Nanocrystalline Rare Earth Phosphates from Glass Dissolution and Precipitation Reactions

A novel method is employed for the formation of rare earth phosphate solid solution compounds with unique mesoscopic structures. Europium‐ and lanthanum‐doped sodium borate glass microspheres and particles, ranging in sizes from 50 to 300 μm, were reacted in 0.25 M K₂HPO₄ solution to form hollow spheres of nanocrystalline rare earth phosphate compounds by dissolution–precipitation reactions. The initially X‐ray amorphous precipitated rare earth phosphate materials were heat‐treated at 700°C for 2 h to form nanocrystalline compounds. Differential thermal analysis (DTA) experiments yield an average activation energy for crystallization of 394 ± 26 kJ/mol. X‐ray diffraction (XRD) data indicate that samples crystallized to the monazite structure (monoclinic P2₁/n) with unit cell volumes ranging from 306.5 Å³ for LaPO₄ to 282.5 Å³ for EuPO₄ and with crystallite grain sizes of 56 ± 14 nm. Compositions containing both rare earth elements formed solid solutions with the composition La_(1?x)Eu_xPO₄. Raman spectroscopy indicates that the P–O symmetric stretching vibrations (ν₁) change systematically from 963 cm^?1 for LaPO₄ to 986 cm^?1 for EuPO₄, consistent with a systematic decrease in average P–O bond length. Photoluminescence measurements show maximum emission intensity for the La_0.65Eu_0.35PO₄ composition.     

The influence of concentration on the formation of BaTiO3 by direct reaction of TiCl4 with Ba(OH)2 in aqueous solution

The formation of fine BaTiO₃ particles by reaction between liquid TiCl₄ and Ba(OH)₂ in aqueous solution at 85 °C and pH⩾13 has been studied for 0.062⩽[Ba²⁺]⩽0.51 mol l⁻¹. The concentration of Ba²⁺ ions has a strong influence on reaction kinetics, particle size and crystallite size. When [Ba²⁺]>≈0.12 mol l⁻¹, the precipitate consists of nanosized (≈30 nm) to submicron (100–300 nm) particles of crystalline BaTiO₃. At lower concentrations, the final product is a mixture of crystalline BaTiO₃ and a Ti-rich amorphous phase even for very long reaction times. A two-steps precipitation mechanism is proposed. Initially, a Ti-rich amorphous precipitate is rapidly produced. Reaction between the amorphous phase and the Ba²⁺ ions left in solution then leads to crystallisation of BaTiO₃. In addition to nucleation and growth of nanocrystals, the final size and morphology of BaTiO₃ particles obtained at low concentration can be determined by aggregation of nanocrystals and heterogeneous nucleation on existing crystal surfaces.     

Particle Size Control of Nanocrystalline Anatase TiO2 Synthesized by Hydrolysis of Titanyl Organic Compounds

The global nanocrystalline anatase TiO₂ particle can be obtained by hydrolysis of titanyl organic compounds. Its particle size is mostly influenced by the titanyl organic compounds' concentration, nitric acid (HNO₃) concentration, reaction time and temperature, and especially the HNO₃ concentration. The formation of nanocrystalline TiO₂ with bigger size can be accelerated by a higher temperature, thick solution of reactant (titanyl organic compounds), and HNO₃. Vice versa, in order to gain smaller particles, such as 6 nm, the reaction conditions should be set at a thin reactant solution, low temperature, and low HNO₃ concentration. The reason lies in the hydrolyzing mechanism of titanyl organic compounds, which is strongly influenced by the temperature and pH.     

Influence of CeO2 addition on crystal growth behavior of CeO2–Y2O3–ZrO2 solid solution

Nanopowders with cubic fluorite-type structure as well as uniform distribution in particle size were synthesized by hydrothermal method in the ternary oxide zirconia–yttria–ceria system with ceria content of 0–25 mol%. X-ray diffraction (XRD), thermogravimetric analysis/differential scanning calorimeter (TG/DSC), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy (Raman), specific surface area (S_BET) and high resolution transmission electron microscopy (HRTEM) were applied to characterize the structure, thermal decomposition, morphological characteristic and crystal growth of the produced powders. Qualitative analyses indicate that the as-synthesized nanoparticles are single-phase crystallites with an average particle size of 4–9 nm. The specific surface area, lattice parameter and microstrain are closely related to Ce⁴⁺ concentration. Moreover, activation energy of crystal growth is significantly dependent on the dopant (CeO₂) concentration. It firstly increased and then decreased with increasing dopant concentration, and the maximum value was observed at the dopant concentration of 5 mol%.     

Sol-Gel Synthesis,Thermal Behavior of Nanopowders and Chemical Stability of La<Subscript>1 – <Emphasis Type="Italic">x</Emphasis></Subscript>Ho<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>PO<Subscript>4</Subscript> Ceramic Matrices

Nanosized powders of orthophosphates in the LaPO₄–HoPO₄–H₂O system have been synthesized to determine the mutual solubility of LaPO₄ · nH₂O and HoPO₄ · nH₂O initial components and to obtain ceramic matrices by sintering them. Formation of hexagonal, monoclinic or tetragonal solid solutions was revealed, and their limits and thermal stability were determined. A series of limited hexagonal LaPO₄ · nH₂O-based solid solutions was observed within the 0 ≤ x ≤ 0.6 concentration range up to 600°C. Further they transformed to monoclinic LaPO₄-based form within the 0 ≤ x ≤ 0.3 concentration range. Solubility of LaPO₄ · nH₂O and LaPO₄ in tetragonal HoPO₄(· nH₂O) is lower (≤10 mol %). Specific surface area of La_1–xHo_xPO₄ · nH₂O powders was in the range of 90.5–165.0 m²/g depending on x. Leaching rate of La³⁺ and Ho³⁺ from La_1–xHo_xPO₄ matrices in nitric acid solution (pH 1–2) was determined to be 10^–5–10^–2 g/(cm² day) for both ions.     

Porous phosphatic materials of agronomical interest:Synthesis and study of their dissolution

Phosphatic materials of agronomic interest were synthesised. These materials of good mechanical quality and of significant porosity show high P₂O₅ contents (up to 28%). They have been prepared by treatment of volcanic pumices with a solution of phosphoric acid (1N, 5N or 10N) at 200°C in a closed system for 72h using a mass solid/liquid ratio of 1:3.After reaction, the precipitation of amorphous phosphates and crystalline aluminium phosphates has been detected by XRD both outside and inside of the material. The morphological study by SEM reveals several crystalline forms of aluminium phosphates: crystals of small size migrate into the internal porosity of the pumices and crystals of berlinite appear (AlPO₄) well developed in a matrix of amorphous phosphate overlapping the initial material.Normal tests of reactivity in water and in 2% citric acid show that the percentages of released P₂O₅ in solution are greater after treatment with weak concentrations of phosphoric acid. Lixiviation of treated materials with citric acid reagent shows that, after three days of reaction, losses in weight are considerable and the kinetic curves of phosphorus liberation present a linear evolution.The fertilizing value of these materials did not decrease during the leaching; so phosphorus will be solubilized with time thus allowing these materials to gain a good agronomical availability and to propose their use as delayed fertilizer types for tropical agricultural soils.     

Preparation of LaPO4:Ce,Tb phosphor with different morphologies and their fluorescence properties

LaPO₄:Ce,Tb phosphor was prepared by firing the LaPO₄:Ce,Tb precipitate derived from co-precipitation of aqueous solution of LaCeTb mixed rare earth nitrate with ammonium dibasic phosphate. Effects of ripening condition of precipitate, flux addition and firing temperature on the morphology and photoluminescence properties of LaPO₄:Ce,Tb phosphor were investigated. It was found that ripening of LaPO₄,Ce,Tb precipitate with mother liquor at pH = 1 and 130 °C for 24 h produced strip phosphor with width of 100 nm and length of 1 μm. While ripening at either 85 or 130 °C after pH adjustment with ammonia generated spherical particles of about 100 nm in size. Types of flux employed significantly influenced the morphology of the phosphor. When H₃BO₃ was used as flux spherical particles of ca. 300 nm were formed. In contrast, when Li₂CO₃ or Li₃PO₄ was employed as flux either separately or a component of double flux, uniform spherical or near spherical particles of ca. 5 μm were generated. Firing temperature of 1100 and 1200 °C in the presence of double flux led to the formation of smooth spherical particles with diameter of ca. 5 μm, and the phosphor prepared with double flux showed higher brightness than the commercial phosphor.     

Characterization and application of activated carbon produced by H3PO4 and water vapor activation

Activated carbons have been prepared from woody biomass birch by using various activation procedures: a) treatment with phosphoric acid and pyrolysis at 600 °C in inert atmosphere, b) the same as in (a) followed by steam activation at the same temperature and c) treatment with phosphoric acid and direct pyrolysis in a stream of water vapor at 700 °C. The surface area and the porosity of the activated carbons were strongly dependent on the treatment after impregnation with H₃PO₄ (pyrolysis in inert atmosphere, steam pyrolysis or combination of both).Activated carbon, prepared by impregnation with phosphoric acid followed by steam pyrolysis (steam activation) had highly developed porous structure and the largest surface area among all prepared carbons (iodine number 1280 mg/g and BET surface area 1360 m²/g). The adsorption capacity of this sample for Hg(II) from aqueous solution was studied in varying treatment conditions: contact time, metal ion concentration and pH. The adsorption followed Langmuir isotherms and the adsorption capacity for Hg(II) at 293 K was 160 mg/g.     

Semi-batch reactive crystallisation of mono-ammonium phosphate: An experimental study

NH₄H₂PO₄, mono-ammonium phosphate (MAP) and (NH₄)₂HPO₄, di-ammonium phosphate (DAP) have become leading phosphate fertiliser products worldwide. Ammonium phosphates are produced by reactions of ammonia and phosphoric acid resulting in the formation of the mono-basic, di-basic, or tri-basic salts. Inefficiencies in the MAP/DAP production process are due to the lack of a fundamental understanding of the crystallisation–reaction mechanisms. A semi-batch reactive crystalliser equipped with a cooling jacket, a dual feed system and a turbine type agitator has been used in this study. Effects of N/P ratio, seed crystals, feeding system, feed flow rate, initial supersaturation, feeding time and mixing intensity to the real-time supersaturation, crystal yield, crystal shape (aspect ratio) and final crystal size distribution (CSD) were studied in order to understand the kinetics. This study aims to provide new insights into the MAP/DAP reaction–crystallisation mechanisms.     

Chemistry of by-product gypsum and plaster. I. Identity of an important solid solution impurity

An investigation has been made into why calcium sulphate β-hemihydrate, derived from the by-product gypsum of wet-process phosphoric acid manufacture, can, when derived from certain types of phosphate rock, be insensitive to hydration retarders. Aluminium and fluorine, which are common impurities in phosphate rock, form complex aluminofluorides in the phosphoric acid reaction liquor; and gypsum crystallising from this system has its crystal habit and physical properties modified by these complexes. The hemihydrate calcined from the gypsum has a setting time that is less amenable to control by retarder addition than the setting time of pure hemihydrate. The gypsum crystals exhibiting this effect incorporate a solid solution impurity which has been identified as AlF₅^2? substituting for SO₄^2? ions in the crystal lattice.     

Controllable hydrothermal synthesis and photoluminescence properties of CaGd2(WO4)4:Eu3+ red-emitting phosphor

CaGd₂(WO₄)₄:Eu³⁺ phosphors with controllable morphology were synthesized via the hydrothermal method. The influences of pH value, reaction time and Eu³⁺ concentration on the crystal structure, morphology, and photoluminescence properties of CaGd₂(WO₄)₄:Eu³⁺ were studied. The pure tetragonal structure CaGd₂(WO₄)₄ is obtained when the pH value is 8 and 9. Furthermore, by altering the pH value of the reaction solution, the morphologies of the CaGd₂(WO₄)₄:Eu³⁺ phosphors evolve from spindle-shaped grains to tetragonal plate-like grains and finally to aggregated bulk particles. Under the 394 nm excitation, the phosphors display a bright red emission corresponding to the characteristic 4f-4f transitions of Eu³⁺, and the intensity of emission peaks depends mainly on the pH value, the reaction time, and the Eu³⁺ concentration. The optimum photoluminescence performance is achieved for CaGd_2-x(WO₄)₄:xEu³⁺ (x = 1) phosphor synthesized at pH = 8 under the reaction time of 16 h. Finally, the thermal stability of the phosphors is analyzed at different ambient temperatures.     

Formation of zircon-type DyCrO4 and its magnetic properties

The formation mechanism of hydrolytic sol-gel synthesized DyCrO₄ with a complexing agent in acidic and basic mediums is thoroughly studied. The role of complexing agents and pH on phase formation temperature is also intensively investigated. The formation temperature for DyCrO₄ is ～500 °C in the absence and presence of complexing agents such as oxalic acid and ethylenediaminetetraacetic acid (EDTA) at pH 10. When critic acid is used, the DyCrO₄ forms with Cr₂O₃ impurity. The crystallite size in the presence of a complexing agent in the basic medium is ～55 nm which is small as compared to only ammonia solution. The various reaction modes lead to tetragonal zircon-type DyCrO₄ at ～500 °C, transforming into orthorhombic perovskite DyCrO₃ at 800 °C. The magnetization curve shows the ferromagnetic behavior of DyCrO₄ below transition temperature T_c ～21 K. This low T_c makes nanocrystalline DyCrO₄ a potential material for cryogenic applications.     

Controllable one-pot hydrothermal preparation of manganese oxide with diverse crystal and morphology for supercapacitors: New strategy for introducing short-chain surfactant

We successfully synthesized nanoflower-like δ-MnO₂, rod-shaped MnOOH, nano-small granular Mn₃O₄, and the cuboidal and spindle shaped MnCO₃ via a facile one-pot hydrothermal method by adjusting the concentration of two sorts of short-chain surfactants with special catechol structures (3-Hydroxytyramine hydrochloride (DA) and 3,4-dihydroxybenzenepropanoic acid (DHCA)) in KMnO₄ solution. The XRD and FTIR data demonstrated that the crystal form, morphology and size of the crystal were controlled by small molecule ligands. Afterwards, the relationship between manganese oxides and electrochemical properties was further explored. The results illustrated that nanoflower-like δ-MnO₂ with layer structures have the highest specific capacitance of 307.4 F g^?1 at a scan rate of 5 mV s^?1.     

Calcium-Magnesium-Aluminosilicate (CMAS) corrosion resistance of high entropy rare-earth phosphate (Lu0.2Yb0.2Er0.2Y0.2Gd0.2)PO4: A novel environmental barrier coating candidate

Single phase (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ was synthesized, and its thermal properties and CMAS resistance were investigated to explore its potential as an environmental barrier coating (EBC) candidate. The high entropy phosphate (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ displays a lower thermal conductivity (2.86 W m⁻¹ K⁻¹ at 1250 K) than all the single component xenotime phase rare-earth phosphates. Interaction of (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ pellets with CMAS at 1300 °C led to the formation of a dense and uniformed Ca₈MgRE(PO₄)₇ reaction layer, which halted the CMAS penetration into the bulk pellet. At 1400 and 1500 °C the (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄-CMAS corrosion showed CMAS penetrating beyond the reaction layer into the bulk pellet via the grain boundaries, and SiO₂ precipitates remaining at the pellet surface. The effects of duration, temperature, and compositions on the resistance against CMAS corrosion are discussed within the context of optimizing materials design and performance of high entropy rare-earth phosphates as candidates for advanced EBC applications.     

Optimization of the Dissolution of Tincal Ore in Phosphoric Acid Solutions at High Temperatures

The aim of the study was to investigate the optimization of the dissolution of tincal ore in phosphoric acid solutions at high temperatures in a batch reactor. The effect of the following parameters on the dissolution process was investigated: the reaction temperature, the phosphoric acid concentration, the particle size, and the solid-to-liquid ratio. The best conditions for the dissolution were determined using the 2⁴ factorial experimental design method. The optimum values of the parameters were experimentally determined. The effective parameters were the reaction temperature, the phosphoric acid concentration, the particle size, and the solid-to-liquid ratio. The optimum conditions resulted in the maximum boron dissolution at an acid concentration of 1 M, reaction temperature of 85°C, particle size of 4.75 mesh, and solid-to-liquid ratio of 1/6 g · mL^?1. Under these optimum conditions, the best dissolution yield was 98.26%.     

Study on photocatalytic deposition of bismuth onto nanocrystalline TiO2 by quartz crystal microbalance and electrochemical method

In situ techniques of quartz crystal microbalance (QCM), differential pulse voltammetry (DPV) and amperometric measurement were employed to investigate the adsorption Bi(III) ions and the photocatalytic deposition Bi process at the surface of nanocrystalline TiO₂. It was obtained that the adsorption of Bi(III) ions onto nanocrystalline TiO₂ accords with the pseudo-second-order reaction and the reaction rate constant k was about 13.3 g mol⁻¹ min⁻¹. In addition, the photocatalytic deposition of Bi onto the surface of TiO₂ was further investigated. It was found that photocatalytic deposition rate at the surface of TiO₂ was enhanced by increasing pH value or initial concentration of Bi(III) ions. The influence of organic hole-scavegeners on the photocatalytic deposition of Bi was also investigated, and it was obtained that formic acid may be the best for the photocatalytic reduction of Bi. The mass ratio between the Bi(III) and Bi metal deposition was calculated as 7.48:1. Therefore, it can be concluded that QCM, DPV and amperometric measurement may be effective and reliable for the investigation of the photocatalytic deposition of Bi onto the surface of nanocrystalline TiO₂.     

Physical Properties of La1−xEuxPO4,0 ≤ x ≤ 1, Monazite‐Type Ceramics

Synthetic La_1?xEu_xPO₄ monazite‐type ceramics with 0 ≤ x ≤ 1 have been characterized by ultrasound techniques, dilatometry, and micro‐calorimetry. The coefficients of thermal expansion and the elastic properties are, to a good approximation, linearly dependent on the europium concentration. Elastic stiffness coefficients range from 182(1) to 202(1) GPa for c₁₁ and from 53.8(7) to 61.1(4) GPa for c₄₄. They are strongly dependent on the density of the sample. The coefficient of thermal expansion at 673 K is 8.4(3)  × 10^?6 K^?1 for LaPO₄ and 9.9(3)  × 10^?6 K^?1 for EuPO₄, respectively. The heat capacities at ambient temperature are between 101.6(8) J·(mol·K)^?1 for LaPO₄ and 110.1(8) J·(mol·K)^?1 for EuPO₄. The difference between the heat capacity of LaPO₄ and the Eu‐containing solid solutions is dominated by electronic transitions of the 4f‐electrons at temperatures above 75 K.     

The Effects of pH and Alkaline Earth Ions on the Formation of Nanosized Zirconia Phases Under Hydrothermal Conditions

The phase composition and crystallite size of zirconia formed under hydrothermal conditions is strongly dependent on crystallization conditions, in particular, pH and a mineralizer. Monoclinic ZrO₂ formed easily in the strong acid and basic media. When pH<1 or >14, monoclinic ZrO₂ was exclusively obtained and its crystallite size increased with increasing pH. In the range of pH 1–14, products of hydrothermal reaction are a mixture of monoclinic and tetragonal ZrO₂. The effects of Mg²⁺, Ca²⁺ and Sr²⁺ ions on the formation of zirconia under hydrothermal conditions were investigated. The presence of these bivalent M²⁺ ions was in favour of the formation of tetragonal (or cubic) ZrO₂. The nanosized ZrO₂ crystallites of cubic and tetragonal symmetry were obtained at pH 10, 220°C for 2 h and in the case of Ca²⁺ and Sr²⁺ as mineralizer, respectively.     

Selectivity and competitive mechanism of cation exchange resin for Fe,Mg, and Al ions in phosphoric acid

The presence of iron, magnesium, and aluminum elements as the primary impurities in wet-process phosphoric acid (WPA) adversely affects the industrial phosphoric acid and subsequent phosphorus chemical products. This study aims to investigate the selectivity and competition mechanism of Sinco-430 cation exchange resin for Fe, Mg, and Al ions in phosphoric acid solution. By studying the effects of different process conditions on the removal efficiency, the suitable conditions for the static removal of metal ions from Fe-Mg, Al-Mg, and Fe-Al binary systems were determined: solid–liquid mass ratio (S/L) of 0.3, phosphoric acid concentration of 27.61 wt.%, system temperature of 50°C, and rotational speeds of 200, 400, and 200 rpm, respectively. By calculating the selectivity coefficients of the resin for metal ions under different experimental conditions and mutual replacement experiments, the semi-empirical formulas for the selectivity coefficients were derived and order of selectivity was determined as follows: Mg²⁺ > Fe²⁺ > Al³⁺. Visual MINTEQ 3.1 software and density functional theory (DFT) calculations demonstrated that at low pH, the main forms of Fe, Mg, and Al present in phosphoric acid were FeH₂PO₄⁺, Mg²⁺, and AlH₂PO₄²⁺, respectively. This finding explained the differences in selectivity of the resin for Fe, Mg, and Al. The dynamic removal of metal ions from phosphoric acid was investigated. The order of metal ion selectivity of the resin by the dynamic method is the same as that of the static method, and the dynamic exchange behaviour was most consistent with the Yan model.     

Power dependent upconversion in Er3+/Yb3+ co-doped BiNbO4 phosphors

In the present article, optical properties and energy upconversion in Er³⁺/Yb³⁺ co-doped BiNbO₄ matrix were investigated. The BiNbO₄ matrix was prepared using the solid-state reaction method. X-ray diffraction of the matrix shows that the crystal structure is consistent with ICSD code 74338. The grain distribution and the behavior of doping with Er³⁺ and Yb³⁺ on the sample surface were obtained by scanning electron microscope. Raman spectral characterization was carried out to examine the behavior of the vibrational modes of the samples. Upconversion emissions in the visible region at 484.5, 522, 541.5 and 670.5 nm in the matrices BiNbO₄:Er,Yb and BiNbO₄:Er were observed and analyzed as a function of 980 nm laser excitation power and rare-earth doping concentration. The results show that BiNbO₄ is a promising host material for efficient upconversion phosphors.