Subsequent Low-temperature Sintering Research Articles

Sol-gel construction of honeycomb-like CuMn2O4 spinels with high infrared emissivity

Spinel materials are gradually becoming high infrared radiation materials with the most potential because of their special crystal structure. However, it is still difficult to design the microstructure reasonably and synthesize it at low temperature. In this paper, the honeycomb-like CuMn2O4 spinels were prepared via a facile sol-gel method with a subsequent low-temperature sintering treatment. Therefore, the CMO samples with numerous pores are conducive to improving absorptivity and emissivity. Moreover, the optimum experimental conditions for high infrared emissivity were acquired by detailed investigation of the synthesis parameters including the molar ratio of trimonium citrate to metal ions, the molar ratio of urea to metal ions and the sintering temperature. Under the synergistic effect of internal factors such as, a suitable amount of impurities, abundant oxygen vacancy, small band gap, and good crystallinity, the obtained sample CMO-800 achieved excellent infrared radiation performance. Its infrared emissivity values at the test temperature of 30 °C and 600 °C separately reached 0.801 and 0.965 in the wavelength range of 3–5 μm. More significantly, this work provides significant guidance for the design and preparation of spinel materials with excellent infrared emissivity.

Hierarchical Structures Formed in an Atmospheric Pressure Plasma Jet by Polymerization from an Aerosol of Oxalic Acid Stabilized WO3–x Nanosheet Colloids: Toward High-Performance Electrochemical Devices and Sensors

The development of microstructures which combine high total surface area and high porosity is crucial for technologies such as electrocatalysis, electrochromics, and sensors. High deposition rate, composition control of deposition, and low processing temperature to retain active compositions are also desirable. To this end, this study describes combining colloidal sol chemistry with a nonthermal atmospheric pressure dielectric barrier discharge plasma jet to print hybrid inorganic/organic tungsten trioxide/oxalic acid (WO3–x/OA) microspheres. Injection of an aerosol of oxalic acid stabilized colloidal tungstic acid into an atmospheric pressure plasma jet results in the deposition of spherical structures in which the colloid is trapped within a plasma-polymerized organic shell. Subsequent low-temperature sintering produces hierarchical spherical shell-like structures comprising tungsten oxide nanosheets. Alteration of the gas flow rate changes the composition of the deposited material. The method has promise for the general preparation from colloidal precursors of porous materials of controlled morphology and composition with hierarchical microstructures, such as are required for applications in electrochemical devices and sensors which need a high ratio of surface area to volume and connectivity throughout the structure, yet also need a microstructure which is open for rapid exchange of reactants.

Low-Temperature Chemical Sintered TiO2 Photoanodes Based on a Binary Liquid Mixture for Flexible Dye-Sensitized Solar Cells

A chemically sintered and binder-free paste of TiO2 nanoparticles (NPs) was prepared using a binary-liquid mixture of 1-octanol and CCl4. The 1:1 (v/v) complex of CCl4 and 1-octanol easily interacted chemically with the TiO2 NPs and induced the formation of a highly viscous paste. The as-prepared binary-liquid paste (<bold>P</bold>BL)-based TiO2 film exhibited the complete removal of the binary-liquid and residuals with the subsequent low-temperature sintering (~150°C) and UV-O3 treatment. This facilitated the fabrication of TiO2 photoanodes for flexible dye-sensitized solar cells (f-DSSCs). For comparison purposes, pure 1-octanol-based TiO2 paste (<bold>P</bold>O) with moderate viscosity was prepared. The <bold>P</bold>BL-based TiO2 film exhibited strong adhesion and high mechanical stability with the conducting oxide coated glass and plastic substrates compared to the <bold>P</bold>O-based film. The corresponding low-temperature sintered <bold>P</bold>BL-based f-DSSC showed a power conversion efficiency (PCE) of 3.5%, while it was 2.0% for <bold>P</bold>O-based f-DSSC. The <bold>P</bold>BL<bold>-</bold>based low- and high-temperature (500°C) sintered glass-based rigid DSSCs exhibited the PCE of 6.0 and 6.3%, respectively, while this value was 7.1% for a 500°C sintered rigid DSSC based on a commercial (or conventional) paste.

Effect of Bi2O3 contents on magnetic and electromagnetic properties of LiZnMn ferrite ceramics

Soft LiZnMn ferrites with low coercive force values and small grain sizes were developed by the solution combustion synthesis and low temperature sintering technique for microwave applications at a high frequencies. Bismuth oxide was used as an additive to lower the sintering temperature. The examination covered the influence of Bi2O3 on the crystal structure, microstructure, primary magnetic and dielectric characteristics of lithium-zinc-manganese ferrites. The most promising sample designed for using in microwave devices was produced by sintering at 1075 ℃ temperature for 8 h with added 1.5 wt% Bi2O3. These production conditions have yielded 2.98 µm average grain size in a ceramic product, the density is 4.84 cu cm/g, and the coercive force, residual induction, and saturation induction are 58 A/m, 2078.4 G, and 3439.1 G, respectively. In addition, this sample demonstrates a high initial magnetic permeability (μi = 168), Curie temperature (Tc = 437.5 ℃), high values of the dielectric loss tangent (tan δɛ = 6.32⸱10−3), ferromagnetic resonance line width (ΔH = 280 Oe) and the resonance line of spin waves (ΔHk = 1.87 Oe). Further increase in the bismuth oxide content allows observing a change in the ceramics microstructure, accompanied by a deterioration in the magnetic and electromagnetic characteristics. Here, the discussion covers the mechanism of change in the functional properties of lithium-zinc-manganese ferrites produced in the conditions of solution combustion with added bismuth oxide. Thus, synthesizing of initial pre-ceramic powder by glycine-nitrate combustion and subsequent low-temperature sintering with added bismuth oxide is a novel efficient technique of producing advanced soft high-frequency LiZnMn ferrites.

Effect mechanism of oxide doping on the microstructure and mechanical properties of Mo–Y2O3 alloys

It is an effective way to enhance the mechanical properties of Mo alloy by adding oxide strengthening phase. However, the oxide second-phase particles in oxide dispersion strengthened Mo (ODS-Mo) alloy prepared via traditional mechanical alloying generally coarsen at grain boundaries (GBs), greatly weakening their strengthening effect. To solve this problem, single and multiple oxide particles strengthened Mo alloys are developed via wet chemical method and subsequent low-temperature sintering in this work. After doping CeO2, the compressive yield strength (340.3 ± 8.1 MPa) and hardness (410 ± 13 HV) of Mo–Y2O3 (Mo–Y) alloy were decreased by 17.9% and 22.7%, respectively. However, after doping La2O3, the compressive yield strength and hardness of Mo–Y alloy are further increased by 89.4% and 21.0%, respectively. According to microstructure observation and theoretical calculation, CeO2 doping decreases the volume fraction of intergranular oxide particles and weakens interfacial drag force, finally promoting Mo grain growth and worsening the mechanical properties of Mo–Y alloy. In contrast, after doping La2O3, more oxide particles are dispersed at GBs, and their coarsening are inhibited. This results in a stronger Zener force. Moreover, interfacial drag force can also be increased through doping La2O3. Two forces synergistically hinder GBs migration. Therefore, La2O3 doping leads to Mo grain refinement and ultimately enhances the mechanical properties of Mo–Y alloy. These results in our work can provide theoretical guidance for choosing proper oxide dopants to prepare ODS alloys with excellent mechanical properties.

Effect of surface modification on the microstructure and sintering characteristics of tungsten nanopowders prepared by a wet chemical method

ABSTRACT In order to prepare high-performance tungsten alloys, surface modification of tungsten nanopowders prepared by a wet chemical method was carried out by acid pickling at room temperature. The low-temperature sintering characteristics of tungsten nanpowders before and after pickling was compared and analysed. The surface modification was carried out by pickling with a solution of hydrofluoric acid, concentrated nitric acid and ammonium fluoride at room temperature. After this surface treatment, uniformly distributed step-shaped defects formed on the surface of the tungsten nanopowders. After low-temperature sintering, the grain size of the tungsten alloy corresponding to the surface-modified powder precursor was reduced by 20% compared to the alloy using untreated powder as precursor. In addition, the step-shaped defects also increase the sintering activity of the pickled tungsten powder and promote its sintering densification, making the final density of pure tungsten sintered at 1600°C as high as 96.7%, and its hardness greatly increased to 521 HV0.2. The results show that room-temperature pickling and subsequent low-temperature sintering is a promising method for preparing high-performance nanotungsten alloys.

Refined microstructure and enhanced mechanical properties in Mo-Y2O3 alloys prepared by freeze-drying method and subsequent low temperature sintering

The ultrafine Mo-Y2O3 composite powders were successfully synthesized by innovative freeze-drying method. Consequently, the freeze-dried Mo-Y2O3 composite powders with high sintering activities possess an average grain size of 54 nm. After low temperature sintering at 1600 °C, the Mo-Y2O3 alloys maintaining a high density (99.6 %) have the finest grain size (620 nm) comparing with available literature about oxide dispersion strengthened molybdenum alloy (ODS-Mo). The oxide particles remain their small size (mainly <50 nm) within Mo grains and at Mo grain boundaries. Furthermore, the Y5Mo2O12 particles were firstly observed within Mo matrix, and its formation can absorb nearby oxygen impurities, which involves the purification of Mo matrix. The mechanical properties show that Mo-Y2O3 alloy possess a high hardness of 487 ± 28 HV0.2, a high yield strength of 902 MPa, a high compressive strength of 1110 MPa, respectively. Our work suggests that freeze-drying and subsequent low temperature sintering can shed light on the preparation of ultrafine ODS-Mo alloys with high performance.

The synthesis of TiC dispersed strengthened Mo alloy by freeze-drying technology and subsequent low temperature sintering

The TiC dispersed strengthened Mo alloys were synthesized by novel freeze-drying technology and subsequent low-temperature sintering. With the introduction of TiC nanoparticles using freeze-drying technology, the sintered Mo-TiC alloys possess ultrafine grain size (2.38 μm), high density (99.3%) and high hardness (402±29 HV0.2). The agglomeration and growth of second phase particles in Mo-TiC alloys were effectively depressed by freeze-drying technology. Moreover, a theory about purification and strengthening is proposed. C-deficient TiC (Ti8C5) will adsorb nearby oxygen impurities and even form TiO2 to purify Mo matrix. Furthermore, TiC (002), Ti8C5 (0-24) and TiO2 (111) can form coherent interfaces with Mo (110) to strengthen their phase boundary and improve material strength. The stable phase interface can pin and limit the growth of these second phase particles, thus maintaining their small size (<50 nm).

Microstructure refinement and mechanical properties improvement in the W-Y2O3 alloys via optimized freeze-drying

Oxide-dispersion-strengthened (ODS) tungsten based alloys with high performance were synthesized by optimized freeze-drying with PVP and PEG polymer dispersant additives and subsequent low temperature sintering. The pre-freezing principle developed firstly in our work shows that fast cooling rate and polymer dispersant additives contribute to the refinement and uniformity of precursor powders. The optimized freeze-dried W-Y2O3 composite powders possess an average grain size of 9 nm, which can be attributed to the steric hindrance provided by polymer dispersants during freeze-drying and voids left by the decomposition of polymer dispersants during calcination. After sintering at low temperature, the W-Y2O3 alloys have ultrafine W grain (210 nm) and nano second phase particles (<50 nm) dispersed in W matrix while maintaining a high relative density of 97.8%. These factors mentioned above lead to the microhardness and compressive strength as high as 721 ± 31 HV0.2, 814 MPa respectively, accompanied by a high total deformation plasticity of above 50%. These results indicate that the optimized freeze-drying and subsequent low temperature sintering is a promising way to fabricate ODS tungsten based alloys with high performance.

Enhanced mechanical properties in oxide-dispersion-strengthened alloys achieved via interface segregation of cation dopants

With significantly enhanced irradiation resistance, high-temperature strength, and creep resistance, oxide-dispersion-strengthened tungsten (ODS-W) alloys present tremendous potential for high-temperature applications. However, the oxide particles tend to segregate at W grain boundary and grow up (even to micron), greatly suppressing their strengthening effect. It is always a great challenge to effectively refine and disperse the oxide particles at W grain boundary. Here, we successfully developed a new type of cation-doped W-Y2O3 alloy via a wet chemical method and subsequent low-temperature sintering. It was found that proper cation doping could not only significantly refine the intergranular Y2O3 second phase particles but also dramatically improve the sinterability of W matrix. These doping effects, as a result, simultaneously enhance the strength and ductility of the W-Y2O3 alloy. It was confirmed that the segregation of cation dopants at the W/Y2O3 interface is the origin of these doping effects. Furthermore, X-ray photoemission spectra (XPS) analyses confirmed that cation dopant segregation also obviously affects the chemical bonding (i.e., W-O bond) along the W/Y2O3 interface. As a result, the rate-limiting mechanism for W grain growth is influenced remarkably, explaining well the difference of W grain size in various cation-doped W-Y2O3 alloys. For the refinement of intergranular Y2O3 particles, it can be understood well from both thermodynamic and kinetic views. Detailedly, W/Y2O3 interfacial energy and atom mobility for Y2O3 coarsening are all limited by cation dopant segregation. More importantly, this cation-doping approach can also be applicable to other ODS alloys for enhancing their comprehensive mechanical properties.

Effects of solution pH value and mineralizer on formation and color property of holmium molybdate pigment via co-precipitation and sintering

A novel functional ceramic or glass pigment of monoclinic holmium molybdate (Ho2MoO6) with ‘allochroic effect’ was synthesized via co-precipitation and subsequent low-temperature sintering with different mineralizers (i.e., lithium chloride, sodium fluoride and boric acid). Effects of solution pH value and mineralizer type/content on Ho2MoO6 formation at different sintering temperatures and time were investigated. The results reveal that the solution pH value in co-precipitation reaction has an influence on the element ratio of Mo and Ho in the precursor and the pigment purity. The sintering temperature of forming sole Ho2MoO6 phase is decreased by 200 °C and the sintering duration is reduced in the presence of 5 wt % lithium chloride due to the decreased crystallization activation energy. Meanwhile, Ho2MoO6 pigment as powder or coating on ceramic tile and glass exhibits various colors (i.e., pale yellow, salmon, mustard and coral) under different illuminants (i.e., incandescent lamp A, solar-light D65, cool white fluorescent lamp F2 and three-band fluorescent lamp F11).

The simultaneous improvements of strength and ductility in W–Y2O3 alloy obtained via an alkaline hydrothermal method and subsequent low temperature sintering

Oxide dispersion strengthened tungsten (ODS-W) alloys are a class of important materials that widely used for high temperature applications due to their unique physical and mechanical properties. However, the insufficient ductility of ODS-W alloys at low or high temperatures limits their widespread application, especially as structural materials. It is always a great challenge to efficiently disperse oxide second phase nanoparticles within W matrix for achieving ODS-W alloy with excellent balance of strength and ductility. Here we report an innovative strategy for the fabrication of high-performance ODS-W alloy with strength of up to 1134 MPa and total elongation over 30% at 700 °C. The innovative route involves the preparation of W–Y2O3 composite powder precursor via alkaline hydrothermal method and subsequent low-temperature sintering in hydrogen atmosphere. The internal flake structure with uniformly dispersed Y component and external coating of W component on this flake structure can be formed in alkaline hydrothermal product and then be passed on to the reduced W–Y2O3 composite nanopowders. As a result, the distribution of second phase oxide particles in the low-temperature sintered alloys using these composite nanopowders as precursor is optimized significantly including an increase in the volume fraction of intragranular oxide particles and a decrease in the size of both intergranular and intragranular oxide particles. Besides, the average W grain size is also refined dramatically (410 nm). More importantly, the intragranular oxide particle / W matrix interface is regulated to be coherent relationship. The combined action of these microstructural factors is responsible for the significantly enhanced strength and ductility of ODS-W alloy. These results indicate that the newly developed alkaline hydrothermal method in our work is a promising way to fabricate high performance ODS tungsten based alloys.

Enhanced critical current density in the low-temperature sintered Nb3Al superconductor with Sn doping

The Nb3(Al1-xSnx) superconductors (x = 0.05, 0.15, 0.25, 0.35) were prepared by mechanical alloying and subsequent low-temperature sintering. It was found that the critical current density (Jc) of the obtained Nb3(Al1-xSnx) sample is enhanced significantly compared to undoped Nb3Al prepared using the same method. The maximum value of the Jc reaches up to 5.10 × 104 A·cm−2 (4.2 K, 5 T) for Nb3(Al0.85Sn0.15) sample. Combined with the phase composition analysis and microstructure observation, it is indicated that Sn enters the A15 crystal structure of Nb3Al forming the Nb3(Al1-xSnx) phase after low-temperature sintering. What's more, Sn doping improves the grain connectivity as well as refines the grain size of Nb3Al, which are the main reasons for the enhancement of Jc in the Sn doped samples. Based on the exploration of the phase evolution of Nb–Al–Sn system during the mechanical alloying and subsequent low-temperature sintering process, it is also confirmed that Nb3Sn forms firstly during the sintering process and then can serve as the perfect nucleus of Nb3Al to increase its nucleation rate, leading to the significant decrease of the grain size of Nb3Al.

Synthesis of W-Y2O3 alloys by freeze-drying and subsequent low temperature sintering: Microstructure refinement and second phase particles regulation

In this work, W-Y2O3 alloys are prepared by freeze-drying and subsequent low temperature sintering. The average size of reduced W-Y2O3 composite powders prepared by freeze-drying method is only 18.1 nm. After low temperature sintering of these composite nanopowders, the formed W-Y2O3 alloys possess a smaller grain size of 510 nm while maintaining a comparatively higher density of 97.8%. Besides a few submicron Y2O3 particles (about 100–300 nm) with a W-Y-O phase diffusion layer on their surface distribute at W grain boundaries, lots of nano Y2WO6 particles (<20 nm) exist in W matrix. Moreover, many Y6WO12 (<10 nm) particles exist within submicron Y2O3 particles. The formation of these ternary phases indicates that some oxygen impurities in the W matrix can be adsorbed by ternary phases, resulting in the purification of W matrix and the strengthening of phase boundaries. The combined action of the above factors makes the hardness of the sintered W-Y2O3 alloys in our work as high as 656.6 ± 39.0 HV0.2. Our work indicates that freeze-drying and subsequent low temperature sintering is a promising method for preparing high performance W-Y2O3 alloys.

W-Y2O3 composite nanopowders prepared by freeze-drying method and its sintering characteristics

In order to prepare high performance oxide-dispersion-strengthened tungsten based alloys, the W-Y2O3 composite powder precursors were synthesized by novel freeze-drying method. The average W grain size of prepared W-Y2O3 composite powder is 14.2 nm, and corresponding grain size distribution is extremely narrow. The low-temperature sintered W-Y2O3 alloys possess an ultrafine W grain size of about 410 nm while maintaining a comparatively high sintering density of 98.1%. The Y2O3 particles (20–50 nm) with a W-Y-O diffusion layer and Y2WO6 particles (<20 nm) distribute uniformly within W grains and especially at W grain boundaries. These W-Y-O phases adsorb free oxygen impurity, resulting in the purification and strengthening of W matrix. What's more, coherent or semi-coherent interfaces are observed between Y2WO6 particles and W matrix, reinforcing the bonding strength of phase boundary. The combined action of the factors mentioned above leads to the hardness of sintered W-Y2O3 alloys in our work as high as 690.2 ± 32.0 HV0.2. These results indicate that the freeze-drying method and subsequent low temperature sintering is a promising method for preparing high performance W-Y2O3 alloys with ultrafine grains.

The effect of milling conditions on the magnetic and topological properties of MgB2 synthesized by high-energy ball mill and sintered at low temperature

Magnesium diboride, MgB2, is known to be an important high-temperature superconductor material. High-energy ball milling with subsequent low-temperature sintering remains an attractive solid-state method for its synthesis. One main optimization parameter of this method is to ensure a balance between cold welding and fracturing at the milling stages. Therefore, in many cases, the use of a process control agent at the milling stage is considered to be the preferred solution in the prevention of particle size enlargement. The aim of this paper is to determine the effect of dry and toluene-assisted milling on the magnetic and surface topology properties of obtained MgB2. The calculated critical current density of 2.89 × 105 A/cm2 at 10 K 0.3 T for a toluene-assisted sample, compared with the 2.37 × 105 A/cm2 for a dry-milled sample as well as the relatively higher mean surface roughness, clearly indicated a notable positive contribution of surfactant use on the true alloying of MgB2 by mechanical activation.

Deposition of nanodiopside coatings on metallic biomaterials to stimulate apatite-forming ability

One of the drawbacks of metals and alloys in biomedical applications is their inefficient fixation to adjacent tissues which can be fairly addressed by applying bioactive ceramic coatings. In this work, colloidal suspensions based on coprecipitation-derived nanoparticulate diopside (CaMgSi2O6) were deposited on stainless steel 316L by dip-coating and subsequent low-temperature sintering. Afterwards, the structure, bioactivity and biodegradation of the samples were in vitro evaluated by spectroscopic and microscopic techniques. The apatite-forming ability of the surface was found to be improved by using the nanodiopside coating, while controlled by a typical ion-exchange reaction mechanism originating from the film's degradability. In this regard, after soaking the coated samples in a simulated body fluid, an integrated leaf-like precipitation of apatite at early stages and a following non-uniform rose-like growth of apatite with an increased level of the carbonate substitution for hydroxyl were detected. It is eventually concluded that nanodiopside coatings deserve further consideration and development in the biomedical field, where a bioactive fixation is needed along the implant/tissue interface.

Fabrication of ceramic membranes on porous ceramic supports by electrophoretic deposition

Nanoporous alumina membrane and continuous zeolite L membrane were fabricated on the inner surface of microporous alumina tubes. In the former case, an electrophoretic deposition (EPD) technique was used for the deposition of bimodal alumina particles for the subsequent low temperature sintering. In the latter case, the EPD was used for the seeding process of zeolite L particles for the subsequent hydrothermal synthesis. A thin layer of polypyrrole was synthesised on the inside wall of the porous tubes by the chemical polymerisation of pyrrole to give the wall electric conduction for the EPD electrode. The thickness of the coating layers was controlled by altering the applied voltage and deposition time. The interfacial connection of the alumina or zeolite coated layer and the substrate was evaluated by SEM observations before and after the thermal treatment. The nanoporous structure of the alumina membrane was also characterised by a pore size analyser.

Significant improvement in the critical current density of MgB2 bulks in situ sintered at low temperature by excess Mg addition

MgB2 bulks with excess Mg addition were rapidly synthesized by sintering at low temperature in present work. It is found that even after ball milling treatment of original powders, the reaction between Mg and B during subsequent low temperature sintering process was uncompleted within 5h and there is still some residual Mg. On the other hand, the presence of residual Mg can make the sintering microstructure more homogeneous and dense, and also reduce lattice defects and mechanical strains. All the factors are contributed to the improvement of the grain connectivity in the samples with excess Mg addition sintered at low temperature compared to the reference MgB2 sample sintered at high temperature. Hence, Jc of these prepared samples is enhanced significantly across the whole measured fields. Especially, at 20K, 2T, the value of Jc in the 5h-sintered MgB2 bulk with Mg addition is above 1×105Acm−2. The technique developed in present work is an effective and low-cost way to further enhance Jc in the MgB2 superconductors without using expensive nanometer-size dopants.

Tough Yttria-Stabilized Zirconia Ceramic by Low-Temperature Spark Plasma Sintering of Long-Term Stored Nanopowders

Weakly agglomerated 1.75 and 3 mol% yttria stabilized zirconia nanopowders were used in this study after six years of storage in vacuum-processed plastic containers. The proper storage conditions of the Y-TZP nanopowders avoided the hard agglomeration. Untreated and bead-milled nanopowders were used to obtain dense ceramics by slip casting and subsequent low-temperature sintering. Fully dense nanostructured 1.75Y-TZP and 3Y-YZP ceramics with and without doping of 1 wt% Al2O3 were produced by an optimized spark plasma sintering (SPS) technique at the temperatures of 1050-1150 degrees C at a pressure of 100 MPa. The SPS has revealed the clear advantage of consolidation of the weakly agglomerated nanopowders without preliminary deagglomeration. The Vickers hardness of both the low-temperature and spark plasma sintered samples was found to lie in the range of 10.98-13.71 GPa. A maximum fracture toughness of 15.7 MPa m(1/2) (average 14.23 MPa m(1/2)) was achieved by SPS of the 1.75Y-TZP ceramic doped with 1 wt% Al2O3 whereas the toughness of the 3Y-TZP ceramics with and without alumina doping was found to vary between 3.55 and 5.5 MPa m(1/2).