Control of crystalline phase and morphology of calcium carbonate by electrolysis: Effects of current and temperature

Calcium carbonate (CaCO₃) can show various properties related to its different crystalline phases and is therefore a useful material for various applications. Wet processes are known to be suitable for preparing metastable CaCO₃ polymorphs. Electrolysis has been proposed as a preparation method at ambient conditions. Although several electrolytic approaches have been reported, the effects of the applied current and temperature of the electrolyte on the crystalline phase and morphology of CaCO₃ remain unclear. In the present study, we attempted the electrochemical preparation of CaCO₃ particles under various electrolysis conditions and discuss the mechanism of CaCO₃ particle formation. The crystalline phases and morphologies of the CaCO₃ precipitates markedly changed depending on the applied current and method of cooling the electrolyte. We assume that these factors were governed by the degree of change in temperature, supersaturation, and pH of the electrolyte that were induced by differences in the electrolysis current.     

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

In this study, the preparation and consolidation of nanocrystalline LaB₆ powders originating from powder blends of La₂O₃, B₂O₃ and Mg were reported. A consecutive route of mechanochemical synthesis (MCS) and purification was utilized for the achievement of nano-sized LaB₆ powders. As-synthesized powders were leached out from intermediate reaction products or impurities. Then, a sequential step of cold pressing (uniaxial pressure at 800 MPa) and pressureless sintering (at 1700 °C for 5 h under Ar gas flow) were utilized for the consolidation of the purified LaB₆ powders. The type of mill (vibratory and planetary high-energy ball mills) was employed as a MCS parameter to reveal its effect on the physical, microstructural and mechanical properties of the LaB₆ powders, and their bulk structures. Compositional, physical and microstructural properties of the products after powder processing were determined via X-ray diffractometer (XRD), particle size analyzer (PSA), differential scanning calorimeter (DSC), stereomicroscope (SM), scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive spectrometer (EDS) coupled with both SEM and TEM, and vibrating sample magnetometer (VSM). The bulk properties of the LaB₆ consolidated from nanocrystalline powders with a minimum 99.99% purity, and ∼62 nm (for vibratory ball mill) or ∼74 nm (for planetary ball mill) average particle size were compared according to various properties. LaB₆ powders were synthesized in planetary mill at an approximately six times longer duration than that of in vibratory mill. According to the results, density, surface area and mean particle size values of the vibratory ball-milled samples (containing paramagnetic powders) are better than those of planetary ball-milled (containing diamagnetic powders) ones. However, mechanical properties such as hardness, surface roughness, wear rate, friction coefficient, and also electrical conductivity were improved in the planetary ball-milled LaB₆ bulks.     

Synthesis of fine dispersed titanium diboride from nanofibrous carbon

This paper presents the experimental data on the synthesis of titanium diboride (TiB₂) fine dispersed powder carried out in laboratory scale. TiB₂ powder was prepared by the reduction of titanium dioxide with boron carbide and nanofibrous carbon in an argon atmosphere. The powders of TiB₂ were characterized by X-ray diffraction (XRD), elemental analyses, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), low-temperature nitrogen adsorption, particle size analysis, simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC). The resulting material contains a single phase – titanium diboride. The particles of the powder were predominantly aggregated. The average size of the particles and the aggregates were 7.4–8.0 µm with a wide size of distribution. The specific surface values of samples obtained were 2.4–5.8 m²/g. The oxidation of titanium diboride began from the temperature of 450 °C. In this work, the optimal synthesis conditions were estimated: the molar ratio was TiO₂:B₄C:C=2:1:3 (according to stoichiometry), the temperature was 1600 °C, the process duration was 20–30 min.     

Hydrothermal synthesis of tellurium nanorods by using recovered tellurium from waste electronic devices

Tellurium (Te) nanostructures with controlled morphology have received the considerable attention in various applications owing to tunable optic, thermoelectric, photoelectronic, piezoelectric, and electrochemical properties. Herein, we introduce the cost-effective and eco-friendly synthesis of Te nanorods (Te NRs) from end of life electronic devices via hydrothermal methods. The Te NRs show the average diameter of 44.6?nm and a length of 358?nm in presence of polyvinylpyrrolidone, as a stabilizing agent. Moreover, the bismuth and intact p-type semiconductor (i.e., Bi_0.5Sb_1.5Te₃) are selectively recovered as intermediated products. The Te NRs exhibit the NO₂ gas sensing properties with concentration as low as 1?ppm at room temperature and fast response/recovery times of 1.59 and 2.10?s at 1?ppm, respectively. We believe that this powerful approach can be expanded to not only selective recovery of valuable materials but synthesis of various nanomaterials from waste electronic devices.     

Hydrothermal synthesis and formation mechanism of tetragonal barium titanate in a highly concentrated alkaline solution

Tetragonal cube-shaped barium titanate (BaTiO₃) was produced by the hydrothermal treatment of a peroxo-hydroxide precursor, a single-source amorphous barium titanate precursor, in a highly concentrated sodium hydroxide solution. Phase pure barium titanate with cube-shaped morphology and particle-sizes in the 0.2–0.5 µm range were formed at temperatures above 80 °C. Also, the cube-shaped morphology of the BaTiO₃ product was preceded by spherical- and plate-like morphologies with, respectively, a Ti-excess and Ba-excess. Coinciding with these morphological observations, changes in the reaction product were also observed. The formation of crystalline BaTiO₃ proceeded alongside secondary BaTi₂O₅ and Ba₂TiO₄ phases. These secondary phases disappeared as the reaction time was increased leaving only BaTiO₃ as the sole reaction product. Kinetic analysis of the formation of hydrothermal BaTiO₃ crystallization by the Johnson-Mehl-Avrami method showed that BaTiO₃ crystallization is a homogeneous dissolution-precipitation reaction. The mechanism is governed by nucleation and growth in the beginning of the reaction and dissolution-precipitation dominating throughout the hydrothermal reaction process.     

Preparation of hierarchical leaf-like cobalt and enhanced magnetic properties by a new low-temperature synthesis method

Hierarchical leaf-like cobalt materials have been synthesized by a simple method at relatively low temperature. The product was characterized by means of XRD, SEM, EDS, and VSM techniques. The effects of temperature and cobalt acetate amount on the final Co were investigated by a series of experiments. It was found that the temperature played an important role in the formation of such novel leaf-like cobalt. When the reaction temperature of the mixture was as low as 40–65 °C, the morphology of final products can be changed from fluffy like to leaf like hierarchical structures. The leaf-like cobalt possessed hexagonal close-packed (HCP) phase structure. The hierarchical leaf-like cobalt exhibited high saturation magnetization (M_s) of 151.6 emu/g and coercivity (H_c) of 158.5 Oe. The low temperature chemical reduction method is quite simple, it will provide possibility for large scale preparation of such leaf-like cobalt. Due to the specific structure and magnetic properties, these cobalt leafs are expected to have potential applications as candidates for microwave absorption and sensors.     

Solution combustion synthesis of bioceramic calcium phosphates by single and mixed fuels—A comparative study

Calcium phosphate based bioceramics have been synthesized by a modified combustion synthetic route using both citric acid and succinic acid separately and in mixture as fuels and nitrate and nitric acid as oxidants. Calcium nitrate and diammonium hydrogen phosphate were used as calcium and phosphate sources. The effects of citric acid to succinic acid ratio on the phase formation have been investigated. The precursors and the calcined products have been characterized by powder X-ray diffraction, Fourier-transform infrared spectroscopy and scanning electron microscopy. Succinic acid has been used as a fuel for the first time to synthesize hydroxyapatite.     

Detonation synthesis of nanoscale silicon carbide from elemental silicon

Direct reaction of precursors with the products of detonation remains an underexplored area in the ever-growing body of detonation synthesis literature. This study demonstrated the synthesis of silicon carbide during detonation by reaction of elemental silicon with carbon products formed from detonation of RDX/TNT mixtures. Continuum scale simulation of the detonation showed that energy transfer by the detonation wave was completed within 2–9 μs depending on location of measurement within the detonating explosive charge. The simulated environment in the detonation product flow beyond the Chapman-Jouguet condition where pressure approaches 27 GPa and temperatures reach 3300 K was thermodynamically suitable for cubic silicon carbide formation. Carbon and added elemental silicon in the detonation products remained chemically reactive up to 500 ns after the detonation wave passage, which indicated that the carbon-containing products of detonation could participate in silicon carbide synthesis provided sufficient carbon-silicon interaction. Controlled detonation of an RDX/TNT charge loaded with 3.2 wt% elemental silicon conducted in argon environment lead to formation of ～3.1 wt% β-SiC in the condensed detonation products. Other condensed detonation products included primarily amorphous silica and carbon in addition to residual silicon. These results show that the energized detonation products of conventional high explosives can be used as precursors in detonation synthesis of ceramic nanomaterials.     

Ion substitution induced formation of spherical ceramic particles

How to precipitate ceramic nano- and microspheres in water based solutions only using inorganic ions is a challenge. In this study, spherical particles of alkaline earth phosphates and fluorides were synthesized using a precipitation reaction. Substituting ions, through inhibition of crystal growth, was used to induce sphere formation and to alter the morphology, size and composition of the spheres. The difference in ionic radius between the substituting ion (Mg, Ca and Sr) and the main cation (Sr and Ba) influenced the critical concentration to allow for sphere formation as well as the crystallinity. The larger difference, the lower was the concentration needed to form spheres. Low concentrations of Mg was enough to generate amorphous spheres of Sr- and Ba-phosphates whereas higher concentrations were needed if the radius difference were smaller. An increasing degree of substitution leads to a decrease in crystallinity of precipitated particles. The degree of substitution was determined to 16–55% where a low degree of ion substitution in the phosphates resulted in the formation of spheres (500–800?nm) with rough surfaces composed of apatite like phases. A higher degree of substitution resulted in amorphous spheres (500?nm- 1?μm) with smooth surfaces.     

Effect of the solid precursors on the formation of nanosized TiBx powders in RF thermal plasma

Continuous synthesis of TiB_x (x≈0.5–2) nanoparticles from various low cost solid precursors such as titanium and titanium dioxide admixed with boron and/or carbon in radiofrequency thermal plasma was studied. Feasibility of TiB₂ formation was predicted by thermodynamic equilibrium calculations in the 500–5000 K temperature range. In all the investigated system high temperature reactions resulted in nanometer-sized TiB_x powders with a mean size varying between 13 and 83 nm. The yield of particular runs ranged from 38% to 97%. Among the synthesized products in addition to TiB_x, oxidized precursor residues were also found in smaller quantities. Although addition of carbon to the precursors could not completely prevent surface oxidation of boride particles, it contributed to the reduction of the mean particle size of the formed TiB₂.     

Synthesis of pure phase BiFeO3 powders in molten alkali metal nitrates

Pure phase BiFeO₃ powders were successfully synthesized in molten alkali metal nitrates (KNO₃–NaNO₃) at 500 °C. The as-prepared BiFeO₃ had a rhombohedral structure which was studied using X-ray diffraction. The plate-like morphologies were investigated through scanning electron microscopy and transmission electron microscopy. The average length and width of BiFeO₃ plates were 400 and 200 nm, respectively. Furthermore, the mechanism of formation of BiFeO₃ was also discussed through X-ray diffraction, thermogravimetry, differential thermal analysis and mass spectrometry.     

Comparison of the microwave-induced combustion and solid-state reaction for the synthesis of LiMn2O4 powder and their electrochemical properties

In the current research, we proposed a new method called microwave-induced combustion synthesis to produce LiMn₂O₄ powders. The microwave-induced combustion synthesis entails the dissolution of metal nitrates, and urea in water, and then heating the resulting solution in a microwave oven. Spinel LiMn₂O₄ powders were successfully synthesized by microwave-induced combustion. The microwave-heated LiMn₂O₄ powders annealed at various temperatures in the range of 600–800 °C were determined. The resultant powders were characterized by X-ray diffractometer (XRD), and scanning electron microscopy (SEM). The annealed samples were used as cathode materials for lithium-ion battery, for which their discharge capacity and electrochemical characteristic properties in terms of cycle performance were also investigated. The LiMn₂O₄ cell provides a high initial capacity of 133 mAh/g and excellent reversibility. The excellent capacity and reversibility were attributed to LiMn₂O₄ powders with small and uniform particle size produced by microwave-induced combustion synthesis.     

Phase formation of REBa2Cu3O7−δ (RE: Y0.5Gd0.5, Y0.5Nd0.5, Nd0.5Gd0.5) superconductors from nanopowders synthesised via co-precipitation

Phase formation of REBa₂Cu₃O_7−δ (RE: Y_0.5Gd_0.5, Y_0.5Nd_0.5, Nd_0.5Gd_0.5) superconductors synthesised via co-precipitation (COP) method were investigated by thermogravimetric analysis (TGA), differential thermal analysis (DTA) and X-ray diffraction (XRD) analysis. All samples showed identical thermal decomposition behaviour from the thermogram in which 5 major weight losses were observed. However, XRD of the samples at different heat treatment temperatures showed different diffraction patterns indicating different thermolytic processes. Meanwhile, transmission electron microscopy and surface area analysis revealed that the powders obtained from COP have particle sizes ranging from 7 to 12 nm with relatively large surface area. Molar ratios of prepared samples obtained were near to the theoretical values as confirmed by elemental analyses using X-ray fluorescence (XRF). The T_C(R=0) for sintered YGd, YNd and NdGd were 87 K, 86 K and 90 K, respectively. Surface morphological study via scanning electron microscope showed the structures of samples were dense and non porous.     

Highly efficient green synthesis of highly pure microporous nanosilica from silicomanganese slag

Highly pure nanosilica was synthesized through a facile hydrometallurgy-based method from silicomanganese slag as a low cost silica source. The synthesis route included short-term nitric acid dissolution at room temperature, gelation, washing, drying, and calcination steps. The experimental dissolution conditions resulted in a dissolution efficiency of 98%. The crystalline structure, chemical composition, chemical bonding, microstructure and elemental analyses, particle size distribution, and surface area of the extracted silica were then studied by appropriate characterization techniques. The characterization findings indicated that the amorphous silica particles had a microporous nature with an average particle size of ~24 nm, exhibiting a high purity of more than 99% and a high specific surface area of ~474 m² g^?1. The overall results indicated that the proposed synthesis route is a promising feasible alternative method to produce highly pure microporous nanosilica from a low cost secondary resource. The proposed method can treat 98% of the slag and uses less chemicals than conventional methods and is therefore a greener nanosilica production process. The current process also competes with the traditional process and other recently introduced processes in terms of process economy and the quality of the produced product.     

Effect of process conditions on spray dried calcium carbonate powders for thermal spraying

Powder preparation is an important stage in the production of thermal spray coatings with the desired characteristics. An important powder feature is flowability, which can be adjusted through particle morphology, particle size and size distribution. Combined, these features dictate the quality of the coating produced. To increase a powder's flowability, spherical particles within a particular size range are ideal. One way to achieve this is through spray drying. The aim of the present study was to investigate the effect of spray drying process parameters on the physical properties of calcium carbonate powder, with the goal of producing large, spherical particles ranging between 50 and 100 μm in preparation for thermal spray experiments. A key aspect was the use of ethanol to aide in the production of large spheres. A 2³ factorial design of experiments (DoE) was utilised to study the following process parameters: gas flow rate, feed flow rate and solids loading. The resulting powders were characterised in terms of particle size, morphology and production yield. Porous, hollow, spherical particles were produced in a suitable size range for thermal spraying, which was attributed to the rapid evaporation of ethanol. Statistical analysis was utilised to interpret trends between the spray drying parameters and powder characteristics quantitatively.     

Preparation and hydrogenation of urchin-like titania using a one-step hydrothermal method

TiO₂ has been widely used in the fields of environmental protection, energy conversion, plastics, coatings, cosmetics, and more. However, it has a wide band gap (3.2 eV) and the easy agglomeration of TiO₂ nanoparticles hinders its application. This study adopts a one-step hydrothermal method to prepare urchin-like TiO₂ photocatalyst and studies the morphology of TiO₂ in relation to hydrothermal time, hydrothermal temperature and hydrogen peroxide (H₂O₂) amount. In addition, the urchin-like TiO₂ is annealed in hydrogen atmosphere to discuss the influence of oxygen vacancies on the band gap and photocatalytic performance improvement.     

CuInS2 nanowires prepared using a hydrothermal process through a polymer-type ion release source

Chalcopyrite copper indium sulfide (CuInS₂, CIS) has a bandgap that is optimal for a solar energy conversion material. In this paper, we used a polymer-type ion release source to control the precursor concentration of the hydrothermal system to grow CIS nanowires. The analytical results indicate that the reaction process is based on the formation of CuS binary compound, which is then followed by indium intercalation, which induces the formation of the CIS chalcopyrite crystal structure. The products were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The CIS nanowires were 100–300 nm in diameter and 2–5 µm in length.     

Influence of carbonation on the low-temperature consolidation by Spark Plasma Sintering of carbonated calcium phosphate bioceramics

Calcium phosphates (CaP) such as biomimetic nanocrystalline apatite or amorphous calcium phosphate are hydrated bioactive compounds particularly suitable for bone repair applications due to their similarity with bone mineral. However, their consolidation in ceramic parts deserves special attention as they are thermodynamically metastable and can decompose into less bioactive phases upon heating. Adapted strategies are needed to obtain bulk bioceramics. Spark Plasma Sintering (SPS) has been shown to allow cold sintering of such compounds at temperatures like 150 °C while preserving the hydrated character and nanosized dimensions of the precursor powders. To this date, however, the role of the degree of carbonation of these precursors on the densification of CO₃-bearing CaP compounds via SPS has not been explored despite the natural carbonation of bone. In this work, several carbonated CaP hydrated compounds were prepared and consolidated by SPS and the characteristics of the obtained ceramics was scrutinized with respect to the starting powders. Two carbonation routes were carried out: via volume carbonation during powder synthesis or via subsequent surface ion exchange. All samples tested led to apatitic compounds after SPS, including amorphous CaP. We show that the degree of carbonation negatively affects the densification rate and propose possible hypotheses explaining this behavior. Evolution in the nature of the carbonate sites (apatitic A-, B-types and labile surface carbonates) before and after SPS is also noticed and commented. The consolidation of such compounds is however proven possible, and gives rise to bone-like apatitic compounds with great potential as bioactive resorbable ceramics for bone regeneration.     

Hydrothermal synthesis of NaNbO3 with low NaOH concentration

NaNbO₃ fine powders were prepared by reacting niobium pentoxide with low NaOH concentration solution under hydrothermal conditions at 160 °C. The reaction ruptured the corner-sharing of NbO₆ octahedra in the reactant Nb₂O₅, yielding various niobates, and the structure and composition of the niobates depended on the [OH⁻] and reaction time. The fine Nb₂O₅ powder first aggregated to large particles and then turned to metastable intermediates with multifarious morphology. The reaction was fast for the situation of [OH⁻] = 2 M. The [OH⁻] determined the structure of final products, and three types of NaNbO₃ powder with the orthorhombic, tetragonal and cubic symmetries were obtained, respectively, depending on the [OH⁻]. The low [OH⁻] was propitious to yield orthorhombic NaNbO₃. The present work demonstrated that higher [OH⁻] was not favored to synthesize NaNbO₃ powders and the conversion speed in this reaction was not in proportion to the [OH⁻].     

Synthesis of LaAlO3 powder using triethanolamine

LaAlO₃ powders were successfully synthesized by pyrolysis of complex compounds of lanthanum and aluminum with triethanolamine (TEA). The precursors and the derived powders were characterized by simultaneous thermogravimetry analysis (TG) and differential scanning calorimetry analysis (DSC), X-ray diffractometry (XRD), specific surface area measurements, and transmission electron microscopy (TEM). Pure LaAlO₃ phase was obtained at 775 °C for 2 h or 750 °C for 4 h, without formation of any intermediate phase. Pores were found from TEM images of LaAlO₃ powders prepared at 800 °C for 2 h.