre). For the fine-grained BFBT-BMN ceramics, Wre greatly increases by 8 times from 0.16 J/cm3 to 1.27 J/cm3, compared to the coarse-grained BFBT-BMN ceramics. Transmission electron microscopy (TEM) and piezo-response force microscopy (PFM) measurements revealed the occurrence of the polar nanoregions (PNRs) in the fine-grained BFBT-BMN ceramics. The dynamic and weakly correlated PNRs structure is responsible for the enhancement of recoverable energy storage density in the fined-grained BFBT-BMN ceramics. This work indicates that the grain size reduction is an effective way to modify the energy storage properties of BiFeO3-based ferroelectric ceramics.

Structural origin of size effect on piezoelectric performance of Pb(Zr,Ti)O3

Grain size effect plays a vital role in piezoelectric performance from both scientific and technological view. However, the underlying structural mechanism related to grain size is still unclear. In the present study, the structural mechanism of grain size effect on piezoelectric performance has been revealed in the prototype Pb(Zr,Ti)O3 system by using in-situ synchrotron X-ray diffraction. The miniaturization of grain size tends to favor the appearance of higher symmetric tetragonal phase, while a single monoclinic phase is determined in the coarse-grained ceramics. The direct structural evidence reveals that both tetragonal and monoclinic phases in the fine-grained ceramics are less sensitive to the electric field, corresponding to the inferior piezoelectric performance, while the single monoclinic phase in the coarse-grained ceramics is more active to be driven by the electric field, generating good piezoelectric behavior. Both domain switching ability and lattice strain are suppressed with decreasing grain size, which directly leads to the deterioration in piezoelectric performance. The current results will benefit the structural understanding of the size effect of piezoelectric and other related systems.

New perspective of fracture mechanics inspired by gap test with crack-parallel compression

The line crack models, including linear elastic fracture mechanics (LEFM), cohesive crack model (CCM), and extended finite element method (XFEM), rest on the century-old hypothesis of constancy of materials' fracture energy. However, the type of fracture test presented here, named the gap test, reveals that, in concrete and probably all quasibrittle materials, including coarse-grained ceramics, rocks, stiff foams, fiber composites, wood, and sea ice, the effective mode I fracture energy depends strongly on the crack-parallel normal stress, in-plane or out-of-plane. This stress can double the fracture energy or reduce it to zero. Why hasn't this been detected earlier? Because the crack-parallel stress in all standard fracture specimens is negligible, and is, anyway, unaccountable by line crack models. To simulate this phenomenon by finite elements (FE), the fracture process zone must have a finite width, and must be characterized by a realistic tensorial softening damage model whose vectorial constitutive law captures oriented mesoscale frictional slip, microcrack opening, and splitting with microbuckling. This is best accomplished by the FE crack band model which, when coupled with microplane model M7, fits the test results satisfactorily. The lattice discrete particle model also works. However, the scalar stress-displacement softening law of CCM and tensorial models with a single-parameter damage law are inadequate. The experiment is proposed as a standard. It represents a simple modification of the three-point-bend test in which both the bending and crack-parallel compression are statically determinate. Finally, a perspective of various far-reaching consequences and limitations of CCM, LEFM, and XFEM is discussed.

Fabrication of mullite nano ceramic through addition of long-chain carbohydrates

Compared to common coarse-grained ceramics, nano-scaled mullite ceramics exhibit unique microstructures and mechanical properties. However, fabrication of homogeneous and ultra-fine ceramic crystals still remains a major challenge. In this study, fine-grain mullite nanocrystals with uniform particle distribution have been successfully synthesized by addition of agarose, which is a long-chain carbohydrate to the precursor. The effect of length of the carbohydrate chain on the size of mullite crystals has been investigated. The results show that long-chain carbohydrate agarose results in smaller grain size of mullite crystals as compared with glucose. Moreover, nano-scaled mullite ceramics with high hardness (18.36 GPa) and fracture toughness values (3.7 MPa m1/2) were obtained owing to the high relative density and small grain size. The present work highlights its potential application as long-chain carbohydrate in producing high-performance nanoceramics.

Dielectric behavior of spark plasma sintered BaTi0.7Zr0.3O3 relaxor ferroelectrics

In the present work we have studied the transport and dielectric behavior of BaTi0.7Zr0.3O3 (BTZ30) relaxor ferroelectrics prepared by spark plasma sintering. Nanocrystalline powder of BTZ30 was obtained by solid state reaction followed by mechanical milling. Fine and coarse-grained ceramics of BTZ30 were prepared by spark plasma sintering at 1100 and 1400 °C, respectively. The obtained materials were characterized by X-ray diffraction and scanning electron microscopy. The grain size of the BTZ30 ceramics increased from 200 nm to 3–5 μm as the sintering temperature was increased from 1100 °C to 1400 °C. The electrical and dielectric properties of BTZ30 ceramics were studied by impedance spectroscopy over −150 to +200 °C temperature range. The dielectric constant exhibits very large values >105 for the spark plasma sintered ceramics associated with very high dielectric loss and electrical conductivity. The insulating nature of the BTZ30 ceramics could be retained by re-oxidation of the samples. The dielectric data of the annealed have been analyzed by the modified Curie-Weiss law and the relaxation process in the materials was fitted by Vogel-Fulcher model.

Correlation of Al2O3-Ceramic Structure with the Mechanism of Surface Layer Formation for Workpieces During Diamond Grinding

Correlation of the structure of Al2O3 ceramics with specimen polished surface roughness and morphology is established. On basis of this the physical nature of surface layer formation during diamond grinding of fine-grained and coarse-grained ceramics is been revealed.

The interrelation of the structure of Al<sub>2</sub>O<sub>3</sub>-ceramics with the mechanism of formation of the surface layer of the workpieces during diamond grinding

The interrelation of the structure of Al2O3ceramics with the roughness and morphology of the polished surface of the samples was established. On this basis, the physical nature of the formation of the surface layer during diamond grinding of fine-grained and coarse-grained ceramics has been revealed. Ill. 2. Ref. 22.

On the origin of grain size effects in Ba(Ti0.96Sn0.04)O3 perovskite ceramics

Over the last 50 years, the study of grain size effects in ferroelectric ceramics has attracted great research interest. Although different theoretical models have been proposed to account for the variation in structure and properties of ferroelectrics with respect to the size of structural grains, the underlying mechanisms are still under debate. Here, we report the results of a study on the influence of grain size on the structural and physical properties of Ba(Ti0.96Sn0.04)O3 (BTS), a ferroelectric compound that represents a model perovskite system, where the effects of point defects, stoichiometry imbalance and phase transitions are minimized by chemical substitution. It was found that different microscopic mechanisms are responsible for the different grain size dependences observed in BTS. High permittivity is achieved in fine-grained BTS ceramics due to high domain wall density and polar nanoregions; high d33 is obtained in coarse-grained ceramics due to a high degree of domain alignment during poling; large electric field-induced strain in intermediate-grained ceramics is an outcome of a favorable interplay between constraints from grain boundaries and reversible reorientation of non-180° domains and polar nanoregions. These paradigms can be regarded as general guidelines for the optimization of specific properties of ferroelectric ceramics through grain size control.

A comparative study of dielectric and ferroelectric properties of sol–gel-derived BaTiO3 bulk ceramics with fine and coarse grains

Fine and coarse-grained BaTiO3 (BTO) ceramics were prepared by conventional sintering of sol–gel-derived BTO nanopowders at 1150 and 1350 °C, respectively. Based on characterization results, fine and coarse-grained BTO ceramics had an average grain size of about 1 and 12 µm, respectively. They exhibited a tetragonal structure with tetragonality (c/a ratio) of 1.0105. The dielectric properties of fine and coarse-grained BTO ceramics were measured in the frequency range of 100 Hz–10 MHz and temperature range of −45–180 °C. A dominant dielectric relaxation was observed at high frequency above 1 MHz for both BTO ceramics. Room-temperature dielectric constant of fine-grained BTO (1502) was greater than that of coarse-grained BTO (1082) at 1 kHz due to the grain size effect. For temperature dependence measurement, dielectric constant of fine-grained BTO was less sensitive with changing temperature at phase transition than coarse-grained BTO. Polarization–electric field (P–E) loop of coarse-grained BTO at room temperature revealed a well-defined hysteresis loop, confirming its ferroelectric switching behavior. In contrast, a lossy hysteresis loop was found for fine-grained BTO owing to its high leakage current. Our results in this work provide a useful information and progress in the dielectric and ferroelectric properties of sol–gel-derived BTO bulk ceramics.

Fabrication of Ti3SiC2 and Ti4SiC3 MAX phase ceramics through reduction of TiO2 with SiC

Ti3SiC2 and Ti4SiC3 MAX phase ceramics were fabricated through high-temperature vacuum reduction of TiO2 using SiC as a reductant, followed by hot pressing of the products under 25MPa of pressure at 1600°C. It was found that both Ti3SiC2 and Ti4SiC3 may be obtained in good yields, depending on the annealing time during the reduction step. In addition to MAX phases, the products contained some amounts of TiC. The hot pressing step did not significantly affect the composition of the products, indicating good stability of Ti3SiC2 and Ti4SiC3 under these conditions. Analysis of the densification behavior of the samples revealed lower ductility in Ti4SiC3 compared to Ti3SiC2. The samples prepared herein exhibited the flexural strength, fracture toughness and microhardness typical of coarse-grained MAX-phase ceramics.

Effects caused by antiferroelectric nanodomains in PZT-based coarse-grained ceramics with compositions from the morphotropic boundary region

Presented results demonstrate importance of taking into account such a phenomenon as the solid solution decomposition at the boundaries separating coexisting phases in lead zirconate-titanate-based solid solutions with compositions belonging to the morphotropic boundary region of the “temperature–composition” phase diagram. It is shown that in the local decomposition of solid solutions in the vicinity of the boundaries separating the tetragonal and rhombohedral phases in lead zirconate-titanate-based solid solutions lead to the changes of the solid solution’s chemical composition and to the formation of segregates. It is also shown that the proper thermoelectric treatment of samples containing these segregates can give substantially higher values of piezoelectric parameters in the lead zirconate-titanate-based compounds.

Energy storage properties of ultra fine-grained Ba0.4Sr0.6TiO3-based ceramics sintered at low temperature

Fine-grained Ba0.4Sr0.6TiO3 ceramics with and without dopants were fabricated by two-step sintering method. Densities of fine-grained ceramics were found lower than those of coarse-grained ceramics, while the breakdown strength improved. With ZnO-Li2O sintering aids doped, the sintering temperature of ceramics decreased to 1200 °C. Their grain size is 219 nm, energy density is 0.564 J/cm3 and AC breakdown strength is 198.8 kV/cm, which is close to Ba0.4Sr0.6TiO3 ceramics obtained by SPS, and much higher than normal BST ceramics. The polarization of ceramics under the same electric field decreases with miniaturization of grain size, but the energy storage efficiency increases. Although smaller polarization due to small grain size is a disadvantage to energy density of bulk material, the features of low sintering temperature, traditional sintering route and small grain size jointly make the utilization in multilayer devices possible, thereby larger energy density can be achieved.