Reduction Sintering Research Articles

An in-situ hybrid laser-induced integrated sensor system with antioxidative copper

Integration of sensors with engineering thermoplastics allows to track their health and surrounding stimuli. As one of vital backbones to construct sensor systems, copper (Cu) is highly conductive and cost-effective, yet tends to easily oxidize during and after processing. Herein, an in-situ integrated sensor system on engineering thermoplastics via hybrid laser direct writing is proposed, which primarily consists of laser-passivated functional Cu interconnects and laser-induced carbon-based sensors. Through a one-step photothermal treatment, the resulting functional Cu interconnects after reductive sintering and passivation are capable of resisting long-term oxidation failure at high temperatures (up to 170 °C) without additional encapsulations. Interfacing with signal processing units, such an all-in-one system is applied for long-term and real-time temperature monitoring. This integrated sensor system with facile laser manufacturing strategies holds potentials for health monitoring and fault diagnosis of advanced equipment such as aircrafts, automobiles, high-speed trains, and medical devices.

Reestablishment of dielectric and nonlinear properties for Bi3+ doped CaCu3Ti4O12 ceramics prepared by reduction-reoxidation method

Multilayer ceramics with base metal internal electrodes should be prepared through reduction sintering to avoid the oxidation of the electrodes, followed by a reoxidation process. In this study, Ca1-xBixCu3Ti4-x/4O12 + 0.03 BN ceramics were fabricated using the reduction-reoxidation method and the reestablishment of dielectric and nonlinear characteristics was investigated. It was observed that the dielectric and nonlinear properties depended crucially on the grain size and the second phases at the grain boundaries. The reoxidation behavior was remarkably improved when grain growth was suppressed and second phases were introduced. Optimal electrical properties were obtained for samples with x = 0.1 sintered at 1050 °C and reoxidized at 800 °C. The dielectric loss decreased to a value of about 0.14, while the permittivity remained a relatively large value of about 9.9 × 103 at 103 Hz. Additionally, the nonlinear coefficient and breakdown field were improved to a value of 2.2 and 981.8 V/cm, respectively. This study illustrates an efficient approach to enhance the reestablished dielectric and nonlinear properties for Bi3+ doped CaCu3Ti4O12-based ceramics fabricated through the reduction-reoxidation method.

Thermally Induced Morphological and Structural Transformations on Eu2+/Eu3+-Coactivated Calcium Silicate Nanophosphors.

This study presents an approach for synthesizing Eu2+/Eu3+-coactivated Ca2SiO4 nanophosphors, by adjusting the ratio of both activators within a singular host material. Utilizing a hydrothermal method complemented by a postreduction sintering process, we fabricated a series of phosphors characterized by uniform 30-50 nm spherical nanoparticles. These engineered phosphors manifest multichannel luminescence properties and exhibit simultaneous blue and red emission from Eu2+ and Eu3+, respectively. Meticulous control of the 5% H2-95% N2 reduction temperature allowed for precise tuning of the Eu2+ and Eu3+ ions within the host lattice, which enabled the strategic adjustment of their luminescent outputs. Utilizing X-ray photoelectron spectroscopy (XPS), we could discern subtle alterations in the europium oxidation state. By using a transmission electron microscope (TEM) and an X-ray diffractometer (XRD), we found that the subsequent changes by reductive sintering to particle size, morphology, and mixed crystal structures influenced the materials' luminescent characteristics. Our findings herald a significant advancement in solid-state lighting, with the potential for the use of Eu2+/Eu3+-coactivated calcium silicate nanophosphors to develop white light emission technologies endowed with enhanced color rendering and luminous efficacy.

Optimized thermoelectric performance driven by A‐site deficient in SrxLa0.1TiO3−δ/TiO2−y composites

AbstractFor thermoelectric applications at high temperatures, perovskite SrTiO3 is a promising contender as an n‐type oxide thermoelectric (TE) material. In this work, SrxLa0.1TiO3−δ/TiO2−y composite ceramics were successfully synthesized by solid state reaction, using a combination of A‐site cation vacancy and composite fabrication. Through X‐ray diffractometer and scanning electron microscopy analysis, a complex microstructure composed of two kinds of titanium oxides was formed after reductive sintering and annealing. In addition, controlling the content of Sr vacancies in the SrxLa0.1TiO3−δ matrix increased the thermoelectric power factor (PF) by optimizing the weighted mobility in the composite, resulting in the highest PF of 425.8 μW m−1 K−2 and the maximum ZT of 0.16 for the Sr0.875La0.1TiO3−δ/TiO2−y composite through the synergistic optimization of electrical and thermal transport properties. The strategy proposed in this study of design A‐site‐deficient composites with nano‐sized metal paved a new way to fabricate high performance oxide thermoelectric materials.

A high performance TiO2 anode modified by germanium and oxygen vacancies for lithium-ion batteries

A TiO2 anode modified by germanium and oxygen vacancies was fabricated using a hydrothermal method with an autoclave at 180 °C, followed by reduction sintering at 600 °C for 5 h in a reducing atmosphere. Germanium was obtained and distributed uniformly on the surface of the anatase TiO2 particles. Oxygen vacancies were identified on the TiO2 surface by electron paramagnetic resonance and X-ray photoelectron spectroscopy. The specific capacity of the Ge@TiO2−x anode was 510 mAh g−1 after 550 charge-discharge cycles. The charge transfer and electrolyte impedance of Ge@TiO2−x were smaller than those of the initial TiO2. The lithium-ion diffusion coefficient in Ge@TiO2−x was 7.87 × 10−13 cm2·s−1, which was significantly higher than that of a commercial TiO2 anode. The germanium and oxygen vacancies in TiO2 provided more active sites for the transport of lithium ions and electrons, thereby reducing the energy barrier, accelerating the charge transfer of lithium ions and enhancing the conductivity and capacity of the Ge@TiO2−x anode.

CaCu3Ti4O12 ceramics with giant permittivity prepared by reduction-reoxidation method

CaCu3Ti4O12 (CCTO) materials are greatly desired in the miniaturized multilayer devices due to the giant permittivity. However, Ni electrode presently cannot be employed as internal electrode for CCTO-based multilayer devices, which is much cheaper than Ag/Pd electrode. This is because Ni electrode should be sintered in protecting atmosphere, which easily led to decomposition of CCTO materials. The decomposition reaction was mainly attributed to the serious Cu-loss during reduction sintering. The Cu-loss can be effectively weakened, when the ceramics were sintered in a close alumina crucible at a relatively low temperature induced by liquid phase. The decomposition reaction was found to be suppressed and dense ceramics were successfully prepared by reduction sintering. Giant permittivity was observed in the reduced CCTO-BN (4.2 × 104), but dielectric loss (8.88) and nonlinear characteristic deteriorated. This is because the grain boundary could not be oxidized during reduction sintering. The reduced CCTO-BN was then reoxidized in air to promote the oxidation of grain boundary. The dielectric loss was decreased more than 102 times to 0.07 and the nonlinear characteristic was reestablished for the reoxidized CCTO-BN. Meanwhile, the relative permittivity still remained a giant value of 1.2 × 104. The findings shed a new light on developing CCTO-based devices with Ni electrode, which helps to significantly reduce the production cost.

Chlorination Treatment for Gold Extraction from Refractory Gold-Copper-Arsenic-Bearing Concentrates

New experimental results have been obtained on the behavior of arsenic and other associated metals (Re and others) under conditions of oxidative and reductive sintering. It has been established that the extraction of arsenic strongly depends on the process temperature during oxidative sintering. The extraction of arsenic into dust media at 873 K is 50% and rhenium is 88–90%. The effect of excess air on the extraction of arsenic and rhenium into dust was studied: the higher the excess air coefficient, the more complete the extraction of arsenic and rhenium into the dust. The obtained data indicate that achieving a high level of arsenic extraction from the initial product is not possible during oxidative sintering. The best arsenic removal results were reached under the conditions of reductive sintering of initial material by natural gas. The extraction of arsenic into dust at 823 K was 88%, and at 1373 K arsenic is almost completely converted into dust. Obtained new experimental results have a fundamental importance for the selection and organization of a comprehensive technology for the processing complex in composition refractory gold-copper-arsenic-bearing products.

Defect-engineered TiO2 nanocrystals for enhanced lithium-ion battery storage performance

TiO2 nanocrystals containing oxygen defects were successfully prepared by a hydrothermal method with reductive sintering in a 5% H2 + 95% Ar mixed atmosphere. The high-resolution transmission electron microscopy and X-ray diffraction results showed that the prepared samples were all anatase TiO2. X-ray photoelectron spectroscopy analysis showed that the reductive sintering converted Ti4+ to Ti3+. The Ti3+/Ti4+ molar ratio of the TiO2 nanocrystals increased with increasing sintering time. The results of the electron paramagnetic resonance spectroscopy analysis showed that the reduced and sintered TiO2 nanocrystals produced strong signals at g = 2.0010 and 1.9478, corresponding to the presence of oxygen defects and Ti3+, respectively. Furthermore, stronger signals were generated with increasing sintering time, indicating a progressively higher concentration of oxygen defects. The TiO2 nanocrystalswere used as anode materials for lithium-ion batteries. One sample, H-TiO2-5, containing a moderate concentration of oxygen defects, presented a higher lithium ion diffusion coefficient (D = 5.1 × 10-13 cm2s−1) compared to the other TiO2 samples. This sample had a Ti3+/Ti4+ molar ratio of 0.179 and excellent cycling stability after 100 cycles. The discharge capacity was 124 mAh.g−1 after 500 cycles at a current density of 1C, which means that it has good cyclic stability.

Controlling the A-site deficiency and oxygen vacancies by donor-doping in pre-reductive-sintered thermoelectric SrTiO3 ceramics

Oxygen vacancies are intrinsic defects in ABO3 perovskite structures, and the reductive sintering of SrTiO3 ceramics combined with donor-doping can create mobile carriers and/or vacancies at A-site to improve their thermoelectric performance. By scanning and transmission electron microscopy analyses, A-site deficiency was detected in undoped and donor-doped SrTiO3 ceramics, reaching 19% of A-site vacancies by co-doping La, Ce and Nb respectively into A- and B-sites. Titania with mixed Ti4+/Ti3+ cations were commonly detected as intergranular phases of Ti4+1−mTi3+mO2−m/2, which amounts to 17 vol.% in the undoped SrTiO3, thus revealing their inheritance from the pre-reductive nano-powders via liquid-phase sintering. Oxygen and A-site vacancies are both detectable by cathodoluminescence analysis from either the nano-powders or sintered phases, which reveal that mixed-donor-doping creates more vacancies at A-site, together with more oxygen vacancies to form Schottky pairs. The combination of pre-reductive sintering and mixed-donor-doping creates abundant vacancies as electronic defects in SrTiO3, while carrier transport were not hindered through the grain boundaries which enables zT value of an oxide to exceed 0.38 at 1000 K.

Ru doped magnetic nanoparticles embedded in mesoporous silica for effective microwave absorption by interface engineering and DFT calculations

Strong absorption, thin thickness, wide-bandwidth as well as lightweight of microwave absorption materials (MAMs) are highly desirable. In this study, the mesoporous silica skeleton embedded by FeRu/(Fe, Ru)3O4 magnetic nanoparticles (FR/FRO-mSiO2) were designed and fabricated by vacuum impregnation and reduction sintering methods. The minimum reflection loss (RLmin) of the FR/FRO-mSiO2 composite with a metal mass ratio of 46% was −61.1 dB at a thickness of 2.0 mm, and the covered absorption bandwidth (RL < −10 dB) reached 6.8 GHz. In combination of Density functional theory (DFT) calculation, microstructure characterizations and electromagnetic characteristics, we found that the interface polarization and magnetic enhancement at the interfaces of FeRu-(Fe, Ru)3O4, the local dipole moment, and the abundant propagation channels for the incident microwaves, endowed by the unique component and construction of FR/FRO-mSiO2, contributed together to the excellent microwave absorption capability of FR/FRO-mSiO2. This work provides a promising porous structure to broaden efficient bandwidth and strengthen microwave absorption intensity with lightweight.

Enhanced electrical property of graphite/Al2O3 composite fabricated by reductive sintering of gel-casted body using cross-linked epoxy polymer

In the present study, graphite/alumina composites are fabricated via reductive sintering of gel-casted green bodies with structurally controlled cross-linked epoxy polymers for the first time. The cross-linking degrees of polymers are tuned by the amount ratio of epoxy monomer/polyvinyl alcohol cross-linker utilized in gel-casting process. Superior electrical properties with respect to 5-fold enhanced electrical conductivity and 2-fold higher carrier mobility are successfully achieved in graphite/alumina composite fabricated from cross-linked epoxy polymer, whose phenomenon is attributed to the excellent conductive path in ceramic matrix established by highly uniform network with improved graphitization degree.

3D Printing of Copper Using Water-Based Colloids and Reductive Sintering.

Copper was manufactured by using a low-cost 3D printing device and copper oxide water-based colloids. The proposed method avoids the use of toxic volatile solvents (used in metal-based robocasting), adopting copper oxide as a precursor of copper metal due to its lower cost and higher chemical stability. The appropriate rheological properties of the colloids have been obtained through the addition of poly-ethylene oxide-co-polypropylene-co-polyethylene oxide copolymer (Pluronic P123) and poly-acrylic acid to the suspension of the oxide in water. Mixing of the components of the colloidal suspension was performed with the same syringes used for the extrusion, avoiding any material waste. The low-temperature transition of water solutions of P123 is used to facilitate the homogenization of the colloid. The copper oxide is then converted to copper metal through a reductive sintering process, performed at 1000°C for a few hours in an atmosphere of Ar-10%H2. This approach allows the obtainment of porous copper objects (up to 20%) while retaining good mechanical properties. It could be beneficial for many applications, for example current collectors in lithium batteries.

Flexible Ni/NiOx-Based Sensor for Human Breath Detection.

We developed a simple methodology to fabricate an Ni/NiOx-based flexible breath sensor by a single-step laser digital patterning process of solution-processed NiOx thin-film deposited using NiOx nanoparticle ink. Laser-induced reductive sintering phenomenon enables for the generation of three parts of Ni electrodes and two narrow NiOx-sensing channels in between, defined on a single layer on a thin flexible polymer substrate. The Ni/NiOx-based breath sensor efficiently detects human breath at a relatively low operating temperature (50 °C) with fast response/recovery times (1.4 s/1.7 s) and excellent repeatability. The mechanism of the gas-sensing ability enhancement of the sensor was investigated by X-ray photoelectron spectroscopy analysis. Furthermore, by decoupling of the temperature effect from the breathing gas, the response of the sensor due to the temperature alone and due to the chemical components in the breathing gas could be separately evaluated. Finally, bending and cyclic bending tests (10,000 cycles) demonstrated the superior mechanical stability of the flexible breath sensor.

Effect of Substrates on Femtosecond Laser Pulse-Induced Reductive Sintering of Cobalt Oxide Nanoparticles.

Direct writing of cobalt/cobalt oxide composites has attracted attention for its potential use in catalysts and detectors in microsensors. In this study, cobalt-based composite patterns were selectively formed on glass, polyethylene naphthalate (PEN), and polyethylene terephthalate (PET) substrates via the femtosecond laser reductive sintering of Co3O4 nanoparticles in an ambient atmosphere. A Co3O4 nanoparticle ink, including the nanoparticles, ethylene glycol as a reductant, and polyvinylpyrrolidone as a dispersant, was spin-coated onto the substrates. Near-infrared femtosecond laser pulses were then focused and scanned across the ink films to form the patterns. The non-sintered nanoparticles were subsequently removed from the substrate. The resulting sintered patterns were found to be made up of Co/CoO composites on the glass substrates, utilizing various pulse energies and scanning speeds, and the Co/CoO/Co3O4 composites were fabricated on both the PEN and PET substrates. These results suggest that the polymer substrates with low thermal resistance react with the ink during the reductive sintering process and oxidize the patterns more easily compared with the patterns on the glass substrates. Such a direct writing technique of cobalt/cobalt oxide composites is useful for the spatially selective printing of catalysts and detectors in functional microsensors.

Laser digital patterning of finely-structured flexible copper electrodes using copper oxide nanoparticle ink produced by a scalable synthesis method

We present a facile and simple method for synthesizing a large-scale, well-dispersed, and high-concentration CuOx nanoparticle (NP) ink. CuOx thin films with ultrafine surfaces were fabricated using the synthesized NP ink by spin coating, which cannot be achieved using commercial NPs. The CuOx NP thin films were subjected to a subsequent laser digital patterning process, yielding finely-structured Cu electrodes on various polymer substrates with the minimum resistivity of 10.5 μΩ cm due to the laser-induced reductive sintering (LRS) phenomenon. Arbitrary Cu electrode patterns were directly generated on various flexible substrates under ambient conditions without any templating process. Cu-grid transparent conducting panels with a low sheet resistance (8.45 Ω sq−1) and high transmittance (87.4% at 550 nm) were prepared. Furthermore, the effect of the amount of polyvinylpyrrolidone, which was used as a dispersing and reducing agent in the NP ink, on the LRS phenomenon was analyzed in detail. Mechanical bending and twisting, cyclic bending, and tape pull tests confirmed the superior electromechanical stability of the Cu electrodes. The long-term oxidation resistance of the Cu electrodes under ambient conditions and the limiting temperature of oxidation resistance were also examined. Finally, a Cu-based flexible transparent touchscreen panel was demonstrated as a possible application.

Tunable Bulk Polymer Planar Bragg Gratings Electrified via Femtosecond Laser Reductive Sintering of CuO Nanoparticles (Advanced Optical Materials 13/2021)

This cover image outlines the fabrication method of a polymer planar Bragg grating electrified via femtosecond laser reductive sintering of CuO nanoparticles (see article number 2002203 by Stefan Kefer and co-workers). Based on this sophisticated methodology, bulk cyclic olefin copolymer substrates can be equipped with integrated photonic structures comprising a waveguide as well as a Bragg grating. Its reflective characteristics can be efficiently tuned by means of the subsequently generated Cu conducting path, whereas the applied femtosecond laser process enables an almost limitless degree of freedom towards conducting path geometries.

Fabrication of Conductive and Gas-Sensing Microstructures Using Focused Deposition of Copper Nanoparticles Synthesized by Spark Discharge

Solvent-free aerosol jet printing has been investigated for fabricating metallic and semiconductor (gas-sensitive) microstructures based on copper nanoparticles on alumina, borosilicate glass, and silicon substrates. The synthesis of nanoparticles was carried out using a spark discharge directly in the printing process without the stage of preparing nano-ink. Printed lines with a width of 100–150 µm and a height of 5–7 µm were formed from submicron agglomerates consisting of primary nanoparticles 10.8 ± 4.9 nm in size with an amorphous oxide shell. The electrical resistivity, surface morphology, and shrinkage of printed lines were investigated depending on the reduction sintering temperature. Sintering of copper oxides of nanoparticles began at a temperature of 450 °C in a hydrogen atmosphere with shrinkage at the level of 45–60%. Moreover, aerosol heat treatment was used to obtain highly conductive lines by increasing the packing density of deposited nanoparticles, providing in-situ transformation of submicron agglomerates into spherical nanoparticles with a size of 20–50 nm. Copper lines of spherical nanoparticles demonstrated excellent resistivity at 5 μΩ·cm, about three times higher than that of bulk copper. In turn, semiconductor microstructures based on unsintered agglomerates of oxidized copper have a fairly high sensitivity to NH3 and CO. Values of response of the sensor based on non-sintered oxidized copper nanoparticles to ammonia and carbon monoxide concentration of 40 ppm were about 20% and 80%, respectively.

Cu Patterning Using Femtosecond Laser Reductive Sintering of CuO Nanoparticles under Inert Gas Injection.

In this paper, we report the effect of inert gas injection on Cu patterning generated by femtosecond laser reductive sintering of CuO nanoparticles (NPs). Femtosecond laser reductive sintering for metal patterning has been restricted to metal and metal-oxide composite materials. By irradiating CuO-nanoparticle paste with femtosecond laser pulses under inert gas injection, we intended to reduce the generation of metal oxides in the formed patterns. In an experimental evaluation, the X-ray diffraction peaks corresponding to copper oxides, such as CuO and Cu2O, were much smaller under N2 and Ar gas injections than under air injection. Increasing the injection rates of both gases increased the reduction degree of the X-ray diffraction peaks of the CuO NPs, but excessively high injection rates (≥100 mL/min) significantly decreased the surface density of the patterns. These results qualitatively agreed with the ratio of sintered/melted area. The femtosecond laser reductive sintering under inert gas injection achieved a vacuum-free direct writing of metal patterns.

Copper and Nickel Microsensors Produced by Selective Laser Reductive Sintering for Non-Enzymatic Glucose Detection.

In this work, the method of selective laser reductive sintering was used to fabricate the sensor-active copper and nickel microstructures on the surface of glass-ceramics suitable for non-enzymatic detection of glucose. The calculated sensitivities for these microsensors are 1110 and 2080 μA mM−1·cm−2 for copper and nickel, respectively. Linear regime of enzymeless glucose sensing is provided between 0.003 and 3 mM for copper and between 0.01 and 3 mM for nickel. Limits of glucose detection for these manufactured micropatterns are equal to 0.91 and 2.1 µM for copper and nickel, respectively. In addition, the fabricated materials demonstrate rather good selectivity, long-term stability and reproducibility.

Influence of ceramic matrix on semi-conductive properties of nano-carbon/ceramic composites fabricated via reductive sintering of gel-casted bodies

In present work, nano-carbon/ceramic composites are fabricated via facile reductive sintering of gel-casted bodies by utilizing different ceramic matrices of alumina and zirconia, respectively. With detailed characterizations of microstructure, bulk density and crystallinity of composites obtained at different sintering temperatures, as well as corresponding residual amount, chemical structure and bonding state of nano-carbon network, the influence of ceramic matrix on semi-conductive properties with respects to electrical conductivity, carrier density and mobility are systemically investigated. It is clarified that ceramic matrix plays an important role in tailoring the state of nano-carbon generated through ceramic particle surface and interface, which determines the resultant semi-conductive properties of composites.