Two-step Sintering Process Research Articles

Sintering of high-entropy nanoparticles obtained by polyol process: A case study of (La0.2Y0.2Nd0.2Sm0.2Gd0.2)2Ce2O7-δ

High-entropy (HE) ceramics nanoparticles have received much attention due to their interesting properties. However, very limited studies have been conducted on their sintering. Here, we report the sintering behavior of HE A2B2O7 type rare earth oxide nanoparticles obtained by polyol process. HE cerate (HECe) (La0.2Y0.2Nd0.2Sm0.2Gd0.2)2Ce2O7-δ is chosen as an exemplary case, which is considered as a good candidate for thermal insulation. HECe nanoparticles with size of 2.6–7.1 nm can be synthesized through polyol process followed by annealing in air at 300–700 °C. HECe nanoparticle compact can be densified by directly sintering at 1500 °C. The sintering temperature could be further decreased using a two-step sintering process, i.e., 1500 °C 5 min-1300 °C 5 h. Our results show that fine particle and abundant oxygen vacancies probably dominate the densification process. By controlling the sintering regime, we can tune the microstructure of HECe ceramics and thermal conductive properties accordingly.

Structural and electronic behavior of yttrium doped zirconolite ceramic; a potential waste form for burning minor actinides

In the study of nuclear materials, zirconolite ceramic doped with yttrium, as a surrogate for minor actinides, is chosen to study the effects of doping on structural and electronic response which are crucial parameters in the determination of the stability and durability of materials under radiation effects. The samples Ca0.9Zr0.9Y0.2Ti2O7 have been synthesized using conventional solid-state route of two-step sintering process. X-ray diffraction and Raman spectroscopy studies are performed on samples to examine the effect of temperatures on formation of phase. The scanning electron microscopy analysis revealed formation of porous micro-structure at 1200 °C while dense-packed morphology at sintering temperature of 1300 °C. Both XRD and Raman spectroscopy on samples sintered at 1300 °C reveal almost single phase (2 M zirconolite) formation with 20% molar concentration of yttrium thus indicating high loading capacity of zirconolite. The formation of 2M-zirconolite phase is also confirmed from Rietveld refinement of the spectra. Analysis of refinement of spectra of sample sintered at 1300 °C indicates substitution of Y3+ ions against both Ca and Zr sites with similar occupancy of ∼10%, while the altered chemical environment is found to affect the bond length. XPS confirms the presence of Y3+ in the structure and illustrates further that oxidation states of all the elements remain unaltered. Finally, the measurements and analysis show that zirconolite can retain its structural integrity by incorporation of 20% molar concentration of yttrium on Ca and Zr sites and can be a promising waste form for burning the minor actinides in advance accelerators.

Enhanced thermal conductivity in Si3N4 ceramics by carbonizing polydopamine coatings

To enhance the thermal conductivity of Si3N4, a polydopamine (PDA) coating was creatively introduced into ceramics sintered with different additive contents through a two-step sintering process consisting of a first treatment at 1500 °C for 8 h followed by 12 h at 1900 °C under 1 MPa nitrogen pressure. After the first-step sintering, the PDA-coated sample exhibited a higher elimination effect of the liquid phase and an increase in the N/O ratio, which allowed the additives to directly interact with the Si3N4 grains, resulting in a microstructure with more and larger rod-shaped grains. After the second-step sintering, the densified samples of the PDA-coating attained slightly coarser rod-shaped grains, lower O content, higher N/O ratio and peak intensity (XRD) of the Y2Si3O3N4 phase, thicker grain boundary film, and secondary phases mainly residing in multigrain junctions. Consequently, the thermal conductivity of all the PDA-coated samples typically showed a 10–12% increment in comparison to the PDA-free samples for each additive content.

Influence of two-step sintering on power loss and permeability dispersion of MnZnNi ferrite

Manganese-zinc-nickel (MnZnNi) ferrite nanoparticles with the chemical composition Mn0.2Zn0.5Ni0.3Fe2O4 were prepared via co-precipitation method. Using the prepared nanoparticles, ring-shaped cores were formed and subjected to two-step sintering. Then, magnetic permeability, magnetic losses, density and microstructure of the samples were examined and compared with those of ferrites produced by the single-step sintering process. It was found that upon reducing the isothermal temperature from 1350 °C to 1250 °C, the density of the samples increased from 4.39 to 4.71 gr/cm3 and the average grain size diminished from 10.1 to 2.2 µm. The samples prepared through the two-step sintering method with a high temperature of 1350 °C and a lower temperature of 1250 °C exhibited a good performance in terms of loss and permeability. The two-step sintering process is an efficient method to produce high density and permeability plus low-loss ferrites.

Microstructural development for optimum fracture toughness of Al2O3-3YSZ composites

ABSTRACT Alumina-based ceramics are widely used for making wear-resistant components and machine parts due to their high hardness, strength, wear-resistance, and high chemical and thermal stability. However, low fracture toughness (KIC ~ 3.0 MPa.m1/2) inhibits alumina’s usage where high toughness is a pre-requisite (e.g. ceramic engines). In this work, yttria-stabilised zirconia (3YSZ) was added to alumina and a two-step sintering process was used to make a composite with enhanced mechanical properties. The key properties – hardness, fracture toughness, and percentage theoretical density (Dth) – could be related to microstructural features. It was revealed that higher numbers of elongated grains formed with increasing additions of 3YSZ. The maximum fracture toughness and hardness achieved were 5.51 MPa.m1/2 and 10.96 GPa, respectively, for the Al2O3-30 wt% 3YSZ composite, believed mainly due to the high proportion of elongated grains. The best-performing sample had a density of 96.1% Dth and the average grain size was 0.35 µm. The thermal expansion coefficient was measured as 6.5 × 10−6 K−1.

Definition of the parameters for the densification of ceramics by two-step solid state sintering

The two-step sintering (TSS) process is generally used to reduce the temperature conventionally required for the sintering of ceramics and to achieve a fine microstructure, but to date a general rule concerning the TSS parameters has not yet been identified. The TSS process was successfully applied to ZnO, Al2O3 and alumina-zirconia composites obtaining a comparable density to that achieved by the conventional process and with limited grain growth. A fine‐grained microstructure was maintained, avoiding the grain boundary migration, by the right selection of the operating conditions (essentially temperatures and holding time). A comparative study of the TSS on different advanced ceramic materials, such as SiC, ZnS and ZrO2, was also proposed, mainly based on the density, the microstructure and the grain size. Moreover, a general rule concerning the process temperatures with respect to the conventional sintering was defined to obtain the same densification preventing the grain growth.

Critical current improvement and resistance evaluation of superconducting joint between Bi2223 tapes

We improved the critical current (I c) of the superconducting joint between the Bi2223 tapes by introducing the two-step sintering process. The in-field transport I c of ∼300 A at 4.2 K and 1 T under a 10−9 Ω criterion was successfully demonstrated. The I c improvement can probably be attributed to the enhancement of the intergrain critical current density for a Bi2223 intermediate layer. Ultra-low in-field joint resistance below 10−14 Ω at 4.2 K and 1 T was also demonstrated using current decay measurement. To the best of our knowledge, this study is the first to demonstrate a practical level of in-field transport I c and ultra-low in-field joint resistance for the superconducting joint between Bi2223 tapes. We believe that this superconducting joint technology will facilitate development of persistent current mode Bi2223 superconducting magnets.

Enhanced electrical properties of (Zn, Mn)-modified BiFeO3–BaTiO3 lead-free ceramics prepared via sol–gel method and two-step sintering

Lead-free 0.7BiFe1–2xZnxMnxO3–0.3BaTiO3 (BFZMx–BT) piezoceramics were prepared through the sol–gel method followed by a two-step sintering process. The crystal structure and microstructure were investigated by X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy, which revealed that the addition of Zn and Mn ions resulted in a noticeable modification of the microstructure. The BF–BT modification through (Zn, Mn) equivalent codoping induces grain size variation and oxygen vacancies, resulting in an enhancement in the Curie temperature, dielectric properties, polarization magnitude, and strain response. A large remnant polarization (2Pr = 94.6 μC/cm2), low leakage current (7.94 × 10−6 A/cm2), high Curie temperature (473 °C), excellent piezoelectric constant (d33 = 271 pC/N), high unipolar strain (0.353%), and high piezoelectric actuator constant (d33* = 441 pm/V) were obtained for BFZMx–BT ceramics with x = 0.04. These results not only demonstrate the remarkable effect of (Zn, Mn) codoping on the BiFeO3-based piezoceramics, but may also shed light on the development of advanced piezoelectric materials with high performance.

Characteristics of six layered Al/Si3N4 functionally graded materials prepared through two-step pressureless sintering process

The rapid advancement of the welding technology has simultaneously increased the demand for the online monitoring system in order to control the process. Among the methods that could be possibly used to assess the weld condition, an air-borne acoustic method grasps the attention from scholars due to its ability to provide a simple, non-contact, and low-cost measurement system. However, it is still lack of resources involving this subject in an attempt to deeply understand the emitted sound behaviour during welding especially when dealing with a complete deviation of a process parameter, welding types, workpiece material as well as the noise from the surrounding. This paper reviews the application of the acoustic method in monitoring the welding process. Specifically, this review emphasized the source of both structure-borne and air-borne acoustic during the welding process and the significance of applying the acoustic method in more detail. By focusing on the liquid state welding process, the scope of discussion converged on the arc and laser welding process. In the last part of this review, the potential future advancement of this method is pointed out before the overall conclusion is made.

Effect of Sintering Time on Mechanical Properties of (Hf-Ta-Zr-Nb-Ti)C

Abstract Advanced high-entropy (Hf-Ta-Zr-Nb-Ti)C ceramics were successfully fabricated by the combination of ball milling and a two-step sintering process at 2100 °C for 5, 10, and 20 minutes from commercially available powders. The microstructure characteristics and mechanical properties of the developed systems were investigated. The high-entropy ceramics exhibit: medium grain size from 8 μm to 11 μm, high compositional uniformity, and high relative density above 99.5 %. Vickers hardness for all systems decreased with increasing applied load, with the highest value HV1 = 22.0 GPa for the system sintered for 5 minutes, while the fracture toughness changed from 2.70 MPa.m1/2 to 3.50 MPa.m1/2. The highest four-point flexural strength of 284 MPa was measured for the system with the smallest grain size.

Increasing the tensile strength of oxide ceramic matrix mini‐composites by two‐step sintering

AbstractAdapting conventional sintering (CS) techniques of monolithic ceramics for the production of oxide ceramic matrix composites (Ox‐CMCs) comes along with a few drawbacks, such as fiber degradation. Thus, the applicability of two‐step sintering (TSS) for the production of Ox‐CMCs based on Nextel™ 610 fibers and porous alumina matrix is investigated in this study for the first time. Uniaxial tensile tests were performed to evaluate the performance of mini‐composites produced by TSS and compared with those produced by CS. Parameters known for influencing the mechanical behavior of the mini‐composites, such as grain size, porosity, shrinkage, as well as matrix properties, were analyzed. Both sintering techniques resulted in similar grain size distributions, whereas TSS showed higher total porosity and lower amount of sintering‐induced cracks. As a result, TSS samples showed a higher tensile strength of 230±27 MPa when compared to 133±8 MPa for CS. In general, it was observed that most of the densification happens during the first phase of TSS, while the matrix is slowly strengthened during the second step. Therefore, the reported TSS process is a very promising and easy‐to‐apply heat treatment for producing Ox‐CMCs with controlled microstructure.

Highly Transparent Eu-Doped 0.72PMN-0.28PT Ceramics with Excellent Piezoelectricity.

The 0.975[0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3]-0.025Eu2O3 ceramics were prepared by a two-step sintering process including oxygen sintering and hot-pressing. An ultrahigh piezoelectric charge coefficient of 1400 pC/N and a superior optical transmittance up to 68% were simultaneously achieved. The underlying mechanism was discussed from a microstructural perspective, where the watermark domain configuration with a small domain size is responsible for the high optical transmission, while the large remanent polarization and dielectric constant and the introduced tetragonal phase with a parallel stripe domain structure are believed to synergistically contribute to the high piezoelectric coefficients. This work demonstrates that the rare-earth dopant in the PMN-PT ceramic system is conducive to enhanced transparency and piezoelectricity.

Binder-jetting additive manufacturing of Mg alloy densified by two-step sintering process

Difficult to effectively densify in a short time is an important issue that limits the application of magnesium (Mg) alloy fabricated by binder jetting additive manufacturing (AM) process. In this study, two-step sintering (TSS) process was utilized to improve the density of printed Mg alloy samples. Results show that the relative density, mechanical properties and corrosion resistance of the samples after different TSS process were dramatically improved in a short time. A suitable sintering temperature of first sintering step can rapidly increase the density of the samples in a short time. During the second step sintering process, little exceeding the liquidus temperature can further increase the density of the sample, but little lower than the liquidus temperature will lead to a sharp increase in grain size. The sample after sintering at 680 °C for 30 min then sintering at 610 °C for 6 h shows the highest relative density (0.995), the highest strength (394 MPa), the lowest corrosion rate (101 mm/year) and the smallest corrosion current density (60.4 μA/cm2). Compared with one-step sintering process, TTS process has higher sintering efficiency.

Influence of Isovalent ‘W’ Substitutions on the Structure and Electrical Properties of La2Mo2O9 Electrolyte for Intermediate-Temperature Solid Oxide Fuel Cells

Lanthanum molybdenum oxide (La2Mo2O9, LAMOX)-based ion conductors have been used as potential electrolytes for solid oxide fuel cells. The parent compound La2Mo2O9 undergoes a structural phase transition from monoclinic (P21) to cubic (P213) at 580 °C, with an enhancement in oxide ion conductivity. The cubic phase is of interest because it is beneficial for oxide ion conduction. In search of alternative candidates with a similar structure that might have a stable cubic phase at lower temperatures, we have studied the variations of the crystal structure and ionic conductivity for 25, 50, 62.5 and 75 mol% W substitutions at the Mo site using high-temperature X-ray diffraction, dilatometry, and impedance spectroscopy. Highly dense ceramic samples have been synthesized by solid-state reaction in a two-step sintering process. Low-angle X-ray diffraction and Rietveld refinement confirm the stabilization of the cubic phase for all compounds in the entire temperature range considered. The substitutions of W at the Mo site produce a decrement in the lattice parameter. The thermal expansion coefficients in the high-temperature range of the W-substituted ceramics, as determined by dilatometry, are much higher than that of the unmodified sample. The impedance spectra have been modeled using a modified genetic algorithm within 300–600 °C. A distribution function of the relaxation times is obtained, and the contributions of ohmic drop, grains and grain boundaries to the conductivity have been identified. Overall, our investigation provides information about cationic substitution and insights into the understanding of oxide ion conductivity in LAMOX-based compounds for developing solid oxide fuel cells.

Digital light processing of Si-based composite ceramics and bulk silica ceramics from a high solid loading polysiloxane/SiO2 slurry

Nowadays, 3D polymer-derived silicon oxycarbide ceramics (SiOC) can be fabricated by the stereolithography method successfully. However, due to their intrinsically poor ceramic content and the large thermal shrinkage during the pyrolysis, it is difficult for the bulk 3D polysiloxane precursors to be pyrolyzed into dense 3D-ceramics. Herein, the ceramic content of the photocurable polysiloxane precursors was increased by adding a large amount of SiO2 powders into the low solid content epoxy-acrylic siloxane. After being added with proper dispersant, the viscosity of the high solid loading polysiloxane/SiO2 slurry can reach a proper level. Bulk 3D-silica ceramics with the wall-thickness around 8 mm can be conveniently fabricated from the polysiloxane/SiO2 slurry by stereolithography and a two-step sintering process. The addition of a proper sintering aid NaF can promote the 3D-PSO/SiO2 precursor to be converted into dense and crack-free 3D-silica ceramics with good mechanical proprieties.

Preparation of nanocrystalline gradient cemented carbide by adding gradient former of V(C, N)

Nanometer WC and Co powders were used as raw materials, gradient former of V(C, N) and traditional gradient former of Ti(C, N) were added respectively to prepare gradient cemented carbide by a two-step sintering process. The results showed that nanocrystalline gradient cemented carbide was prepared by adding gradient former of V(C, N), and ultrafine gradient cemented carbide was prepared by adding gradient former of Ti(C, N). Growth of WC grains were inhibited and the gradient layer was thickened by adding the gradient former of V(C, N). The mechanical properties of the alloy with V(C, N) were better than those of alloy with Ti(C, N).

Electrical properties of (1-x)BiFe0.94Zn0.03Ti0.03O3-xBaTiO3 lead-free ceramics obtained via sol-gel route and two-step sintering process

(1-x)BiFe0·94Zn0·03Ti0·03O3-xBaTiO3 (BFZN-BT) ceramics are prepared via sol-gel method, followed by conventional solid-state sintering or two-step sintering processes. Then, systematic experimental characterization and performance evaluation are carried out to investigate this technique and its effects on eight different ceramic samples. Owing to morphotropic phase boundary and optimal sintering conditions, 0.7BiFe0·94Zn0·03Ti0·03O3-0.3BaTiO3 ceramic sintered at T1 = 1025 °C and T2 = 900 °C shows the largest rhombohedral distortion (90°−αR = 0.14°) and tetragonal distortion (cT/αT = 1.021), as well as low leakage (1.19 × 10−5 A/cm2), excellent d33 = 149 pC/N, 2Pr = 85.1 μC/cm2 and Curie temperature of 461 °C. Moreover, large unipolar strain of 0.307% and d33∗ value of 516 p.m./V are also obtained for this ceramic. Overall, the proposed strategy exhibits promise in the advancement of eco-friendly lead-free piezoceramics.

The preparation of high-performance 96W-2.7Ni-1.3Fe alloy parts by powder extrusion 3D printing

The tungsten heavy alloy green parts are successfully manufactured by an innovative indirect 3D printing technology, powder extrusion printing. Then the subsequent debinding-sintering process of the printed green parts is optimized to obtain a high-density and suitable-contiguity alloy. Finally, a 96W-2.7Ni-1.3Fe alloy with a high density of ~99.1%, suitable contiguity of ~ 0.63, strength of ~801 MPa and elongation of ~22.1% is prepared by a two-step sintering process under H2 atmosphere. After proper heat treatment, the strength and elongation of the as-sintered specimen are further improved to 838 MPa and 26.1%, respectively, which can be attributed to the reduction in the contiguity value of specimen, the enhancement of interfacial bonding force of W- γ interface, and the formation of the bridging mechanism of cracks. The indirect 3D printing technology developed in this work provides a new technical approach for the additive manufacturing of tungsten-based alloys and/or other refractory metals.

Fabrication and mechanical properties of homogeneous and gradient nanocomposites by two-step sintering

Homogeneous and functional gradient nanocomposites were prepared using a two-step sintering process at a pressure of 30 MPa. The effects of macro –micro fracture morphologies and element distributions on the mechanical properties of the composites were studied. The results showed that the fracture toughness of the inner layers of the gradient material was almost the same as that of the corresponding homogeneous material. The composition content difference between the gradient layers had a significant influence on the bending strength and hardness. There was little Ni and Mo diffusion in the gradient nanocomposites because of the relatively low contents and suitable processing parameters. A gradient distribution of the metals was ensured. The surface layer thickness had little influence on the flexural strength of the gradient nanocomposite, but it had an obvious influence on the hardness and fracture toughness of the surface. The five-layered gradient composite with a surface layer thickness of 85 μm exhibited the best mechanical properties, including a flexural strength of 1021 ± 35 MPa, fracture toughness of 7.60 ± 0.21 MPa m1/2 and surface hardness of 19.26 ± 0.62 GPa.

Post‐processing technologies of copper–polylactic acid composites obtained by 3D printing fused deposition modeling

The goal of the present study is to obtain a new structured material starting from a copper–polylactic acid (PLA) composite filament 3D printed using fused deposition modeling method, followed by a sintering process in vacuum atmosphere. Metal-reinforced filaments made of 80 wt.% copper encased in an environmentally friendly, biodegradable and carbon neutral PLA binder were used as feedstock materials. The thermal stability and the melting temperature of the filaments were evaluated through thermogravimetric analysis. The printing parameters were chosen according to the producer's specifications. Furthermore, the printed samples were submitted to a two-step sintering process in a vacuum furnace, assuring a complete removal of the polymeric material and the diffusion of the copper particles. The post-treatment revealed a porous structured material, similar to a cellular solid. Microstructural analysis and preliminary mechanical testing show that the porosity and hardness of the end product are heavily influenced by the initial polymer content.