Showing papers on "Sintering published in 2015"

Unfolding grain size effects in barium titanate ferroelectric ceramics

Abstract: Grain size effects on the physical properties of polycrystalline ferroelectrics have been extensively studied for decades; however there are still major controversies regarding the dependence of the piezoelectric and ferroelectric properties on the grain size. Dense BaTiO3 ceramics with different grain sizes were fabricated by either conventional sintering or spark plasma sintering using micro- and nano-sized powders. The results show that the grain size effect on the dielectric permittivity is nearly independent of the sintering method and starting powder used. A peak in the permittivity is observed in all the ceramics with a grain size near 1 μm and can be attributed to a maximum domain wall density and mobility. The piezoelectric coefficient d33 and remnant polarization Pr show diverse grain size effects depending on the particle size of the starting powder and sintering temperature. This suggests that besides domain wall density, other factors such as back fields and point defects, which influence the domain wall mobility, could be responsible for the different grain size dependence observed in the dielectric and piezoelectric/ferroelectric properties. In cases where point defects are not the dominant contributor, the piezoelectric constant d33 and the remnant polarization Pr increase with increasing grain size.

Enhanced energy storage density of Ba0.4Sr0.6TiO3–MgO composite prepared by spark plasma sintering

Abstract: (100 − x ) wt%Ba 0.4 Sr 0.6 TiO 3 – x wt%MgO composite ( x = 3, 5, 10) were prepared by spark plasma sintering (SPS) and their phase constitutions, microstructure and electrical energy storage properties were investigated. It was found that the reaction between Ba 0.4 Sr 0.6 TiO 3 and MgO was suppressed due to low sintering temperature and short sintering period. The Curie temperature of BST/MgO composite is independent of the MgO content, which confirms the absence of chemical interdiffusion between BST and MgO. Diffuse phase transition was observed due to the reduced grain size and lack of sufficient chemical diffusion during SPS process. The dielectric breakdown strength of Ba 0.4 Sr 0.6 TiO 3 /MgO composite was greatly improved, which results in a significant improvement of maximum energy storage density. The high dielectric breakdown strength of 300 kV/cm, highest maximum energy storage density of 1.50 J/cm 3 and high energy storage efficiency of 88.5% were obtained in 95 wt%Ba 0.4 Sr 0.6 TiO 3 –5 wt%MgO composite.

Sintering of Lead-Free Piezoelectric Sodium Potassium Niobate Ceramics

Abstract: The potassium sodium niobate, K0.5Na0.5NbO₃, solid solution (KNN) is considered as one of the most promising, environment-friendly, lead-free candidates to replace highly efficient, lead-based piezoelectrics. Since the first reports of KNN, it has been recognized that obtaining phase-pure materials with a high density and a uniform, fine-grained microstructure is a major challenge. For this reason the present paper reviews the different methods for consolidating KNN ceramics. The difficulties involved in the solid-state synthesis of KNN powder, i.e., obtaining phase purity, the stoichiometry of the perovskite phase, and the chemical homogeneity, are discussed. The solid-state sintering of stoichiometric KNN is characterized by poor densification and an extremely narrow sintering-temperature range, which is close to the solidus temperature. A study of the initial sintering stage revealed that coarsening of the microstructure without densification contributes to a reduction of the driving force for sintering. The influences of the (K + Na)/Nb molar ratio, the presence of a liquid phase, chemical modifications (doping, complex solid solutions) and different atmospheres (i.e., defect chemistry) on the sintering are discussed. Special sintering techniques, such as pressure-assisted sintering and spark-plasma sintering, can be effective methods for enhancing the density of KNN ceramics. The sintering behavior of KNN is compared to that of a representative piezoelectric lead zirconate titanate (PZT).

High thermoelectric performance in copper telluride

Abstract: Recently, Cu2-δS and Cu2-δSe were reported to have an ultralow thermal conductivity and high thermoelectric figure of merit zT. Thus, as a member of the copper chalcogenide group, Cu2-δTe is expected to possess superior zTs because Te is less ionic and heavy. However, the zT value is low in the Cu2Te sintered using spark plasma sintering, which is typically used to fabricate high-density bulk samples. In addition, the extra sintering processes may change the samples’ compositions as well as their physical properties, especially for Cu2Te, which has many stable and meta-stable phases as well as weaker ionic bonding between Cu and Te as compared with Cu2S and Cu2Se. In this study, high-density Cu2Te samples were obtained using direct annealing without a sintering process. In the absence of sintering processes, the samples’ compositions could be well controlled, leading to substantially reduced carrier concentrations that are close to the optimal value. The electrical transports were optimized, and the thermal conductivity was considerably reduced. The zT values were significantly improved—to 1.1 at 1000 K—which is nearly 100% improvement. Furthermore, this method saves substantial time and cost during the sample’s growth. The study demonstrates that Cu2-δX (X=S, Se and Te) is the only existing system to show high zTs in the series of compounds composed of three sequential primary group elements. A time-saving procedure for boosting the performance of experimental thermoelectric energy harvesters has been developed by a team in China. Recently, copper sulfide (CuS) and copper selenide (CuSe) compounds have garnered interest as thermoelectric generators because their extraordinarily low thermal conductivities enable highly efficient conversion of temperature swings into electricity. However, copper telluride (CuTe) compounds, which have even lower lattice thermal conductivities than CuS or CuSe compounds, have so far displayed only moderate thermoelectric capacities. Xun Shi at the Chinese Academy of Sciences and co-workers solved this mystery by eliminating the spark plasma sintering procedure normally used to produce high-density CuS and CuSe thermoelectrics. The researchers raised the thermoelectric efficiency by a few times by directly annealing CuTe crystals. They attribute this increase to better control over carrier concentrations in the samples’ crystal structure. Enhanced thermoelectric figure of merit in the fully densified Cu2Te bulk materials by direct annealing method.

High-Energy Storage Performance of (Pb0.87Ba0.1La0.02)(Zr0.68Sn0.24Ti0.08)O3 Antiferroelectric Ceramics Fabricated by the Hot-Press Sintering Method

Abstract: (Pb0.87Ba0.1La0.02)(Zr0.68Sn0.24Ti0.08)O3 (PBLZST) antiferroelectric (AFE) ceramics have been prepared by hot-press sintering method and conventional solid-state reaction process, and the dependence of microstructure and energy storage properties of the ceramics on sintering approaches has been studied. The results reveal that not only the microstructure, but also the electrical properties of the PBLZST AFE ceramics are significantly improved by using the hot-press sintering method. Samples resulting from the hot-press sintering process have high breakdown strength of 180 kV/cm which results from the increase of density. Coupled with large polarization, the hot-pressed AFE ceramics are shown to have a high recoverable energy density of 3.2 J/cm3. The recoverable energy density of the hot-pressed PBLZST AFE ceramics is 100% greater than the conventional sintered specimens with recoverable energy density of 1.6 J/cm3.

Characterization of hot-pressed graphene reinforced ZrB2 -SiC composite

Abstract: In this paper, the hot pressing of monolithic ZrB2 ceramic (Z), ZrB2–25 vol% SiC composite (ZS), as well as 5 wt% graphene reinforced ZrB2–25 vol% SiC composite (ZSG) is investigated. The hot pressing at 1850 1C for 60 min under a uniaxial pressure of 20 MPa resulted in a near fully-dense ZSG composite (499% relative density). In addition, the influence of graphene reinforcement on the sintering process, microstructure, and mechanical properties (Vickers hardness and fracture toughness) of ZrB2–SiC composite is discussed. It was disclosed that the grain growth of the ZrB2 matrix was effectively stopped by SiC particles and graphene nano-platelets. The fracture toughness of ZSG composite (6.4 MPa m 1/2 ) was strongly enhanced by incorporating the mentioned reinforcements into the ZrB2 matrix, which is higher than that of Z ceramic (1.8 MPa m 1/2 ) and ZS composite (4.3 MPa m 1/2 ). After the hot pressing process, the fractographical outcomes revealed that some graphene nano-platelets were kept in the composite microstructure, apart from SiC grains, which lead to the toughening of the composite through graphene nano-platelets wrapping and pull out, crack deflection, and crack bridging. & 2014 Elsevier B.V. All rights reserved.

Effect of grain size on dielectric and ferroelectric properties of nanostructured Ba 0.8 Sr 0.2 TiO 3 ceramics

Abstract: Barium strontium titanate (Ba0.8Sr0.2TiO3, BST) nanocrystalline ceramics have been synthesized by high energy ball milling. As the sintering temperature increases from 1200 °C to 1350 °C, the average grain size of BST ceramics increases from 86 nm to 123 nm. The X-ray diffraction (XRD) studies show that these ceramics are tetragonal. The phase and grain size of the sintered pellets have been estimated from the XRD patterns, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images. The effect of grain size on dielectric and ferroelectric properties is studied. The dielectric and piezoelectric parameters are greatly improved at room temperature with increase in grain size. The Curie transition temperature is found to shift slightly towards higher temperatures as the grain increases from 86 nm to 123 nm. The coercive field decreases and the remnant polarization and spontaneous polarization increase as the grain size of BST nano ceramics increases. These ceramics are promising materials for tunable capacitor device applications.

Comparison of laser and intense pulsed light sintering (IPL) for inkjet-printed copper nanoparticle layers

Abstract: In this contribution we discuss the sintering of an inkjet-printed copper nanoparticle ink based on electrical performance and microstructure analysis. Laser and intense pulsed light (IPL) sintering are employed in order to compare the different techniques and their feasibility for electronics manufacturing. A conductivity of more than 20% of that of bulk copper material has been obtained with both sintering methods. Laser and IPL sintering techniques are considered to be complementary techniques and are highly suitable in different application fields.

Influence of layer-by-layer laser sintering conditions on the quality of sintered surface layer of products

Abstract: The influence of technological modes of sintering: the displacement velocity of laser beam V, laser power P, scanning step S and preheating temperature of powder material t on the quality of sintered surface layer of aluminum powder PA-4, copper powder PMS-1 and cobalt- chromium-molybdenum powder DSK-F75 were studied.

Evidences of grain boundary capacitance effect on the colossal dielectric permittivity in (Nb + In) co-doped TiO2 ceramics

Abstract: The (Nb + In) co-doped TiO2 ceramics were synthesized by conventional solid-state sintering (CSSS) and spark plasma sintering (SPS) methods. The phases and microstructures were studied by X-ray diffraction, Raman spectra, field-emission scanning electron microscopy and transmission electron microscopy, indicating that both samples were in pure rutile phase while showing significant difference in grain size. The dielectric and I–V behaviors of SPS and CSSS samples were investigated. Though both possess colossal permittivity (CP), the SPS samples exhibited much higher dielectric permittivity/loss factor and lower breakdown electric field when compared to their CSSS counterparts. To further explore the origin of CP in co-doped TiO2 ceramics, the I–V behavior was studied on single grain and grain boundary in CSSS sample. The nearly ohmic I–V behavior was observed in single grain, while GBs showed nonlinear behavior and much higher resistance. The higher dielectric permittivity and lower breakdown electric field in SPS samples, thus, were thought to be associated with the feature of SPS, by which reduced space charges and/or impurity segregation can be achieved at grain boundaries. The present results support that the grain boundary capacitance effect plays an important role in the CP and nonlinear I–V behavior of (Nb + In) co-doped TiO2 ceramics.

Bimodal Sintered Silver Nanoparticle Paste with Ultrahigh Thermal Conductivity and Shear Strength for High Temperature Thermal Interface Material Applications

Abstract: A bimodal silver nanoparticle (AgNP) paste has been synthesized via the simple ultrasonic mixing of two types of unimodal AgNPs (10 and 50 nm in diameter). By sintering this paste at 250 °C for 30 min, we obtained an ultrahigh thermal conductivity of 278.5 W m–1 K–1, approximately 65% of the theoretical value for bulk Ag. The shear strength before and after thermal cycling at 50–200 °C for 1000 cycles was approximately 41.80 and 28.75 MPa, respectively. The results show that this excellent performance is attributable to the unique sintered structures inside the bimodal AgNP paste, including its low but stable porosity and the high density coherent twins. In addition, we systematically discuss the sintering behavior of this paste, including the decomposition of the organic layers and the formation of the coherent twins. On the basis of these results, we confirm that our bimodal AgNP paste has excellent potential as a thermal interface material for high temperature power device applications.

Separating bulk from grain boundary Li ion conductivity in the sol–gel prepared solid electrolyte Li1.5Al0.5Ti1.5(PO4)3

Abstract: Lithium aluminium titanium phosphate (LATP) belongs to one of the most promising solid electrolytes. Besides sufficiently high electrochemical stability, its use in lithium-based all-solid-state batteries crucially depends on the ionic transport properties. While many impedance studies can be found in literature that report on overall ion conductivities, a discrimination of bulk and grain boundary electrical responses via conductivity spectroscopy has rarely been reported so far. Here, we took advantage of impedance measurements that were carried out at low temperatures to separate bulk contributions from the grain boundary responses. It turned out that bulk ion conductivity is by at least three orders of magnitude higher than ion transport across the grain boundary regions. At temperatures well below ambient long-range Li ion dynamics is governed by activation energies ranging from 0.26 to 0.29 eV depending on the sintering conditions. As an example, at temperatures as low as 173 K, the bulk ion conductivity, measured in N2 inert gas atmosphere, is in the order of 8.1 × 10−6 S cm−1. Extrapolating this value to room temperature yields ca. 3.4 × 10−3 S cm−1 at 293 K. Interestingly, exposing the dense pellets to air atmosphere over a long period of time causes a significant decrease of bulk ion transport. This process can be reversed if the phosphate is calcined at elevated temperatures again.

Alumina coated nickel nanoparticles as a highly active catalyst for dry reforming of methane

Abstract: Alumina coated nickel nanoparticles were prepared employing atomic layer deposition (ALD) on nickel oxide (NiO) nanoparticles and subsequent reduction of the NiO core. The materials showed impressive activity and stability for dry reforming of methane at elevated temperatures (700–800 °C), especially when compared to the uncoated and reduced NiO nanoparticles. The stabilization against sintering at high temperatures is the crucial factor explaining the high catalytic activity of alumina coated Ni nanoparticles.

Effects of Li source on microstructure and ionic conductivity of Al-contained Li6.75La3Zr1.75Ta0.25O12 ceramics

Abstract: Cubic garnet-type Al-contained Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 (Al-LLZTO) ceramics were prepared by conventional solid-state reaction using either Li 2 CO 3 or LiOH·H 2 O as Li source. The effects of Li source on the lithium ionic conductivity of Al-LLZTO ceramics were systematically investigated by comparing the synthesis processes and characteristics of the mother powders and the microstructures of the sintered pellets made with different Li sources. Different calcination processes of the raw materials mixtures with different Li sources are revealed by thermogravimetry-differential scanning calorimetry investigation. The mother powder derived from LiOH·H 2 O is of higher sintering activity than those made from Li 2 CO 3 at the same calcination temperature. For the LiOH·H 2 O-derived pellets, higher relative density and enhanced lithium-ion conductivity is achieved at a much lower sintering temperature of 1100 °C as compared with Li 2 CO 3 -derived pellets. The highest conductivity obtained at room temperature is 9.28 × 10 −4 S cm −1 , which is the highest value reported to date for the same Ta doping level.

Sintering boron carbide ceramics without grain growth by plastic deformation as the dominant densification mechanism.

Abstract: A new ceramic sintering approach employing plastic deformation as the dominant mechanism is proposed, at low temperature close to the onset point of grain growth and under high pressure. Based on this route, fully dense boron carbide without grain growth can be prepared at 1,675–1,700 °C and under pressure of (≥) 80 MPa in 5 minutes. The dense boron carbide shows excellent mechanical properties, including Vickers hardness of 37.8 GPa, flexural strength of 445.3 MPa and fracture toughness of 4.7 MPa•m0.5. Such a process should also facilitate the cost-effective preparation of other advanced ceramics for practical applications.

A shortcut to garnet-type fast Li-ion conductors for all-solid state batteries

Abstract: Ga-doped Li7La3Zr2O12 garnet structures are among the most promising electrolytes for all-solid state Li-ion-batteries. The synthesis and processing of garnet-type fast Li-ion conductors depend on conventional sol–gel and solid state syntheses and sintering that are usually done at temperatures above 1050 °C to reach the high Li-ion conducting cubic phase. This process results in micron-sized particles and potential Li-loss, which are unfavorable for further processing and electrode–electrolyte assembly. Here, we tackle this problem and report a novel low temperature synthesis-processing route to stabilize the cubic phase of Li7La3Zr2O12, while keeping the nanocrystallites at ∼200–300 nm. Li7La3Zr2O12 phases are obtained at temperatures as low as 600 °C by a modified sol–gel combustion method utilizing mainly nitrate precursors, and the sintering temperature is lowered by ∼200 °C compared to the state-of-art. Through a new model experiment, we also shed light on the conditions influencing the tetragonal to cubic phase transformation via homogeneous Ga-diffusion and incorporation occurring at a surprisingly low temperature of ∼100 °C for a post-annealing step. The sintered pellets of the newly obtained Li6.4Ga0.2La3Zr2O12 deliver high bulk Li-ion conductivities in the range of ∼4.0 × 10−4 S cm−1 at 20 °C, and a wide thermal operation window is accessible through its characteristic activation energy of ∼0.32 eV. We report that there is an optimum in sintering-processing conditions for the cubic c-Li6.4Ga0.2La3Zr2O12 solid state electrolytes and their Li-ionic conductivity and the (Raman) near order characteristics that can be tracked through changes in Li–O vibrational modes. Based on this alternative route, low-temperature synthesized powders can be sintered to relatively dense pellets at around only 950 °C. At higher sintering temperatures (e.g. 1100 °C), Li-losses progress as confirmed by structural studies and a reduction of both ceramic pellet density and ionic conductivity, as well as distortions in the Li-sublattice, are found. Through this work, an alternative low temperature processing route for Ga-doped Li7La3Zr2O12 garnet type electrolytes for all-solid state batteries is suggested. The new synthesis method and the use of c-Li6.4Ga0.2La3Zr2O12 nanoparticles could open pathways in terms of preventing Li-loss during the process and advancing future solid electrolyte–electrode assembly options for all-solid state Li-ion batteries.