Fine-grained Ceramics Research Articles

Spherical Submicron Powders with a Nanopolycrystalline Superstructure—a Promising Raw Material for Obtaining Fine-Grained High-Density Ceramics

Spherical Submicron Powders with a Nanopolycrystalline Superstructure—a Promising Raw Material for Obtaining Fine-Grained High-Density Ceramics

Chemical Stability of Fine-Grained Ceramics Based on Nd0.33Zr2(PO4)3 Phosphate with Kosnarite Structure at Elevated Temperatures

Chemical Stability of Fine-Grained Ceramics Based on Nd0.33Zr2(PO4)3 Phosphate with Kosnarite Structure at Elevated Temperatures

Ultrathin green-emitting LuAG:Ce-Al2O3 composite fine-grained ceramics for high-brightness chip-scale packaging LEDs

To realize green-emitting chip-scale packaging LEDs (CSP-LEDs) with high-brightness, the ultrathin green-emitting phosphor ceramic (PC) with high luminous flux (LF) and luminous efficiency (LE) is urgently explored. LuAG:Ce-Al2O3 fine-grained ceramics (about 150 μm thick) were prepared via vacuum solid-state sintering. Ultrathin LuAG:Ce PC owns LE up to 199.26 lm W−1@50 W as high Ce3+ concentration x = 0.02. Fine-grained composite structure induced by the introduction of Al2O3 results in an increase of LE to 209.95 lm W−1@50 W and an improvement of thermal stability from 91.7 % to 97.2 % at 200℃. It is noteworthy that corresponding optimal packaging CSP-LEDs demonstrates the LF of green lighting up to 978.3 lm@3500 mA. Thus these type of LuAG:Ce-Al2O3 PC is a promising candidate for the color converter of CSP-LEDs.

Fine-grained BCZT piezoelectric ceramics by combining high-energy mechanochemical synthesis and hot-press sintering

Different stoichiometries of lead-free BaZr0.2Ti0.8O3–Ba0.7Ca0.3TiO3 (BCZT) prepared by mechanosynthesis and sintered by either conventional sintering (CS) or hot pressing (HP) techniques were studied to establish the dependence of piezoelectric and dielectric properties on sintering parameters and microstructure. All synthesized stoichiometries showed a pseudocubic perovskite phase with homogeneously distributed A- and B-cations in the structure. The BCZT retained the pseudocubic symmetry after sintering and an average grain size <1.8 µm was obtained in all cases. HP sintering hindered the secondary phase segregation observed in the CS ceramics and increased the relative density. Piezoelectric coefficients (d33) ranging from 5.1 to 21 pC/N and from 10.0 to 88.0 pC/N were obtained for CS and HP ceramics, respectively, despite the pseudocubic symmetry and the fine grain size. The higher d33 values for the HP ceramics are a consequence of the higher density, better chemical homogeneity and lower sintering temperature and time required for the mechanosynthesized BCZT powders with high sintering activity.

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.

Fine grained textured BaTiO3-based piezoelectric ceramics with outstanding strain properties for the lead-free multilayer actuator

Textured BaTiO3-based piezoelectric ceramics show the excellent application potential as their piezoelectric characteristics almost approach the commercial lead-based ceramics. Piezoelectric multilayer actuators (MLAs) with low driving voltages and compact structures are great in line with the equipment trend of miniaturization and functional integration. However, the coarse textured grains result in few grain layers in each thin ceramic layer of MLAs, hardly guaranteeing the electromechanical dependability. In this work, we combined the composition regulation with a modified template grain growth (TGG) to prepare fine-grained textured ceramics. The Zr4+ and Sn4+ co-substituted (Ba0.95Ca0.05)(Ti0.94Zr0.05Sn0.01)O3 (BCZTS) was chosen as the matrix composition. Unreacted BaCO3 (BC, 7.3 % in content) was artificially introduced into matrix to realize the partial reaction template grain growth (RTGG), reducing textured grains from 9.7 μm to 6.9 μm. Textured BCTZS-BC MLAs achieved high texture degree of 92 % and further smaller grain size of 3.4 μm at 1300 °C, due to inhibition on mass transfer and improvement on thermal conductivity of Pt electrodes. The high strain of 0.12 % in textured MLAs is almost 2.3 times enhancement compared to non-textured counterparts, and the ultralow strain hysteresis Hs of 5.6 % at 30 kV/cm is equivalent to that in PZT MLAs. The microscopic coercive voltage is 3.1 V with a maximum displacement of 210 p.m., which is mainly ascribed to the smaller domain sizes and the favorable “4O” domain configuration in textured MLAs.

How does cold sintering affect the microstructural evolution of high-entropy pyrochlore during pressureless sintering?

The challenge of producing dense fine-grained high-entropy ceramics is hereby addressed by a combined cold and conventional pressureless sintering method for the first time. It provides nearly fully dense high-entropy pyrochlores with better microstructural homogeneity, and only half the grain size of conventional pressurelessly sintered samples with identical density. Both the finer and more homogeneous microstructures benefit better mechanical properties in hardness and reduced Young’s modulus. The kinetics and activation energies of both densification and grain growth were quantified and discussed to understand how cold sintering affects the microstructural evolution during the following pressureless sintering. More importantly, cathodoluminescence analysis suggested that cold sintering could cause more defects on the surface of the original particles, which could promote the grain boundary diffusion and have drag effects on grain boundary motion. The lessons learned here offer scientific understanding and technological guidance towards pressureless sintering of dense bulk high-entropy ceramics.

Downscaling grain size toward the nanometre range – A key-factor for tuning the crystalline structure, phase transitions, dielectric and ferroelectric behaviour in Ba0·8Sr0·2TiO3 ceramics

The present study aims to describe the role of the grain size on the properties of submicron- and nano-structured Ba0·8Sr0·2TiO3 (BST) ceramics. Dense (1 − 2% porosity) ceramics with average grain sizes in the range of (77 − 234) nm were consolidated under different spark plasma sintering conditions starting from nanopowders with a mean particle size of 70 nm, synthesized via the acetate variant of the sol-gel method. The structural analysis based on XRD data revealed a mixture of cubic and tetragonal modifications at room temperature for the precursor powders and for all the investigated ceramics. The structural heterogeneity of the individual ceramic grains with coexistence of cubic and tetragonal polymorphs was confirmed by HR-TEM investigations. Accordingly, a “brick-wall" model with cubic grain boundary regions and tetragonal grain cores is proposed. By increasing the grain size, from 77 to 234 nm, a decrease of the phase transitions diffuseness accompanied by an increase of the permittivity maxima (from 650 to 4500) and dielectric losses (from 5 to 7.5%, at 100 Hz), was detected by broadband dielectric spectroscopy. No variation of the Curie temperature in the investigated Ba0·8Sr0·2TiO3 ceramics was detected, unlike typically reported for BaTiO3 ceramics with similar grain sizes. The Curie-Weiss temperature and the Curie constant decrease when grain size is diminished, indicating an overall reduction of the ferroelectric active volume, as a scaling effect. The ferroelectric switching was demonstrated for all the selected fine-grained BST ceramics, either at nanoscale or macroscopically, with an increased ferroelectric character for the coarser submicron-structured ceramics, with respect to the nanocrystalline one. The observed properties of the fine-grained Ba0·8Sr0·2TiO3 ceramics are explained in the frame of multiphase coexistence and ferroelectricity “dilution” due to the increasing role of non-ferroelectric grain boundaries when reducing grain size and complete the knowledge on the scale-dependent properties of dense fine-grained BaTiO3-based ceramics.

Химическая устойчивость мелкозернистой керамики на основе фосфата Nd0,33Zr2(PO4)3 со структурой коснарита при повышенных температурах

Chemical stability of Nd0.33Zr2(PO4)3 fine-grained ceramics, which can be used for immobilization of REE that are part of the HLW was studied. Single-phase Nd0.33Zr2(PO4)3 submicron powders with the structure of the mineral kosnarite were obtained by colloid-chemical synthesis. Powders were obtained by successive annealing at 600, 800, and 900 °C for 6 h at each stage. Ceramics Nd0.33Zr2(PO4)3 has been obtained by Spark Plasma Sintering (SPS) method. The relative density of ceramics was 89.9 %; the average grain size was 5 – 20 μm. The chemical stability of the ceramics in the static mode at 90 °C in distilled water, mineral waters, as well as in acidic media (0.1 M HCl) and alkaline media (0.01 M NaOH) was studied. Ceramics have high hydrolytic stability. The influence of the contact medium on the rate leaching and mechanism of Nd leaching from the surface of Nd0.33Zr2(PO4)3 fine-grained ceramic samples has been studied. The de Groot-van der Sloot model was used to analyze the obtained time dependences of the leaching rate Ri. Nd leaching in acidic medium occurs due to dissociation of Nd from the ceramic surface, in alkaline medium and mineral water due to diffusion from the inner layers, in distilled water due to the dissolution of the ceramic surface layer.

Сравнительный анализ структуры и свойств керамик на основе оксида алюминия, полученных методами свободного и электроимпульсного плазменного спекания

The effect of the initial size of alumina particles on the density, microstructure, hardness, and fracture toughness of ceramics obtained by conventional and spark plasma sintering (SPS) has been studied. We studied ceramics obtained from commercial Al2O3 powders with an initial particle size of 40 – 50 nm, 0.2 mm, and 1 mm, and domestic fine powders with an initial particle size of 0.2 – 3 mm, and Al2O3 + 0.25 vol. % MgO and Al2O3 + 10 vol. % ZrO2. It is shown that the density of alumina ceramics nonmonotonically depends on the initial size of Al2O3 powder particles. It has been established that an increase in the grain size leads to a nonmonotonic change in the hardness of alumina ceramics. It has been established that the addition of 0.25 vol. % MgO accelerates the sintering of alumina. The addition of 10 vol. % ZrO2 makes it possible to provide an optimal combination of hardness and fracture toughness. It is shown that fine-grained ceramics obtained by the SPS method have a higher hardness. It has been suggested that SPS of submicron alumina powders with an amorphous layer on the surface, additionally stabilized by zirconia particles, is promising for further increasing the hardness of alumina ceramics.

2Y-TZP ceramics with high strength and toughness by optimizing the microstructure

Low-yttria doped zirconia polycrystals (Y-TZP) have recently emerged as new fine-grained zirconia ceramics exhibiting a promising combination of high fracture toughness and strength. In this investigation, we optimized the strength and fracture toughness of 2Y-TZP ceramics prepared by gelcasting and pressureless sintering. The 2Y-TZP ceramics with optimized processing reached a mean biaxial strength of 1070MPa, similar to the strength obtained for the standard 3Y-TZP ceramics. However, a higher transformability of tetragonal grains in 2Y-TZP ceramics resulted in a stronger transformation toughening effect compared with 3Y-TZP ceramics. The fracture toughness obtained for 2Y-TZP exceeded that of 3Y-TZP (7.7 vs. 5.2MPa m0.5). The differences in the microstructure and toughening mechanism between 2Y-TZP and 3Y-TZP were described, and the potential benefits of the 2Y-TZP were discussed.

Fine-grained NaNbO3-based relaxor antiferroelectric ceramics with high energy-storage performance

Fine-grained NaNbO3-based relaxor antiferroelectric ceramics with high energy-storage performance

Fine-grained high-performance Ba0.85Ca0.15 Zr0.1Ti0.9O3 piezoceramics obtained by current-controlled flash sintering of nanopowders

Due to environmental concerns, extensive research has been carried out to develop high-performance lead-free piezoceramics capable of replacing commercial lead-based materials. The lead-free (Ba0.7Ca0.3)TiO3 −Ba(Zr0.2Ti0.8)O3 system has emerged as a candidate for room temperature transducer applications because a high piezoelectric charge coefficient is achieved in this system for compositions at the morphotropic phase boundary. However, conventional ceramic processing of these eco-friendly piezoceramics demands high energy consumption because long-lasting, high-temperature heat treatments are needed, which often lead to microstructural degradation that compromises the material reliability. Field-assisted flash sintering has started to be explored since the application of an adequate electric field was shown to significantly reduce the sintering time and temperature, thereby controlling grain growth. In this work, Ba0.85Ca0.15Zr0.1Ti0.9O3 ceramics are obtained by current-controlled flash sintering of mechanosynthesized nanopowders. Exhaustive control of the sintering parameters allows tailoring of the microstructure, which allows dense fine-grained flash-sintered ceramics exhibiting a high electric field-induced strain response to be obtained.

Designing barium titanate ceramics with high energy storage and mechanical properties

Designing the fine-grained ceramics with high recoverable energy storage density and excellent mechanical performance, still presents great challenges. Because the ceramics with large pore size and low relative density were obtained by the traditional sintering method, they always exhibit small breakdown strength and poor mechanical properties, which limits the miniaturization and operation of devices in harsh environments. In this article, we designed the barium titanate ceramics (BT) ceramics with grain size of 252 nm and relative density of 0.92 can be obtained via co-sintering of two sizes of BT particles at 1000 °C for 10 h. The cubic phase BT particles with 80 nm are doped in the tetragonal phase BT particles with 200 nm to obtain tetragonal phase fine-grained BT ceramics. When doped with 80 nm BT particles, the sintering of 200 nm BT particles is promoted. The enhanced sinterability is due to the phase transformation and metastability, which act as sintering aids. The obtained BT ceramics with nanodomains, high energy storage, and mechanical properties. The discharge energy density is of 0.81 J/cm3, and the Vickers hardness is 1.75 GPa.

Structures and colossal dielectric behavior of Na1/3Ca1/3La1/3Cu3Ti4O12 ceramics fabricated via a sol−gel process

The dielectric response of a novel giant dielectric ceramic with good dielectric and electrical properties, Na1/3Ca1/3La1/3Cu3Ti4O12, was investigated in this work. This ceramic, with a perovskite−like structure, was successfully synthesized using a sol−gel method. Fine-grain ceramics were found in all sintered specimens. These ceramics produced large dielectric permittivities, 9.89 × 103−1.04 × 104, and low dielectric loss tangent values of 0.040–0.057. First-principles calculations indicated that Na+ is substituted at the Cu2+ sites. Additionally, La3+ likely occupies Ca2+ sites. Experimental and computational techniques demonstrated the presence of an oxygen vacancy in the Na1/3Ca1/3La1/3Cu3Ti4O12 lattice that results in generation of Cu+ and Ti3+. The IBLC phenomenon and small polaron hopping between valence electrons of Cu and Ti ions have been recognized as the causes of the giant dielectric behavior of these materials.

Improved microstructure and optical properties of Nd:YAG ceramics by hot isostatic pressing

Hot isostatic pressing (HIP) has unique advantages in the preparation of high-density, fine-grained ceramics. In this work, 1.0 at.% neodymium-doped yttrium aluminum garnet (Nd:YAG) powders were prepared via solid-state reaction (SSR) and co-precipitation (CP) methods, and differences in their microstructure after being subjected to the same sintering process were compared. After calcination of raw materials at different temperatures, there was significant phase change at 800 °C-1400 °C for SSR powders. However, this process did not occur for CP powders. Average grain size of Nd:YAG ceramics prepared via CP method was smaller than that of ceramics prepared via SSR method. HIP provided significant improvement in both microstructure and in-line transmittance of ceramics. Ceramics pre-fired under vacuum at 1700 °C showed slight increase in grain size after HIP, with densities close to 100%, and clean, narrow grain boundaries. Moreover, in-line transmittance of ceramics increased from 69.91% to 80.37% (at 1064 nm) via HIP.

Fine-grained transparent Dy2O3 ceramics fabricated from precipitated powders without sintering aids

As a promising material for Faraday isolator used in visible and infrared range, Dy2O3 ceramics with the average grain size of ∼800 nm and the in-line transmittance of 79.3% at 2000 nm and 70.0% at 633 nm were successfully fabricated. The Dy2O3 wet-chemically precipitated powders were densified in two steps consisting of vacuum pre-sintering and hot isostatic pressing (HIP) post-treatment. The precipitated precursor was calcined at 1150–1250 °C for 4 h in air to obtain Dy2O3 powders. The influences of calcination temperature on the morphology and sintering behavior of Dy2O3 powders were studied. The optical quality and microstructure of Dy2O3 ceramics with different pre-sintering temperatures were investigated. The Verdet constant of Dy2O3 ceramics at 633 nm is −322.1 ± 3.5 rad T−1 m−1, about 2.4 times that of commercial Tb3Ga5O12 crystals. Transparent Dy2O3 ceramics with high Verdet constant and suitable transparent windows present a prospective application for Faraday rotators.

Sintering pure polycrystalline boron carbide bulks with enhanced hardness and toughness under high pressure

Boron carbide ceramics possess high hardness, but they generally are subjected to low toughness due to strong covalent bonds. However, the fabrication of dense and pure polycrystalline boron carbide bulks with excellent mechanical properties by conventional methods remains a significant challenge. Here, the fine-grained and pure polycrystalline boron carbide bulks have been successfully fabricated utilizing submicron-sized boron carbide powder as starting materials under the condition of high pressure (5.5 GPa) and low temperature range (1000–1400 °C). The well-sintered boron carbide bulk recovered at 1000 °C exhibits a mean grain size of 205.2 ± 191.2 nm and relative density of up to ∼97%, and the measured Vickers hardness and fracture toughness achieve 35.0 ± 2.8 GPa and 4.6 ± 0.5 MPa m0.5, respectively. The outstanding trade-off between hardness and toughness demonstrated that the synergistic effect between fine grain size and dislocation, and the interplay between amorphous grain-boundary phases and pores via grain-boundary sliding have critical role in contributing to improve hardness and toughness, respectively. This study has essential implications in sintering fine-grained and pure polycrystalline engineering ceramics and superhard materials, which provide a tuning approach to enhance the mechanical properties.

Microscopic origin and relevant grain size effect of discontinuous grain growth in BaTiO3-based ferroelectric ceramics

Barium titanate [BaTiO3 (BT)]-based ceramics are typical ferroelectric materials. Here, the discontinuous grain growth (DGG) and relevant grain size effect are deeply studied. An obvious DGG phenomenon is observed in a paradigmatic Zr4+-doped BT-based ceramic, with grains growing from ∼2.2–6.6 to ∼121.8–198.4 μm discontinuously near 1320 ℃. It is found that fine grains can get together and grow into large ones with liquid phase surrounding them above eutectic temperature. Then the grain boundary density (Dg) is quantitatively studied and shows a first-order reciprocal relationship with grain size, and the grain size effect is dependent on Dg. Fine grains lead to high Dg, and then cause fine domains and pseudocubic-like phase structure because of the interrupted long-range ferroelectric orders by grain boundary. High Dg also causes the diffusion phase transition and low Curie dielectric peak due to the distribution of phase transition temperature induced by internal stress. Local domain switching experiments reveal that the polarization orientation is more difficult near the grain boundary, implying that the grain boundary inhibition dominates the process of polarization orientation in fine-grain ceramics, which leads to low polarization but a high coercive field. However, large-grain ceramics exhibit easy domain switching and high & similar ferroelectricity. This work reveals that the grain boundary effect dominates the grain size effect in fine-grain ceramics, and expands current knowledge on DGG and grain size effect in polycrystalline materials.

Fused Filament Fabrication of Thermal-Shock-Resistant Fine-Grained Refractories for Steel-Casting Applications

Three-dimensionally printed fine-grained refractory ceramics ready for use in contact with liquid steel based on developed one-step thermal debindable ceramic filaments that do not require any chemical solvent pre-debinding are investigated. This work exhibits the most favourable debinding and sintering regimes with an excellent form stability and reproducibility of printed products ensured. The structure of the sintered products was examined with computed tomography. The designed inner geometry with micro-porosity introduced during debinding combined with pre-designed printed macro-cavities enabled the outstanding thermal-shock performance of the specimens. The functionality of the sintered refractory products in the form of casting nozzles was preliminarily tested in contact with steel melt using a hot-stage microscope. The structure of the specimen was subsequently examined with laser scanning microscopy and scanning electron microscopy. The mechanical properties of printed samples were studied via mercury intrusion porosimetry, compressive strength testing, and spatial tensile strength testing. According to the results, the cold crushing strength of the 3D-printed specimens in the printing direction was comparable to that of pressed fine-grained alumina specimens (50–60 MPa). The measured porosity was 21.5 vol% with a pore size less than 10 µm, which is suitable for applications in contact with molten steel. In order to show thermal-shock resistance of the 3D-printed casting nozzle, a 100 kg steel-melt flow test was performed in a steel-casting simulator with the nozzle surviving all related thermal shocks as well as the ferrostatic pressure of the melt. The evaluated composition and production route of the filaments can be utilized to produce one-step, thermally debindable, thermal-shock-resistant refractory parts with a complex inner structure that are applicable in an industrial environment.