Assisted Sintering Method Research Articles

High-Performance Single-Crystal Lithium-Rich Layered Oxides Cathode Materials via Na2WO4-Assisted Sintering.

Single-crystal lithium-rich layered oxides (LLOs) with excellent mechanical properties can enhance their crystal structure stability. However, the conventional methods for preparing single-crystal LLOs, require large amounts of molten salt additives, involve complicated washing steps, and increase the difficulty of large-scale production. In this study, a sodium tungstate (Na2WO4)-assisted sintering method is proposed to fabricate high-performance single-crystal LLOs cathode materials without large amounts of additives and additional washing steps. During the sintering process, Na2WO4 promotes particle growth and forms a protective coating on the surface of LLOs particles, effectively suppressing the side reactions at the cathode/electrolyte interface. Additionally, trace amounts of Na and W atoms are doped into the LLOs lattice via gradient doping. Experimental results and theoretical calculations indicate that Na and W doping stabilizes the crystal structure and enhances the Li+ ions diffusion rate. The prepared single-crystal LLOs exhibit outstanding capacity retention of 82.7% (compared to 65.0%, after 200 cycles at 1C) and a low voltage decay rate of 0.76mV per cycle (compared to 1.80mV per cycle). This strategy provides a novel pathway for designing the next-generation high-performance cathode materials for Lithium-ion batteries (LIBs).

Low-temperature synthesis of K0.5Na0.5NbO3 ceramics in a wide temperature window via cold-sintering assisted sintering method and enhanced electrical properties

High temperatures (≥ 1100 °C) and narrow temperature window (∼ 20 °C) for sintering dense K0.5Na0.5NbO3 ceramics always deteriorate their electrical properties. Here, via cold-sintering assisted sintering method, dense K0.5Na0.5NbO3 ceramics were obtained in a wide temperature span between 800 °C and 1000 °C. An aqueous solution of NaOH and KOH mixture was used as transient liquid. Effects of liquid content (LC), molar concentration (MC) of liquid, cold-sintering temperature (TCS), and post-annealing temperature (TAN) on densification and electrical properties of the ceramics were investigated in detail. The ceramics prepared using LC = 10 wt%, MC = 10 mol/L, TCS = 350 °C, and TAN = 900 °C exhibit excellent electrical properties with d33 = 123 pC/N, εr = 609, tanδ = 0.021, Pr = 28.0 μC/cm2, Pm = 39.2 μC/cm2, and Ec = 20.3 kV/cm. Compared to the ceramics with same or similar compositions via conventional solid-state sintering, the present K0.5Na0.5NbO3 ceramics exhibit excellent electrical properties. The study endows the cold-sintering assisted sintering the successful method to prepare K0.5Na0.5NbO3 ceramics at low temperatures and in a wide temperature window.

The preparation of Li7La3Zn2O12(LLZO) battery ceramic and its conductivity

For Li7La3Zr2O12 (LLZO) solid electrolytes, higher density usually means higher ionic conductivity. Researchers tried many preparation methods to get high density samples and at same times to realize industrial production, low cost, scalable and fast synthesis techniques. In this paper, the mainstream preparation methods of LLZO was given, as polymerized complex method, sol-gel method, field assisted sintering, combustion technique, auto-consolidation method, water-based solvent method. Among these methods, the last four methods can always increase the density to more than 93%. Especially the field assisted sintering method can make the relative density to be high as 99.8%. And all of these methods can make the ion conductivity to be higher than 1.4 × 10-4 S cm-1. Also, most of these methods introduced the Al element into LLZO to realize the liquid sintering.

Controlled current-rate AC flash sintering of uranium dioxide

Uranium dioxide (UO2) pellets with controlled microstructures were densified up to 93.4% of their theoretical density in less than 25 minutes at a furnace temperature of 873 K, utilizing controlled current-rate alternating current (AC) flash sintering (FS). Using this AC-FS method it was possible to control the sintering rate and thermal gradients, resulting in dense pellets with no appreciable hourglassing and good mechanical integrity. Moreover, the apparent sintering activation energy for FS and for conventionally sintered samples was estimated to be 108 kJ mol−1 and 380 kJ mol−1, respectively using the master sintering curve method. The apparent activation energy for FS was remarkably close to those reported in the literature for spark plasma sintering of UO2. Both these field assisted sintering methods utilize fast heating rates and electrical effects that are likely enhancing the grain boundary diffusion mechanism. Finally controlled current-rate AC-FS has been demonstrated as a technological advancement, capable of producing ceramic nuclear fuels in a fraction of the conventional processing time.

Flash sintering of sodium niobate ceramics

Dense and stoichiometric NaNbO3 ceramics were difficult to be prepared by conventional sintering methods because Na2O easily volatilized at elevated temperature. In this work, the volatility of Na2O was suppressed by applying an electrical field assisted sintering method. Flash sintering was preformed successfully on NaNbO3 ceramics under the electrical fields ranging from 400 to 700 V/cm. The Na/Nb ratio of dense ceramic remained similar to that of the green sample. Our work demonstrated that flash sintering was an alternative technique for fabricating dense and high-purity ceramics with highly volatile species.

Exploration of photoanode characteristics of a mixed ferroelectric ZnSnO3 and semiconducting Zn2SnO4 phase for photovoltaic applications

The spontaneous and reversible polarization characteristics of ferroelectric materials gain considerable interest in photovoltaic applications. Intrinsic polarization of ferroelectric materials induces the built-in potential which potentially facilitates the separation of photoexcited electron–hole pairs in the devices. In an effort to understand the photo-physical characteristics, ball milling assisted sintering method was employed to prepare the compound. Mixed phase of ZnSnO3 and Zn2SnO4 shows an absorbance maximum in the range between 235 and 400 nm in UV–Vis spectrum. Synthesized mixed phases of ZnSnO3 and Zn2SnO4 is used as a photoanode in the Dye-sensitized solar cell and the effect of mixed phases in the photovoltaic property is analyzed. By using Ruthenium dye and LiI electrolyte, the photo conversion efficiency is examined. The observed open circuit voltage is 0.64 V and the short circuit current density is 4.2 mAcm−2. The observed Fill factor and conversion efficiency are 0.56 and 1.5%, respectively.

La2MgNi7.5Co1.5 Hydrogen Storage Alloy Prepared by the Magnetic Field Assisted Sintering

La2MgNi7.5Co1.5 hydrogen storage alloy was prepared by magnetic field assisted sintering method. The phase structure and hydrogen absorption properties of the alloy were systematically studied by means of isothermal constant volume method, XRD, SEM, EDS and HPDSC etc. The results showed that the main phase is PuNi3 type structure of (La, Mg) (Ni, Co)3. The impure phase La (Ni, Co)5 decreased firstly and increased later with the magnetic field increased. Addition of Co can improve the hydrogen absorption and desorption properties of the alloy obviously, the maximum hydrogen storage capacity reached 1.538 wt.%. The dehydrogenation DSC curve of the alloy has a multimodal characteristics. The magnetic field changed hydrogen desorption thermal behavior of (La, Mg)(Ni, Co)3 phase significantly. The alloy after activation showed excellent hydrogen absorption and desorption kinetics performance.

Structural properties of MgO–ZrO2 ceramics obtained by conventional sintering, arc melting and field assisted sintering technique

MgO–ZrO2 ceramics were obtained by three different methods: conventional sintering, arc melting and field assisted sintering. The present study was undertaken in order to determine the influence of the different sintering techniques on the structural properties of MgO–ZrO2 ceramics. The phase compositions of the obtained samples consisted of MgO and ZrO2cub. ss (doped by MgO). The amount of all phases in the final product and their average crystallite sizes are strongly influenced by the sintering method. The smallest crystallites were realized for ceramics consolidated by the field assisted sintering method (20.1±2.5nm for MgO and 27.0±2.7nm for ZrO2cub. ss). About ten times larger crystallites resulted by conventional sintering. The average crystallite size in the material obtained by arc melting technique was around 116±20.4nm. It was observed that the microstructures of the samples prepared by arc melting and field assisted sintering are similar.

Micro–Macro Analysis of Capacitor Discharge Sintering in Copper–Diamond Bead

Capacitor Discharge Sintering (CDS) is an ultrafast electric current assisted sintering method (ECAS) suited for electrically conductive metallic and metal‐ceramic composite powders. The technique is a solid state near net shape sintering process which is in general characterized by a very short discharge time (tens of milliseconds), very high current intensity peak (up to 350 kA) and relatively high pressure (up to 1 GPa) with minimum shrinkage. Computer modeling comes to aid in the setup of industrial processes to produce enhanced sintered products such as copper–diamond beads for stone cutting applications. The simulation of the CDS process introduces a new class of nonlinear problems, involving a complex bulk and contact multiphysics with an intricate coupling of micro–macro phenomena. The work presents a micro–macro CDS model along with the solution strategy developed to assess the densification behavior of a copper–diamond bead for stone cutting applications. The overall CDS model implements a four‐way coupling strategy which combines the relevant partial differential equations governing the electrical, thermal, and stress–strain fields over the macroscopic scale of the CDS system, with the densification kinetic model governing the particle scale phenomena. The results are presented in terms of field and shrinkage variables as a function of space and time.

Conductivity enhancement of yttria doped ceria by spark plasma and ac assisted sintering methods

CeO2 doped with lower valence cations, such as Y3+, Sm3+, Gd3+, etc., is considered as electrolyte material for low temperature solid oxide fuel cells. Doping introduces additional oxygen vacancies and improves ionic conductivity. This work describes the preparation of yttria–calcia doped ceria precursor material, its sintering in DC and AC field methods, and characterisation of sintered electrolytes with respect to the ionic conductivity. Doped precursor materials were prepared by a controlled co-precipitation process. Sintering of prepared, pelletised particulates was carried out using three different methods such as: conventional, spark plasma sintering, and an alternating current field at 1125°C (AC assisted sintering). The conductivity was determined as a function of defect density, grain boundary area and phase stability. It was found that the applied field during sintering, either via pulsed DC (spark plasma), or AC, produced marginal effects on yttria doped ceria microstructure. However, it should be noted that because the sintering conditions were not optimised there is room for substantial conductivity improvements.On considère le CeO2 dopé avec des cations à valence plus faible, comme le Y3+, le Sm3+, le Gd3+, etc., comme matériau électrolyte pour les piles à combustible d’oxyde solide à basse température. Le dopage introduit des lacunes additionnelles d’oxygène et améliore la conductivité ionique. Ce travail décrit la préparation de matériau précurseur d’oxyde cérique dopé à l’oxyde d’yttrium-calcia, son frittage par des méthodes de champ CD et CA et la caractérisation des électrolytes frittés par rapport à la conductivité ionique. On a préparé les matériaux précurseurs dopés au moyen d’un procédé de co-précipitation contrôlée. On a effectué le frittage des particules préparées en boulette en utilisant trois méthodes différentes: conventionnelle, frittage au plasma à étincelles et un champ de courant alternatif à 1125°C (frittage assisté au CA). On a déterminé la conductivité en fonction de la densité de défaut, de la superficie de joint de grain et de la stabilité de phase. On a trouvé que le champ appliqué lors du frittage, soit par CD à impulsion (plasma à étincelles) ou par CA, produisait des effets marginaux sur la microstructure de l’oxyde cérique dopé à l’oxyde d’yttrium. Cependant, on doit noter que parce que les conditions de frittage n’étaient pas optimisées, il y a place pour des améliorations substantielles de la conductivité.

Improving thermal stability of KSrPO4:Tb3+ phosphors prepared by microwave assisted sintering

In this paper, KSr0.94PO4:Tb0.063+ phosphor was synthesized successfully by microwave assisted sintering method at 1200°C for 1h under an air atmosphere. Photoluminescence results show KSr0.94PO4:Tb0.063+ phosphor prepared by microwave assisted sintering method presents an observable improved effect on the enhancement of thermal stability. The emission intensity reduces slowly and marginally by approximately 7% for the maximum emission peak with the temperature increasing from 30°C to 200°C indicating great thermal stability KSrPO4:Tb3+ phosphors are. Additionally, the activation energy (ΔE) of thermal quenching of KSrPO4:Tb3+ phosphor was calculated to be 0.2eV. Moreover, the chromaticity (x, y) located at (0.30, 0.52) was kept without variation when the temperature increased from 30°C to 200°C.

Microwave Assisted Sintering of Thermally Stable BaMgAl10O17:Eu2+ Phosphors

Eu2+ doped BaMgAl10O17:Eu2+ (BAM) phosphors suffer from serious instability, although they have been widely used in various lighting and display applications. In this paper, high thermally stable Si–N-codoped BAM phosphors have been prepared by the microwave assisted sintering method at a relatively low temperature of 1450°C, and they were synthesized at 1550°C by the conventional solid-state reaction method for comparison. The microwave assisted sintering could reduce the activation energy and thus enhance the diffusion rate, and greatly improve the substitution amount of Si–N pairs for Al–O ones in the BAM host lattice. The Si–N doping significantly increased both the photoluminescence and the thermal stability of the BAM phosphors under ultraviolet (UV) and vacuum ultraviolet (VUV) excitations, comparable to the samples prepared by the conventional solid state reaction method. The proposed method is expected to be potentially applicable to other phosphors with higher photoluminescence and stability.

W/steel joint fabrication using the pulse plasma sintering (PPS) method

The paper presents application of pulse plasma sintering method (PPS), developed at the Faculty of the Materials Science and Engineering of Warsaw University of Technology. Unlike other electric-field assisted sintering methods, the PPS method employs pulse high-current electric discharges for heating and activating the material to be sintered. The phenomena, taking place during the high-current pulses, which heat the powder during the PPS treatment and activate the sintering process, are similar to those occurring in SPS technique. However, in PPS, thanks to much higher energy the pulse discharge, these phenomena run much more intensively. The aim of the present study was to fabricate by the PPS a joint between tungsten and Eurofer 97 steel. Because of the large difference in thermal expansion coefficients of the joined materials, stresses are induced at the joint interfaces. To reduce these stresses a thin interlayer was incorporated between the joined materials. Four different materials were tested. The experiments allowed to establish the optimal PPS sintering parameters. It was shown that the interlayers between W and Eurofer 97 steel fabricated at 1000 °C for 10 min were highly dense and no delamination at joint interfaces occurred. The results of the thermocycle tests proved a high strength of the joints produced by PPS.

Nanostructured Bulk Ceramics (Part IV. Polymer Precursor Derived Nanoceramics)

In the last (fourth) section, the discussion will entail a silicon-nitride/silicon-carbide nanocomposite, produced by pyrolysis of liquid polymer precursors, demonstrating one of the lowest creep rates reported so far in ceramics at the comparable temperature of 1400℃. This was first achieved by avoiding the oxynitride glass phase at the intergrain boundaries. One important factor in the processing of these nanocomposites was the use of the electrical field assisted sintering method.

Dense and Homogenous Silicon Nitride Composites Containing Carbon Nanotubes

Silicon nitride (Si3N4) materials with 1.8 and 5.3 vol.% of multi-walled carbon nanotubes (MWCNTs) were densified using 7 wt% of sintering additives (Y2O3 +Al2O3). The mixing and sintering procedures produced quite homogenous and dense MWCNT/Si3N4 composites. The nanotubes condition was followed by micro-Raman spectroscopy and no alteration was observed in spite of the relatively high sintering temperatures (approximately 1600 degrees C). Mechanical parameters (hardness, elastic modulus and fracture toughness) of the composites and comparative blank specimens were measured by instrumented indentation and discussed in parallel. Thermal conductivity was also estimated for these specimens. The nanotube orientation effect inherent to pressure assisted sintering methods and the weak interfacial bond between nanotubes and Si3N4 are important factors to explain the mechanical and thermal behaviours of these composites.

Nanostructured Bulk Ceramics (Part II. Superplasticity and High Strain Rate Superplasticity)

In Part II, the paper will describe a three-phase alumina-based nanoceramic composite demonstrating superplasticity at a surprisingly lower temperature and higher strain rate. One important factor in the processing of these nanocomposites was the use of the electrical field assisted sintering method, SPS. These improvements in mechanical properties were briefly discussed in the context of the results from the microstructural investigations. SPS forming approach provides a new route for low temperature and high-strain-rate superplasticity for nanostructured materials and should impact and interest a broad range of scientists in materials research and superplastic forming technology.

Nanostructured Bulk Ceramics (Part I)

The processing and characterization of ceramic nanocomposites, which produce bulk nanostructures with attractive mechanical properties, have been emphasized and introduced at Prof. Mukherjee’s Lab at UC Davis. The following subjects will be introduced in detail in Part Ⅱ, Ⅲ, and Ⅳ. In Part Ⅱ, the paper will describe a three-phase alumina-based nanoceramic composite demonstrating superplasticity at a surprisingly lower temperature and higher strain rate. The next part will show that an alumina-carbon nanotube-niobium nanocomposite produced fracture toughness values that are three times higher than that of pure nanocrystalline alumina. It was possible to take advantage of both fiber-toughening and ductile-metal toughening in this investigation. In the fourth section, discussed will be a silicon-nitride/silicon-carbide nanocomposite, produced by pyrolysis of liquid polymer precursors, demonstrating one of the lowest creep rates reported so far in ceramics at the comparable temperature of 1400℃. This was first achieved by avoiding the oxynitride glass phase at the intergrain boundaries. One important factor in the processing of these nanocomposites was the use of the electrical field assisted sintering method. This allowed the sintering to be completed at significantly lower temperatures and during much shorter times. These improvements in mechanical properties will be discussed in the context of the results from the microstructural investigations.

Sintering and Densification; New Techniques: Sinter Forging

In this chapter pressure assisted sintering methods will be described. Attention will mainly be paid to sinter forging as a die-wall free uniaxial pressure sintering technique, where large creep strains are possible. Sinter forging is an effective tool to reduce sintering temperature and time and to obtain a nearly theoretically dense ceramic. In this way grain size in tetragonal zirconia ceramics can be reduced down to 100 nm. Another important phenomenon is the reduction of the number density and size of cracks and flaws resulting in higher strength and improved reliability, which is of utmost importance for engineering ceramics. The creep deformation during sinter forging causes a rearrangement of the grains resulting in a reduction of interatomic spaces between grains, while grain boundary (glassy) phases can be removed. The toughness and in some cases the wear resistance is enhanced after sinter forging as a result of the grain-boundary-morphology improvement.