Two-step Sintering Research Articles

New insight on two-step sintering process of lead-free (K, Na)NbO3-based piezoelectric ceramics

New insight on two-step sintering process of lead-free (K, Na)NbO3-based piezoelectric ceramics

Effect of Two-Step Sintering on the Mechanical and Electrical Properties of 5YSZ and 8YSZ Ceramics.

Yttria-stabilized zirconia (YSZ) has been widely used in structural and functional ceramics because of its excellent physicochemical properties. In this paper, the density, average gain size, phase structure, and mechanical and electrical properties of conventionally sintered (CS) and two-step sintered (TSS) 5YSZ and 8YSZ are investigated in detail. As the grain size of YSZ ceramics became smaller, dense YSZ materials with a submicron grain size and low sintering temperature were optimized in terms of their mechanical and electrical properties. 5YSZ and 8YSZ in the TSS process significantly improved the plasticity, toughness, and electrical conductivity of the samples and significantly suppressed the rapid grain growth. The experimental results showed that the hardness of the samples was mainly affected by the volume density, that the maximum fracture toughness of 5YSZ increased from 3.514 MPa·m1/2 to 4.034 MPa·m1/2 in the TSS process, an increase of 14.8%, and that the maximum fracture toughness of 8YSZ increased from 1.491 MPa·m1/2 to 2.126 MPa·m1/2, an increase of 42.58%. The maximum total conductivity of the 5YSZ and 8YSZ samples under 680 °C increased from 3.52 × 10-3 S/cm and 6.09 × 10-3 S/cm to 4.52 × 10-3 S/cm and 7.87 × 10-3 S/cm, an increase of 28.41% and 29.22%, respectively.

Powder Metallurgy Route to Ultrafine-Grained Refractory Metals.

Ultrafine-grained (UFG) refractory metals are promising materials for applications in aerospace, microelectronics, nuclear energy, and many others under extreme environments. Powder metallurgy (PM) allows to produce such materials with well-controlled chemistry and microstructure at multiple length scales and near-net shape manufacturing. However, sintering refractory metals to full density while maintaining a fine microstructure is still challenging due to the high sintering temperature and the difficulty to separate the kinetics of densification versus grain growth. Here an overview of the sintering issues, microstructural design rules, and PM practices towards UFG and nanocrystalline refractory metals are sought to be provided. The previous efforts shall be reviewed to address the processing challenges, including the use of fine/nanopowders, second-phase grain growth inhibitors, and field-assisted sintering techniques. Recently, pressureless two-step sintering has been successfully demonstrated in producing dense UFG refractory metals down to ≈300nm average grain size with a uniform microstructure and this technological breakthrough shall be reviewed. PM progresses in specific materials systems shall be next reviewed, including elementary metals (W and Mo), refractory alloys (W-Re), refractory high-entropy alloys, and their composites. Last, future developments and the endeavor towards UFG and nanocrystalline refractory metals with exceptionally uniform microstructure and improved properties are outlined.

Annealing of AlN guided by estimation on oxygen content

In this paper, an estimation method revealing relationship between residual oxygen content and Y2O3 additive amount used in AlN sintering was presented. It is revealed that AlN sample added with 3 wt% Y2O3 showed better performance than the others while the oxygen content is 0.765 wt% in the initial powders. Nevertheless, short-time sintering does not make AlN to obtain ultra-high performance consistent with the estimation. By two-step sintering regimes, the distribution of second phase in AlN ceramic was changed from continuous distribution to island-like distribution significantly. The bending strength increases first and then decreases, reaching a higher value of 418 MPa after 2-h annealing. The residual oxygen content continues to decrease as more liquid second phase precipitated with long-time annealing. After reaching a state with lower residual oxygen content than the estimation, high-performance AlN is prepared with thermal conductivity of 230 W/m·K and lower bending strength of 358 MPa.

A comparative study of microstructure and electrical properties of (Al, Nb) co-doped TiO2 ceramics by different sintering methods

(Al, Nb) co-doped TiO2 colossal dielectric ceramics were effectively made using conventional sintering (CS), flash sintering (FS) and two-step flash sintering (TSFS), respectively. The structure and electrical characteristics of ceramic samples were thoroughly investigated. FS and TSFS can decrease the processing temperature by 20% and shorten the sintering time by 30 times compared with CS. All sintered samples were pure rutile TiO2 structure. The TSFS samples with finer grain size had higher permittivity than the CS samples. With increasing sintering temperature, the permittivity of the co-doped TiO2 ceramic increased first and then decreased. When the furnace temperature was 1080 °C, the TSFS ceramic sample showed the best overall performance: the dielectric constant (ε’) of 1.4 × 105, the dielectric loss (tanδ) of 0.57 at 1 kHz, and the nonlinear coefficient of 5.8. XPS and IS spectra indicated that the excellent dielectric properties are attributed to the internal barrier layer capacitance model. Therefore, the TSFS method provides an ideal way to prepare co-doped TiO2 colossal dielectric ceramics.

Control of the nanostructure of MgO–Y2O3 composite ceramics using two-step sintering for high temperature mid infrared window applications

Transparent Y2O3–MgO nanocomposite ceramic has been processed for mid-IR window applications. The powder was synthesized by a sol-gel route. The final temperature of this process has an impact on the crystallite size and crystallization ratio. A homogeneous powder with around 10 nm crystallite size was made with a final temperature of 600 °C. The powder was then sintered by the SPS technique performing a two-step sintering process at 1200 °C/50 MPa instead of a conventional bearing. A fully dense ceramic (>99%) with an average grain diameter of 150 nm (compared to 350 nm with one-stage sintering) was obtained. After HIP at 400 MPa and annealing in air for 100 h post-treatments, the IR transmittance in the 3–5 μm wavelength range exceeds 80% for a thickness of 1 mm. Transparency loss at 5 μm is less than 10% at 1000 °C, which is more suitable for the IR's band II than materials such as sapphire, spinel or AlON.

Multi-scale collaborative optimization of SrTiO3-based energy storage ceramics with high performance and excellent stability

SrTiO3 (ST)-based ceramics are considered as promising candidates for energy storage applications. However, the low polarization intensity in ST-based materials limits their energy storage performance, rendering materials that usually exhibit a low recoverable energy-storage density. In the present study, we have optimized the energy storage performance of ST-based ceramics by using a combined optimization strategy of structural engineering and microstructural regulation. High performance (Sr1-x-y-2φNayBixCaφ□φ)TiO3 (abbreviated as zSNBCT, where □ represents the Sr vacancies) ceramics were thereby designed. During composition optimization, the phase state of zSNBCT was adjusted to a critical point where the relaxor ferroelectric-paraelectric phase transition occurred around room temperature. It thus induced a strong relaxation behavior with the formation of ferroelectric polar nano-regions, yielding a high recoverable energy-storage density (Wrec) of ∼6 J/cm3 and a high energy-storage efficiency (η) of ∼92% under a large breakdown electric field of 440 kV/cm, for z = 0.2 sample. Moreover, the breakdown strength (BDS) of the 0.2SNBCT ceramic was further improved by adopting a two-step sintering and spark plasma sintering approach for the microstructural refinement. Simulation models containing grains and grain boundaries were well established using phase-field simulation and finite element analysis. These simulations came to a similar conclusion for the enhanced BDS. Namely, the fine-grained microstructure significantly hindered the growth of breakdown cracks under an applied electric field. Most importantly, the 0.2SNBCT sample showed excellent frequency stability (1−1000 Hz), thermal stability (20−140 °C), and cycling stability (105 cycles), rendering it a promising candidate for energy storage systems. Our designed strategy of structural engineering and microstructural regulation may provide a new paradigm for the design of high-performance energy storage ceramics for pulse power applications.

Effect of NiTi content on the mechanical properties and microstructure of TiB2–TiN-based composite ceramic material prepared using spark plasma sintering

In this study, TiB2–TiN-based composite ceramic materials modified with NiTi were prepared by a two-step spark plasma sintering process (900 °C/3 min + 1650 °C/5 min). The effects of adding different volume fractions of NiTi (0–12 vol%) on the mechanical properties and microstructure of the ceramic material were investigated. The results showed that the optimal mechanical properties of the composite ceramic were obtained when the NiTi content was 9 vol%. Its flexural strength, fracture toughness and hardness were 942.5 MPa, 5.96 MPa m1/2 and 19.8 GPa, respectively. The addition of NiTi promoted densification during the sintering process, which improved the sinterability and refined the grain size of the materials. The toughening mechanisms were attributed to branching, deflection and bridging of cracks. Furthermore, a new toughening mechanism, phase change toughening was observed due to the addition of NiTi. Its specific manifestation in TiB2–TiN–NiTi cermet materials was the limitation of crack extension, which caused NiTi to transform from austenite to martensite when the crack extended into the region containing NiTi, and this consumed most of the fracture energy. The fracture toughness of the TiB2–TiN–NiTi composite ceramic materials was improved by the ability of NiTi to hinder crack growth.

Effect of particle grading on the properties of photosensitive slurry and BaTiO3 piezoelectric ceramic via digital light processing 3D printing

To improve the density and piezoelectric constant of BaTiO3 ceramics prepared by Digital Light Processing 3D printing, the properties of photosensitive slurry were investigated from the perspective of particle grading, and the nitrogen-air two-step debinding and sintering process on the relative density and electrical properties were explored. It was found that as the mass ratio of coarse particles increased, the viscosity, shear stress and cure depth of the slurry decreased. When the mass ratio of fine and coarse particles was 2:8 and sintered at 1350 °C, the ceramic had better performance, with relative density reaching 95.39 ± 0.63 %. The piezoelectric constant d33 was 215 ± 13 pC/N, 29.52 % higher than the single-peak powder. The relative permittivity (εr) and polarization (Pr) were 978 and 16.656 μC/cm2. Finally, BaTiO3 ceramics with Triply Periodic Minimal Surface structures were prepared as piezoelectric sensors, which had the highest output voltage at the same displacement when the mass ratio was 2:8.

Preparation of Gd2Zr2O7 nanoceramics by flash sintering and two-step flash sintering

Preparation of Gd2Zr2O7 nanoceramics using a simple and low energy-consuming approach is significant for their wide range of applications. In this work, flash sintering (FS) and two-step flash sintering (TSFS) were first utilized to prepare Gd2Zr2O7 nanoceramics. The effects of electric field intensity and current limit on the densification and grain growth were investigated. The grain size was decreased with the increase of electric field intensity. Meanwhile, both grain size and density were increased with the increase of current limit. The Gd2Zr2O7 nanoceramics with an average grain size of 98 nm and a relative density of 92.8% were prepared by FS. By contrast, the Gd2Zr2O7 nanoceramics with the higher relative densities of 96.3–97.5% and the smaller average grain sizes of 85–90 nm were prepared by TSFS. Compared to FS, TSFS can achieve suppressed grain growth and fast densification. The as-prepared Gd2Zr2O7 nanoceramics showed excellent hardness and fracture toughness. The sample breakage caused by current localization can be avoided by controlling the current ramp after the occurrence of the flash event.

Fine-Grained Tb3Al5O12 Transparent Ceramics Prepared by Co-Precipitation Synthesis and Two-Step Sintering

In recent years, transparent terbium aluminum garnet (TAG) ceramics have attracted much attention for use in high-power Faraday isolators. Fine-grained ceramics usually possess better mechanical properties and accordingly better service performance. In this work, transparent TAG ceramics with fine grains were prepared using a two-step sintering procedure based on the low-temperature sintering process to suppress grain growth. The composition of TAG precursor and powders calcined at different temperatures was studied in detail. The microstructure and relative density of air pre-sintered TAG ceramics were studied to meet the requirements of hot isostatic pressing (HIP) post-treatment. Driven by the low pre-sintering temperature in air, the average grain sizes of the obtained TAG ceramics after HIP treatment are about 2.9–5.3 μm. The TAG ceramics (1.2 mm thick) pre-sintered at 1450 °C with HIP post-treatment at 1550 °C for 3 h under a 176 MPa Ar atmosphere possess the highest in-line transmittance of 80.3% at 1064 nm. The Verdet constant of the TAG ceramics at 632.8 nm is −180.5 rad·T−1·m−1 at room temperature, which is about 1.3 times larger than that of the commercial Tb3Ga5O12 single crystals.

Improvement of energy storage density and energy harvesting performance of amphoteric Pr ion-modified lead-free Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCZT) ceramics

Mechanical energy harvesting and energy storage through lead-free piezoelectric materials is an inevitable source of eco-friendly sustainable powering of electronic devices. Herein, we have synthesized amphoteric rare-earth element praseodymium (Pr) modified Ba0.85Ca0.15Ti0.9Zr0.1O3(BCZT) ceramics, with a cost-effective solid-state-reaction based two-step sintering method for the controlled grain growth. Their crystalline structures and surface morphology were investigated by using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The dielectric and ferroelectric properties of the ceramic capacitors were investigated and correlated with structural parameters. In the vicinity of the monotonic phase transition region, with the coexistence of orthorhombic and tetragonal symmetry, the Pr ion addition in BCZT improves the tetragonal phase, which widens its energy harvesting and storage arena. The appreciable energy harvesting ability was found for an optimized energy harvester with a composition of 0.04 wt% Pr added BCZT ceramic, with an open-circuit voltage of about 5.1 V (corresponding power density of 1.213 mW/cm3) from a simple finger tapping and bring about output voltage stability with a maximum output voltage of about 9 V over 10,000 cycles when periodic force is applied by a machine tapping. Furthermore, this optimized Pr added BCZT ceramic capacitor is capable of storing a substantial recoverable energy density of 81.9 mJ/cm3with a considerable energy storage efficiency of 76.4%. These upshots offer a head start in implementing these ceramic capacitors for effective energy harvesting and energy storage applications for powering futuristic miniature electronics.

Solid State Processing of BCZT Piezoceramics Using Ultra Low Synthesis and Sintering Temperatures.

Lead-free (Ba0.92Ca0.08) (Ti0.95 Zr0.05) O3 (BCZT) ceramics were prepared by a solid-state route (SSR) using ultra-low synthesis (700 °C/30 min and 700 °C/2 h) and sintering temperatures (from 1150 °C to 1280 °C), due to prior activation and homogenization by attrition milling of the starting high purity raw materials for 6 h before the synthesis and of the calcined powders for 3 h before the sintering. The comparison of the thermal analysis of the mixture of the starting raw materials and the same mixture after 6 h attrition milling allowed to evidence the mechanisms of activation, resulting in a significant decrease of the perovskite formation temperature (from 854 °C down to 582 °C). The secondary phases that limit the functional properties of the ceramic and their evolution with the sintering conditions were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), which allowed the design of a two-step sintering method to eliminate them. A pure tetragonal BCZT perovskite phase (P4mm, c/a = 1.004) and homogeneous ceramic microstructure was obtained for synthesis at 700 °C for 2 h and sintering with the use of a two-step sintering treatment (900 °C for 3 h and 1280 °C for 6 h). The best electromechanical properties achieved were d33 = 455 pC/N, kp = 35%, Qm = 155.

Probing microstructure of solid-state synthesized LiCoO2 with MAS NMR spectroscopy

LiCoO2 is an important category of active cathode materials in lithium-ion batteries due to its high compacted electrode density, good thermal stability, and stable voltage platform. Recent works on LiCoO2 have focused on the realization of higher charging voltages to fully utilize its high theoretical capacity. However, an unambiguous atomic-level local probe is essential for the understanding of structure-function correlation. Here we employ high-resolution solid-state nuclear magnetic resonance (NMR) spectroscopy to study the local atomic environments in LiCoO2 synthesized with three common sintering methods. While one-dimensional 7Li NMR shows distinct linewidth and subtle dependence on lithium over-stoichiometry, both 7Li and 59Co relaxation times are highly dependent on the sintering method. We prove that the two-step sintering method favors the elimination of unreacted Co3O4, thereby enabling the best discharge capacity in all-solid-state lithium batteries assembled with LiCoO2/LGPS/LiIn, which is in accordance with its narrowest 7Li linewidth and the longest 7Li/59Co T1.

The grain growth mechanisms for 0.8(Bi,Na)TiO3-0.2(Sr,Ti)O3 ceramics prepared using a two-step sintering process

ABSTRACT In this study, lead-free 0.8(Bi,Na)TiO3-0.2(Sr,Ti)O3 piezoelectric ceramics were prepared using a two-step sintering process to analyze their sintering mechanisms. Two-step sintering process has benefits of being able to be conducted at lower temperatures than conventional sintering process, but the complicated sintering mechanisms involved in this process have not been yet fully investigated. Therefore, in the present study, two-step sintering mechanism for lead-free piezoelectric ceramics was analyzed by estimating the activation energy required depending on the sintering conditions. Using the two-step sintering process, the piezoelectric charge and electromechanical coupling coefficients improved from 146 pC/N and 0.347 to 163 pC/N and 0.377, respectively. By comparing the grain-growth mechanisms for the conventional and two-step sintering processes, it appeared that the sintering mechanisms differed. By introducing the two-step sintering process, piezoelectric ceramics with improved piezoelectric charge and electromechanical coupling coefficients were produced.

Consolidation of alumina toughened zirconia by two-step sintering

Sol-gel-synthesized ZrO2-based nanoparticles, containing 1.5mol% Y2O3 used as dopant and 25mol% Al2O3 (abbreviated as ZrO2(1.5Y)-25Al2O3), were densified by pulsed electric current sintering (PECS) accompanied with a two-step heating profile. Particularly, sintering was carried out in the temperature range from 1050 to 1200?C for 10min, followed by dwell at 1325?C for 10min. The ZrO2-based ceramics achieved by the two-step sintering was fully dense, containing 98 vol.% of tetragonal ZrO2 phase. Morphology of the sintered samples showed the submicron grains with uniform size distribution. Although the second-step sintering temperature was fixed at 1325?C, the mean grain size was increased along with the increase in the first-step sintering temperature. In addition, the material characterization of the samples after the first-step heating was tested to investigate the influence of the first-step sintering on the consolidation of ceramics achieved via the two-step sintering.

Coarse-grained molecular dynamics modeling and analysis of graded porous electrodes of reversible solid oxide cells sintered in two steps

Two-step sintering is proposed for preparing a tri-layer skeleton composed of graded nanoparticles. Effects on the sintering behavior, microstructure and stress for graded nanoparticles are investigated by a coarse-grained molecular dynamics method.

Stereolithography-based additive manufacturing of RB-SiC ceramics by a two-step sintering method

Digital light processing technology (DLP) is an effective method to manufacture ceramics with high precision. It is a big challenge to prepare SiC green body with high solid content by DLP method due to the limitation of high light-absorbance of SiC powder in stereolithograhy. Here, we present one novel route for DLP additive manufacturing of high-performance SiC ceramics by introducing low light-absorbance SiO 2 powders and a two-step sintering process including carbothermal reduction and reactive melt infiltration. As-prepared SiC ceramic exhibited higher flexural strength of 268.66 ± 10.19 MPa, compared with reported stereolithograhy-based SiC ceramics. This method demonstrates the potential of preparing SiC ceramic composites with high mechanical properties by stereolithography additive manufacturing.

Efficient thin-film perovskite solar cells from a two-step sintering of nanocrystals.

Creating semiconductor thin films from sintering of colloidal nanocrystals (NCs) represents a very important technology for high throughput and low cost thin-film photovoltaics. Here we report the creation of all-inorganic cesium lead bromide (CsPbBr3) polycrystalline films with grain size exceeding 1 μm from the bottom up by sintering of CsPbBr3 NCs terminated with short and low-boiling-point alky ligands that are ideal for use in sintered photovoltaics. The grain growth behavior during the sintering process was carefully investigated and correlated to the solar cell performance. To achieve precise control over the microstructural development we propose a facile two-step sintering process involving the grain growth via coarsening at a relative low temperature followed by densification at a high temperature. Compared with the one-step sintering, the two-step process yields a more uniform CsPbBr3 bulk film with larger grain size, higher density and lower trap density. Consequently, the photovoltaic device based on the two-step sintering process demonstrates a significant enhancement of efficiency with reduced hysteresis that approaches the best reported CsPbBr3 solar cells using a similar configuration. Our study specifies a rarely addressed perspective concerning the sintering mechanism of perovskite NCs and should contribute to the development of high-performance bulk perovskite devices based on the building blocks of perovskite NCs.

ELECTRICAL AND CORROSION BEHAVIOUR OF ALUMINIUM-ALUMINIUM NITRIDE PARTICULATE COMPOSITE PRODUCED USING POWDER METALLURGY

The properties of aluminium nitride (AlN) such as good coefficient thermal expansion, good wear and corrosion resistance, high mechanical strength, good resistance to thermal shock and good electrical properties have continued to attract research attention in recent times. In this paper, the interest is to investigate the influence of AlN on the electrical conductivity and corrosion properties of Al-AlN particulate composite produced using powder metallurgy. The samples were compacted using the pressure of 450Mpa and sintered at 550 oC for 30 minutes using the two-step sintering method. Four-point probes machine was used to determine the electrical resistivity and subsequently the electrical conductivity of the samples. The samples were immersed in 0.05M NaOH for 24 hours to determine their corrosion behaviour using the weight loss method. The corroded samples were characterized using X-ray Diffractometer (XRD) and SEM. The result of the electrical conductivity of the samples increased with both the amount of reinforcement and magnesium added. The unreinforced aluminium sample recorded the lowest value of electrical conductivity while the sample with composition Al-16.25wt%AlN-1.0wt%Mg gave the highest value, 131.33(Ωm)-1. The results of XRD analyses revealed peaks of aluminium hydroxide (Al(OH)3) as the main corrosion product on the samples. The results show that corrosion rates of the samples decrease with weight fractions of AlN and Mg. The sample containing Al-16.25wt%AlN-1.0wt%Mg offered the best corrosion resistance with a corrosion rate equivalent to 22.74mm/y; while the unreinforced aluminium sample corroded the most.