Sintering Step Research Articles

Refitting of Zirconia Toughening into Open-Cellular Alumina Foams by Infiltration with Zirconyl Nitrate.

The present work describes the combination of the well-established dispersion infiltration of the hollow struts in reticulated porous ceramics (RPCs) and the salt solution infiltration of the remaining strut porosity. This approach is applied on alumina foams, which are loaded subsequently with a dispersion of sub-micrometer alumina particles and a ZrO(NO3)2 solution. The zirconyl nitrate is converted into a ZrO2 transformation toughening phase during the final sintering step. As a consequence of the complex microstructure evolution during the consecutive infiltration cycles, the reinforcement phase concentrates selectively at the weak spots of RPC structures—namely, the hollow strut cavities and longitudinal cracks along the struts. As a consequence, a severe improvement of the compressive strength is observed: The average compressive strength, normalized to a porosity of 91.6 vol.%, is 1.47 MPa for the Al2O3/ZrO2 infiltrated foams, which is an improvement by 40% with respect to alumina-only loaded foams (1.05 MPa) or by 206% compared to uninfiltrated alumina RPCs (0.48 MPa). The compressive strength results are correlated to infiltration parameters and the properties of the infiltration fluids, for example the rheological behavior and the size of the Zr solute species in the respective ZrO(NO3)2 solution.

Effects of different milling conditions on the properties of lanthanum hexaboride nanoparticles and their sintered bodies

In this study, the preparation and consolidation of nanocrystalline LaB 6 powders originating from powder blends of La 2 O 3 , B 2 O 3 and Mg were reported. A consecutive route of mechanochemical synthesis (MCS) and purification was utilized for the achievement of nano-sized LaB 6 powders. As-synthesized powders were leached out from intermediate reaction products or impurities. Then, a sequential step of cold pressing (uniaxial pressure at 800 MPa) and pressureless sintering (at 1700 °C for 5 h under Ar gas flow) were utilized for the consolidation of the purified LaB 6 powders. The type of mill (vibratory and planetary high-energy ball mills) was employed as a MCS parameter to reveal its effect on the physical, microstructural and mechanical properties of the LaB 6 powders, and their bulk structures. Compositional, physical and microstructural properties of the products after powder processing were determined via X-ray diffractometer (XRD), particle size analyzer (PSA), differential scanning calorimeter (DSC), stereomicroscope (SM), scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive spectrometer (EDS) coupled with both SEM and TEM, and vibrating sample magnetometer (VSM). The bulk properties of the LaB 6 consolidated from nanocrystalline powders with a minimum 99.99% purity, and ∼62 nm (for vibratory ball mill) or ∼74 nm (for planetary ball mill) average particle size were compared according to various properties. LaB 6 powders were synthesized in planetary mill at an approximately six times longer duration than that of in vibratory mill. According to the results, density, surface area and mean particle size values of the vibratory ball-milled samples (containing paramagnetic powders) are better than those of planetary ball-milled (containing diamagnetic powders) ones. However, mechanical properties such as hardness, surface roughness, wear rate, friction coefficient, and also electrical conductivity were improved in the planetary ball-milled LaB 6 bulks.

Special Features of Powder Technology for the Production of Lithium-Ion Batteries

Abstract: Electrodes of lithium-ion batteries show similarities with classic ceramic products. In both, oxide or carbonaceous materials in powder form are used and the processing is carried out by a slurry-based coating process. However, electrodes lack a densifying sintering step. This results in consequences from the application of additives to the final cell properties. Additionally, within the process chain, mixing and drying have the crucial task of setting the desired electrode structure.

Preparation of high density ZnO ceramics by the Cold Sintering Process

Abstract High-density ZnO ceramics were prepared by a Two-Step Cold Sintering Process (TS-CSP) at an ultralow-temperature. The densification process of ZnO ceramics was demonstrated by TS-CSP. And the density, microstructure and electrical properties of the ZnO ceramics were investigated. The results indicate that most of the crystallization of ZnO ceramic can be completed in the first step of sintered at 150 °C/200 MPa for 30 min, with a relative density approaching 97.36%. This relative density was further improved to 99.43% after the second sintering step was deployed at 200 °C/200 MPa for 30 min, which displayed an average grains size of 1.5 μm and resistivity of 0.125 Ω cm. This work demonstrated an effective method to reduce CSP temperature and pressure in the production of high-performance ZnO ceramics.

Morphology of Aluminum Alloy Foams Produced with Dolomite via Partial Sintering of Precursors.

Highly expanded, low-cost aluminum-based foams were successfully produced via powder metallurgy using dolomite as foaming agent. Nickel additions (5–15 wt.%) were explored in order to reduce the temperature disparity between dolomite decomposition and the melting range of the metallic matrix. Specific Al–Ni compositions provide appropriate viscosities for effective encapsulation of CO2 gas released during dolomite decomposition. A partial sintering step of compacted precursors was introduced prior to foaming, which resulted in high porosity levels (~86%) and significant volume expansion (~250%) in the final product. The partial sintering technique was a key determining factor in obtaining stable, highly expanded cellular structures with homogeneous pores, averaging 3 mm in size and being morphologically comparable with ALPORASTM foams.

Effect of Sintering Holding Time on Tetragonal Phase Stability of Yttria Stabilized Zirconia Ceramics

In this study, the effect of second step sintering holding time on LTD of 3 mol% yttria tetragonal zirconia polycrystals (3Y-TZP) ceramic was studied. The Zirconia powder was die pressed followed by cold isostatically pressed (CIP) at 200 MPa, and the samples were initially heated to a first step temperature of 1400°C with constant heating rate 10°C/min and the samples were stand still for 1 minute. Then cooled down to the second step temperature of 1200°C and maintained at that temperature for 2 and 10 hours before cooled down to the room temperature. The other sample was sintered by using Single Step Sintering (SSS) at 1400°C for two hours. The phase content in the zirconia samples was measured by using X-ray diffractometer. Average grain size was calculated by using Field Emission Scanning Electron Microscope (FESEM). The low temperature degradation study was conducted in an autoclave containing superheated steam at 180°C and 10 bar pressures for periods up to 60 hours. Results revealed that the two-step sintering (TSS) effectively controlled the grain growth than SSS and subsequently ageing. Increase in second step sintering holding time from two to ten hours also increased the densification and hydrothermal ageing resistance of Y-TZP ceramics.

Upside - down composites: Fabricating piezoceramics at room temperature

Piezoelectric materials have a multi-billion dollar impact on the electromechanical transducers market. Their conventional synthesis includes a sintering step (at over 1000 °C) that often hampers direct integration into monolithic devices and confines their applications to heterostructures made using tedious multi-step assembly or to composites with poor electromechanical behavior. Here, we demonstrate a new and easy to scale method for the integration of piezoelectric ceramics at ultra-low temperatures without compromising their functionality. We show that all-ceramic “upside-down” composites with exceptionally high fractions of piezoelectric filler (˜75 vol. %) and low porosity can be achieved using aqueous dispersion of lithium molybdate as a binder. The method is based only on coating, mixing, moulding and drying sequences. The measured piezoelectric charge coefficient, d33 ˜84 pC·N−1, outperforms any other known composite, whereas the voltage constant, g33 ˜33 mV m·N−1, competes with bulk materials, thus paving the way for versatile applications not previously considered.

Microwave dielectric properties of zirconia fabricated using NanoParticle Jetting™

Additive manufacturing of ceramics has been actively investigated with the objective of fabricating complex structures that compete in terms of material performance with traditionally manufactured ceramics but with the benefit of increased geometric freedom. More specifically, zirconia provides high fracture toughness and thermal stability. In addition, its dielectric permittivity may be the highest among materials available for 3D printing, and may enable the next generation of complex electromagnetic structures. NanoParticle Jetting™ is a new material jetting process for selectively depositing nanoparticles and is capable of printing zirconia. Dense, fine-featured parts can be manufactured with layer thicknesses as small as 10 μm and jetting resolution of 20 μm after a final sintering step. For this study, 3D printed zirconia using NanoParticle Jetting™ was characterized in terms of chemistry, density, crystallography, sintering shrinkage and dielectric properties as a foundation for developing high performance radio frequency (RF) components. The experimental results indicate a yttria-stabilized ZrO2 structure exhibiting a bulk relative permittivity of 23 and a loss tangent of 0.0013 at microwave frequencies. A simple zirconia dielectric resonator antenna is measured, confirming the measured dielectric properties and illustrating a practical application of this material.

Impact of sodium excess on electrical conductivity of Na3Zr2Si2PO12 + x Na2O ceramics

In order to industrialize NaSICON materials, modern fabrication techniques have to be used and one of those techniques for producing large-scale electrolyte sheets with 10–300 μm thickness is tape casting. Such technique however requires a sintering step at high temperatures leading to sodium depletion due to evaporation. The sodium loss becomes more significant for large-area and thin components. In order to investigate and compensate the sodium loss, NaSICON compositions with sodium excess were prepared, i.e. Na3Zr2Si2PO12 + x Na2O (0 ≤ x ≤ 0.2). The sodium loss can be reduced by applying a two-step sintering process (1250 °C for only 0.5 h and then at 1230 °C for 5 h). Several characterization techniques were used to analyze the resulting ceramics, the sodium depletion and its consequence on electrical conductivity. Chemical analyses indicated that all compositions were sodium deficient. Furthermore, the weight loss was investigated by thermogravimetric analysis confirming the reduction of weight loss by a factor 2 by applying a two-step sintering procedure with lower second sintering temperature. Initial thermodynamic calculations of the phase equilibria at high temperatures confirm the predominant evaporation of sodium. The highest electrical conductivity (1.6 ⋅ 10−3 S cm−1 at 25 °C) was measured for the composition showing the least sodium deficiency (x = 0.2). Furthermore, the activation energy of bulk and grain boundary conductivity decreased with increasing x in system.

Experimental investigation of boiling regimes in a capillary-fed two-layer evaporator wick

Vapor chambers with transformative evaporator wick designs capable of passively dissipating high heat fluxes over large areas, while maintaining low thermal resistances, can meet the thermal management needs of next-generation power semiconductor devices. Our prior work proposed a two-layer evaporator wick structure to enhance the performance of vapor chambers operating at high heat fluxes. The current study experimentally characterizes the capillary-fed boiling heat transfer behavior in such a two-layer evaporator wick, compared to a homogeneous (single-layer) wick, over a 1 cm2 evaporator area. The two-layer design comprises a thin base wick layer that is fed with liquid from a thick cap wick layer above using an array of vertical posts. The two-layer wick is fabricated using a sequence of sintering and laser-machining steps to form the base wick layer (200 µm), array of liquid-feeding posts, and cap wick layer (800 µm) using 90–106 µm copper particles. A test facility is constructed to replicate the conditions that exist at the evaporator of a vapor chamber; the novel facility design uses a physical restriction to prevent flooding of the wicks during testing. Two-layer wicks having 5 × 5 and 10 × 10 arrays of liquid feeding posts are characterized, along with a 200 µm-thick single-layer evaporator wick. The 10 × 10 array provides a >400% enhancement in the dryout heat flux compared to the single-layer wick. High-speed visualizations are used to identify the characteristic regimes of boiling operations for the wicks. The single-layer wick exhibits a partial dryout mode of operation, where a dry spot formed in the center of the heated evaporator area causes an increase in the thermal resistance with heat flux. In contrast, the distributed feeding provided by the two-layer wicks mitigates the development of this partial dryout regime and maintains a constant low resistance (∼0.1 K/W) during capillary-fed boiling until a complete dryout event occurs. This study demonstrates the significant enhancement in dryout heat flux offered by the liquid-feeding approach realized in the two-layer evaporator wicks characterized here.

Cermet Plasma TiC–Cr3C2–NiCr–Mo–C Coatings

To determine the possibility of maintaining the maximum possible carbon content in TiC carbide, a powder additionally doped with chromium carbide, molybdenum, and carbon was produced by plasma spraying of the cermets of the TiC–NiCr system. To reduce the effect of gases from the air atmosphere, plasma spraying was carried out using a standard plasmatron, supplemented with a special nozzle. The analysis of the oxygen, nitrogen, and carbon content in the powder manufacturing stages and in the coatings was carried out. The content of O and N is reduced in the sintering step for spraying a powder with respect to the content in the starting components, but increases again during spraying. In the coating, the quantitative distribution of the carbide phases was determined by their size. A change in the phase composition and dimensions of the crystal lattices of the phases in the powder for deposition and coatings was determined. The share of the main hardening TiC phase in the coating decreased by 9.7% with respect to the initial mixture, but taking into account the newly formed carbide phases with the participation of chromium and molybdenum, the proportion of all the carbide phases in the coating increased by 12.7%. The microhardness of the plasma coating at loads of 200 and 20 g on the indenter was 14.9 and 28.7 GPa, respectively. The reasons for the decrease in the actual microhardness of the cermet coating in relation to the theoretically possible one based on the volume fraction of the strengthening phases are analyzed.

Two-step sintering of Sm2O3 doped ceria stabilized zirconia

Effect of Sm2O3 addition and two-step sintering of Ceria Stabilized Zirconia (CSZ) on microstructure and mechanical properties were investigated in the present work. Samaria doped CSZ (SmCSZ) nanopowders were prepared by co-precipitation synthesis from their respective nitrate salts. Synthesized powders were calcined at 1000 °C for 2 h and then compacted to ϕ10 mm pellets using a uniaxial hydraulic press. Single step & two-step sintering methods were used to sinter the compacted pellets. Powders and sintered pellets were characterized for phase and microstructure using X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) technique. Rietveld method was used for quantification of obtained phases. The hardness of the sintered samples was evaluated by Vicker's hardness tester, and toughness was estimated by indentation fracture toughness method. Samples sintered using two-step sintering method shown optimum hardness and toughness (up to 1288 HV10 and 5.37 MPa m1/2) values compared to conventionally sintered samples because of reduced grain size.

Grain growth in Na0.5Bi0.5TiO3-based solid solutions

This paper discusses effects of different dopants, sintering technique and parameters on microstructure and properties of pure and Yb, Er-doped Na0.5Bi0.5TiO3 (NBT). All stoichiometric compositions follow the abnormal grain growth mechanism (AGG) and exhibit a bimodal grain size distribution. Bi over-stoichiometry, two step sintering and hot pressing are effective inhibitors of AGG. Microstructure of sintered NBT greatly influences such properties as dielectric permittivity and depolarization temperature.

Electrocatalytic activity of perovskite-based cathodes for solid oxide fuel cells

A very active cathode material for intermediate temperature - solid oxide fuel cells (IT-SOFCs) is obtained by mixing La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) powders. Three different volume ratios are considered: BSCF-LSCF 50-50 v/v% (BL50), BSCF-LSCF 70-30 v/v% (BL70) and 30–70 v/v% (BL30).The electrodes are slurry coated on Ce0.8Sm0.2O2-δ electrolyte and sintered at 1100 °C. After the sintering step XRD-analyses highlight relevant cation inter-diffusion within the mixed powders. As a result, an enhanced activity of BL30-BL70 electrodes towards oxygen reduction reaction is detected in comparison to LSCF or BSCF pure powders. A polarization resistance of 0.021 Ω cm2 at 650 °C for BL70 is obtained, one of the lowest value reported in literature for SOFC cathodes. Furthermore, all the electrodes show lower activation energy than the two reference materials in the considered temperature range (500–650 °C) and two different kinetic regimes are identified at the extremes of this range. Effect of the applied overpotential (0–0.3 V) on the electrode kinetic is also investigated.After a preliminary ageing, performed at 650 °C for 200 h by applying a current density of 200 mA cm-2, the electrodes preserve a remarkable performance as IT-SOFC cathodes, despite an initial degradation. A stable value of 0.048 Ω cm2 of polarization resistance for the sample richer in BSCF is recorded.

Solution-Based Synthesis of Li3PS4 Solid Electrolytes

Sulfide-based solid electrolytes have attracted much attention due to their high conductivities, which are far beyond those of oxide-based solid electrolytes.[1,2] However, They (Li2S-P2S5 system, i.e., LPS) have been normally synthesized by solid state synthesis such as mechanical ball milling. These methods require rigorous control of reaction environment as well as high temperature heat treatment and repeated pelletizing steps. In contrast, solution-based synthesis methods can induce chemical reaction among precursor particles (Li2S and P2S5) at low temperatures resulting in the formation of conductive phases of β-Li3PS4 and Li3P7S11 with only moderate thermal treatment.[3,4] The method deserves great attention since it simplifies synthesis process, yields products of great purity, and may facilitate the fabrication of composite electrodes with improved interfaces. In this work, we have developed an efficient method to form a thin solid electrolyte layer directly on Li metal using the liquid coating techniques. The formation of LPS (Li2S-P2S5) based electrolyte is achieved by rational design of the solvent and the Li, P, and S precursor ratios. The solution electrolyte can be directly coated and formed on Li metal through the in-situ formation of the solid electrolyte layer, which does not require the complex synthesis process and high temperature sintering step. Layers of thickness of < 50 mm can be fabricated and electrochemical cycling of lithium is achieved. This liquid-phase coating is a simple and straightforward technique for making a thin solid electrolyte and can be applicable to anode surface with complex contours. The new liquid coating technique holds the promise to overcome the limitations of current state solid electrolytes. [1] Kamaya, N. et al. A lithium superionic conductor. Nature Mater. 10, 682-686 (2011). [2] Yamane, H. et al. Crystal structure of a superionic conductor, Li7P3S11. Solid State Ion. 178, 1163-1167 (2007). [3] Ito, S. et al. A synthesis of crystalline Li7P3S11 solid electrolyte from 1,2-dimethoxyethane solvent. J. Power Sources 271, 342-345 (2014). [4] Liu, Z. et al. Anomalous High Ionic Conductivity of Nanoporous β-Li3PS4. J. Am. Chem. Soc. 135, 975-978 (2013).

Dopant‐Free Partial Rear Contacts Enabling 23% Silicon Solar Cells

AbstractOver the past five years, there has been a significant increase in both the intensity of research and the performance of crystalline silicon devices which utilize metal compounds to form carrier‐selective heterocontacts. Such heterocontacts are less fundamentally limited and have the potential for lower costs compared to the current industry dominating heavily doped, directly metalized contacts. A low temperature (≤230 °C), TiOx/LiFx/Al electron heterocontact is presented here, which achieves mΩcm2 scale contact resistivities ρc on lowly doped n‐type substrates. As an extreme demonstration of the potential of this heterocontact, it is trialed in a newly developed, high efficiency n‐type solar cell architecture as a partial rear contact (PRC). Despite only contacting ≈1% of the rear surface area, an efficiency of greater than 23% is achieved, setting a new benchmark for n‐type solar cells featuring undoped PRCs and confirming the unusually low ρc of the TiOx/LiFx/Al contact. Finally, in contrast to previous versions of the n‐type undoped PRC cell, the performance of this cell is maintained after annealing at 350–400 °C, suggesting its compatibility with conventional surface passivation activation and sintering steps.

High piezoelectricity of BiScO3-PbTiO3 ceramics prepared by two step sintering

(1-x)BiScO3-xPbTiO3 (BSPT) ferroelectric ceramics were prepared by two-step sintering. Dielectric measurements in combination with diffraction contrast transmission electron microscopy (TEM) were used to confirm the phase diagram of BSPT system. From these results, the low temperature phase diagram for BSPT was achieved. In present study, high piezoelectric properties with piezoelectric coefficients (d33 ≈ 545 pC/N, d33* ≈ 920 pm/V) and planar electromechanical coupling factor (kp ≈ 57%) are achieved in the ceramics at the composition x = 0.64, indicating a future high-temperature piezoelectric application.

Sintering processes in direct ink write additive manufacturing: A mesoscopic modeling approach

Direct ink write (DIW) is an emerging additive manufacturing technique that allows for the fabrication of arbitrary complex geometries required in many technologies. DIW of metallic or ceramic materials involves a sintering step, which greatly influences many of the microstructural features of the printed object. Herein, we explore solid-state sintering in DIW through a mesoscopic modeling framework that is capable of capturing bulk and interface thermodynamics and accounting for various mass transport mechanisms. Simulation results of idealized geometries identify regimes in materials parameter space, where densification rates are enhanced. With the aid of several statistical and topological descriptors, the role of particle size distribution (PSD) on the microstructural evolution is explored and quantified. More specifically, it is found that a bi-dispersed PSD enhances pore shrinkage kinetics. However, bi-dispersity yields microstructures with pores that are highly eccentric, an effect that could be detrimental to the mechanical properties of the printed material. On the whole, our modeling approach provides a capability to explore the phase space of DIW process parameters and determine ones that lead to optimal microstructures.

Objective criterion for evaluation of the quality decoration in ceramic floor tile production prior to the sintering process

Abstract In the ceramic tile decoration process, the set of variables that can cause hue variation is quite wide and independent of the methodology used in the production. The understanding of the effects of these variables over the final expected result is of great importance to the producers, especially if it could be done before the ceramic tile’s sintering stage. The quality of the decoration is linked to the deposited dot shape and its filling with ink. It is proposed in this paper the determination of a ‘quality index’, comparing the theoretical deposition pattern and a real deposition obtained with samples. Measuring the values of the areas with and without ink, using the statistical tool of the ROC curve, it was obtained the values for sensibility=0.791, specificity=0.830, precision=0.802 and quality index=0.768. The method is intended to be used prior to the sintering step for hue variation control.

A Reversible Protonic Ceramic Cell with Symmetrically Designed Pr₂NiO4+δ-Based Electrodes: Fabrication and Electrochemical Features.

Reversible protonic ceramic cells (rPCCs) combine two different operation regimes, fuel cell and electrolysis cell modes, which allow reversible chemical-to-electrical energy conversion at reduced temperatures with high efficiency and performance. Here we present novel technological and materials science approaches, enabling a rPCC with symmetrical functional electrodes to be prepared using a single sintering step. The response of the cell fabricated on the basis of P–N–BCZD|BCZD|PBN–BCZD (where BCZD = BaCe0.5Zr0.3Dy0.2O3−δ, PBN = Pr1.9Ba0.1NiO4+δ, P = Pr2O3, N = Ni) is studied at different temperatures and water vapor partial pressures (pH2O) by means of volt-ampere measurements, electrochemical impedance spectroscopy and distribution of relaxation times analyses. The obtained results demonstrate that symmetrical electrodes exhibit classical mixed-ionic/electronic conducting behavior with no hydration capability at 750 °C; therefore, increasing the pH2O values in both reducing and oxidizing atmospheres leads to some deterioration of their electrochemical activity. At the same time, the electrolytic properties of the BCZD membrane are improved, positively affecting the rPCC’s efficiency. The electrolysis cell mode of the rPCC is found to be more appropriate than the fuel cell mode under highly humidified atmospheres, since its improved performance is determined by the ohmic resistance, which decreases with pH2O increasing.