CaF2 AlF3 Research Articles

Enhancing light output of WSLED with microlens array fabricated on Tb³⁺/Sm³⁺ co-doped fluorophosphate glass via hot embossing

In this study, Tb3+/Sm3+ co-doped NaPO3–BaF2–AlF3–CaF2 glasses were synthesized using the melt quenching method. These glasses exhibit a glass transition temperature below 380 °C and maintain optical transparency close to 90% within the visible spectrum. Furthermore, under UV excitation, the emission shows a wide spectrum and varies with the rare-earth doping ratio or excitation wavelength. Hence, this glass was introduced into the wide-spectrum light-emitting diode (WSLED) application. Moreover, a microlens array was fabricated on the glass surface using the hot embossing technique. In fact, the microlens array enhanced the light output power of the WSLED by 5% at a driving current of 150 mA.

Determination of the energy transfer parameters in modeling the luminescence kinetics of Er3+ and Yb3+ ions in fluorophosphate glasses with a low phosphate content

The fluorophosphate glasses with composition of Ba(PO3)2 – AlF3–CaF2 – MgF2–BaF2 – SrF2–YbF3 – ErF3 were synthesized and their spectroscopic properties were studied. The main attention here was paid to studying the luminescence kinetics Yb3+ (1040 nm) and Er3+ (1550 nm, 655 nm and 540 nm). It has been found that the population of the 4I13/2 (Er3+) level and the relaxation of the 2F5/2 (Yb3+), 4F9/2, and 2H11/2+4S3/2 (Er3+) levels are close to exponential and the probabilities of these processes are well approximated by linear functions with respect to the ytterbium concentration. All this led to the conclusion that the migration-accelerated energy transfer makes a significant contribution to the population and deactivation of these energy levels. A scheme of the main energy transfer channels in an ensemble of ytterbium and erbium ions and the corresponding system of rate equations are proposed. On the basis of these equations, mathematical modeling of the luminescence kinetics of ytterbium and erbium ions was carried out and the energy transfer parameters were estimated.

Impact of the O/P ratio on the optical properties and structures of fluoride–phosphate glass

AbstractCa(PO3)2–AlF3–CaF2–BaF2–BaO glasses were prepared by the melt quenching method, and the effects of the O/P ratio on the optical properties and glass structure were investigated. The bandgap energy showed no significant change at O/P = 3.0–3.4 but drastically decreased with the increase in the O/P from 3.6 to 4.0. In addition, the refractive index dispersion was analyzed based on the Lorentz model, and it was found that the decrease in the resonance frequency in the ultraviolet region with the increase in the O/P ratio resulted in an increase in the refractive index and dispersion. Analysis of the infrared absorption and Raman scattering spectra revealed that the phosphate chains were broken, and isolated Q0 units were generated with the increase in the O/P ratio from 3.6 to 4.0. Based on the structural change of the glass, the origin of the nonlinear dependence of the optical properties on the O/P ratio was discussed.

Effect of Gd2O3 in Li2O–AlF3–CaF2–P2O5–Eu2O3 glasses for laser medium and X-rays detection material applications

The glass formula of 20Li2O+10AlF3+2.5CaF2+(66.5-x)P2O5+xGd2O3+1Eu2O3 (PLACEu1Gdx), where x = 2,4,6,8,10 mol% have been prepared by conventional melt-quenching technique. These glasses' optical absorption, photoluminescence, decay time, and radioluminescence have been measured at room temperature. Judd-Ofelt's (J-O) analysis of PLACEu1Gd8 has been investigated and the magnitude of parameters follows the trend as Ω2>Ω4>Ω6. The important laser parameters such as radiative properties are calculated using J-O parameters and luminescence spectra. The decay time with the transition from the 5D0 level was estimated under 274 nm and 394 nm excitation using bi-exponential fitting. CIE coordinates, CCT and intensity ratio have been calculated for glass samples. The integral scintillation efficiency of PLACEu1Gd8 is 31% compared with BGO crystal. The PLACEu1Gdx glasses are promising for laser medium and x-rays detection material applications.

Viscosity of conventional cryolite-alumina melts

The study covers the viscosity of NaF–AlF3–CaF2–Al2O3 conventional cryolite-alumina melts with a cryolite ratio CR = 2.3 depending on the CaF2, Al2O3 content and temperature. The viscosity of cryolite-alumina electrolyte samples prepared under laboratory conditions and electrolyte samples of industrial electrolytic cells was measured by the rotary method using the FRS 1600 rheometer («Anton Paar», Austria). The laminar flow region of the melt determined according to the dependence of viscosity on shear rate at a constant temperature was 10–15 s–1 for all the studied samples. The temperature dependence of cryolite-alumina melt viscosity was measured at a shear rate of 12 ± 1 s–1 in the temperature range from liquidus to 1020 °C. It was shown that the change in the viscosity of all samples in the investigated temperature range (50–80 °С) can be described by a linear equation. The average temperature coefficient of linear equations describing the viscosity of cryolite-alumina electrolytes prepared in laboratory conditions was 0.005 mPа· s/°С, which is 2 times less compared to industrial cell electrolytes. Thus, the change in the viscosity of industrial cell electrolytes with increasing temperature is more significant. Both alumina and calcium fluoride additives increase the cryolite melt viscosity. The viscosity of samples prepared with the conventional composition NaF–AlF3–5%CaF2–4%Al2O3 (CR = 2.3) is equal to 3.11 ± 0.04 mPа· s at an electrolysis operating temperature of 960 °C, while the viscosity of industrial cell electrolytes with the same cryolite ratio is 10–15 % higher and falls in the range of 3.0–3.7 mPа· s depending on the electrolyte composition.

Effect of Al2O3 and CaF2 additives on the viscosity of conventional cryolite melts

The viscosity of cryolite melts of conventional composition NaF–AlF3–CaF2–Al2O3 was studied by rotational viscometry using the FRS 1600 high-temperature rheometer. The cryolite ratio of the NaF–AlF3 melt was 2.1, 2.3, and 2.5; the Al2O3 content varied from 2 to 6.6, and CaF2 – from 0 to 8 wt%. The measurements were carried out in the temperature range from liquidus to 1200 °C. The conditions for the laminar flow of the investigated melts were determined, based on the measurements of the cryolite melts viscosity as a function of the shear rate at a constant temperature. A shear rate of 12 ± 1 s–1 was chosen for studying the viscosity temperature dependence for all samples. The viscosity temperature dependence of cryolite melts is described by a linear equation. The temperature coefficient b in this equation has negative values and varies in the range of (–0.01)–(–0.06) mPa·s/deg. It was found that the viscosity of cryolite melts of conventional composition in the range of operating temperatures of aluminum electrolysis (950–970 °C) varies from 2.5 to 3.7 mPa·s (depending on the composition and temperature). The viscosity of cryolite-alumina melts increases with the rise of alumina content: 1 wt% Al2O3 increases the viscosity, on average, by 1%. However, the influence of CaF2 is more significant: the addition of 1 wt% CaF2 leads to an increase in viscosity by 3%. A decrease in the CR of the melt by 0.1 (in the range of 2.1–2.5) leads to a decrease in the viscosity of cryolite melts by 2.3%. A viscosity regression equation for the cryolite melts of conventional composition as a function of several independent parameters (temperature, CR, CaF2 and Al2O3 content) is obtained by the multivariable approximation of experimental data. The equation satisfactorily (within 1.5%) describes the viscosity of conventional industrial electrolytes and can be used for estimation of their viscosity.

On the Use of Photometric Separation for the Processing of Techno-Genic Raw Materials

This work presents studies on the material composition and physico-mechanical characteristics of an alumina-containing estimate formed during the production of aluminum on electrolyzers with self-baking anodes during technological operations. The material is a mixture of fine powder 2.5 mm in size (62.78% on average mass) with the presence of pieces of material ranging in size from 0.5 to 6 cm (average 20.26% by weight). The samples contain pieces of hardened aluminum with sizes from 5 to 20 mm (16.96%). The largest content in the sample has the fineness classes-0.315 + 0.16 mm in the volume of 29.85% and the largest class +2.5 mm-37.22%. B It was established that aluminum is concentrated in the fractions-0.315 + 0.16 mm (45.7%) and 0.16 + 0 mm (48.8%), silicon in the fraction-0.63 + 0.315 mm (1.91%), iron at-1.25 + 0.63 mm (0.601%) and-0.63 + 0.315 mm (0.62%). The material consists of cryolite (Na3AlF6), chiolite (Al3F14Na5), quartz, feldspar, carbonaceous matter and the technogenic phase of the composition (NaF) 1.5CaF2 AlF3. The material is characterized as non-abrasive (working index Ai - 0.0184) and very soft in relation to impact crushing (working index CWi - 3.64), the working index of ball grinding Bond (BWi - 6.47) characterizes a very low resistance to ball grinding. The implementation of the crushing operation of an alumina-containing estimate will allow the use of dry cascade-gravity and centrifugal classification to separate impurities in the form of SiO2, and Fe2O3 for the use of alumina-containing material in primary aluminum technology. On the basis of laboratory tests, it is established that alumina-containing raw materials can be separated and photometric and gravitational separation methods can be used. A mathematical model of the motion of particles of primary and prepared alumina-containing raw materials in a cascade-gravity classifier was developed. The criteria and factors characterizing the alumina-containing material, as well as influencing and determining the maximum material enrichment, are established.

Data mining assisted prediction of liquidus temperature for primary crystallization of different electrolyte systems

Liquidus temperature for primary crystallization is an important physical and chemical property for electrolyte system. It plays a crucial role on the stability of the electric cell in electrolysis production process. So how to accurately predict the liquidus temperature for primary crystallization of electrolyte based on the composition of electrolyte is a meaningful research subject. In this work, data mining assisted prediction of liquidus temperature for primary crystallization of electrolyte systems was proposed. The essential differences between the complex industrial electrolyte system and electrolyte system prepared in laboratory were revealed by means of comparing the micro-morphology, phase composition and thermal analysis. To some extent, it was verified that the empirical formula has no versatility in the two different electrolyte systems. The prediction model of liquidus temperature for primary crystallization of different electrolyte systems was constructed by using SVM(support vector machine), BPANN(back-propagation artifical neural networks), RFR(random forest regression) and GBR(gradient boosting regression) algorithm, respectively. The electroyte system inculdes Na3AlF6(CR)-Al2O3–AlF3–CaF2, Na3AlF6(CR)-Al2O3–MgF2–CaF2–LiF, Na3AlF6(CR)-Al2O3-MgF2-CaF2-KF-LiF, and Na3AlF6(CR)-Al2O3-AlF3-CaF2-MgF2-LiF-KF-NaF. For different electrolyte systems, ANN, SVM, RFR and other models all have good performances, they can effectively predict the liquidus temperature for primary crystallization of each electrolyte systems. For some electrolyte systems, ANN, SVM, RFR models are obviously superior to the prediction level of empirical formula described in the literature. It can be seen that data mining has a good application prospect in the prediction of the liquidus temperature for primary crystallization of electrolyte systems. We provide a new method for predicting the liquidus temperature for primary crystallization of different electrolyte systems based on the electrolyte composition dataset in this work.

Hot corrosion behavior of two-step sintered magnesium aluminate spinels in molten electrolyte

Magnesium aluminate spinel (MAS) has been considered a promising candidate ledge material used in aluminum electrolytic cell. In this study, the compacts of MgAl2O4 were firstly sintered by conventional sintering and two-step sintering methods and then the corrosion behavior of sintered samples in molten electrolyte (Na3AlF6–AlF3–CaF2–Al2O3) at 970 °C for different times was investigated. The results show that a MAS with a high relative density of 99.3% and a homogeneous microstructure consisting of small grains (560 nm) is achieved under optimum two-step sintering condition (T1 = 1500 °C/0 h and T2 = 1400 °C/20 h). The densification has improved the resistance of MgAl2O4 spinel against molten-electrolyte penetration. The corrosion mechanisms of MAS in Na3AlF6–AlF3–CaF2–Al2O3 bath are mainly the penetration of fluorides and bulk dissolution of MAS.

Rare-earth doped fluoride phosphate glasses: structural foundations of their luminescence properties.

We report a detailed structural investigation of a series of fluoride-phosphate glasses with different phosphate/fluoride ratios in the system xSr(PO3)2-(100 - x)[AlF3-CaF2-SrF2-MgF2] with x = 5, 10, 20, 40. Raman and multinuclear solid NMR spectroscopies confirm that the polyphosphate network structure is successively transformed to a structure dominated by Al-O-P linkages with increasing AlF3 content. Average numbers of Al-O-P linkages have been quantified by 27Al/31P NMR double-resonance techniques. The majority of the fluoride species are found in an alkaline earth metal/aluminum rich environment. The local environments for rare-earth ions have been characterized by EPR spectroscopy of Yb3+ ion spin probes and by photoluminescence experiments on Eu3+ dopant ions, including the 5D0 → 7F2 and 5D0 → 7F1 transition intensity ratio, the normalized phonon sideband intensities in the excitation spectra, and the lifetime of the 5D0 excited state. The results indicate clear correlations between these parameters as a function of composition, and confirm that even at the highest fluoride levels, there is still some residual rare-earth phosphate coordination.

Two-step sintering of magnesium aluminate spinels and their corrosion in Na3AlF6–AlF3–CaF2–Al2O3 bath

New types of refractory materials need to be developed for designing the man-made ledge of the Hall–Héroult cell for aluminum electrolysis, which are currently constructed by frozen ledge. Magnesium aluminate spinel (MAS) as potential candidate materials was prepared by two-step sintering. The densification and grain growth of the MAS were investigated by the Archimedes drainage method and scanning electron microscope (SEM). All the specimens were corroded in a Na3AlF6–AlF3–CaF2–Al2O3 bath to assess the corrosion resistance. The results show that a MAS material with a high relative density of 99.2% and a homogeneous microstructure is achieved under two-step sintering conditions. The corrosion mechanisms of MAS in Na3AlF6–AlF3–CaF2–Al2O3 bath are mainly proposed by dissolution of MAS, formation of aluminum oxide, and diffusion of fluorides. The MAS prepared by two-step sintering exhibits good corrosion resistance to the Na3AlF6–AlF3–CaF2–Al2O3 bath.

Determination of the Al2O3 content in NaF-AlF3-CaF2-Al2O3 melts at 950 °C by Raman spectroscopy.

The in situ control of the chemical composition of industrial aluminum smelter is a challenge mainly for physicochemical reasons: high temperature, high surrounding electromagnetic field, and the highly corrosive molten salt electrolyte to deal with. In previous works, we proposed that Raman spectroscopy is a method of choice that could be adapted to real smelters. The laboratory study presented here relies on reproducible Raman spectra recorded on molten mixtures whose compositions are identical to those used during the production of aluminum. A normalization procedure for the Raman spectra is proposed based on the equilibria taking place in the bath. In addition, we discuss two quantitative models to determine the alumina content from the Raman spectra of the molten NaF-AlF3-CaF2-Al2O3 electrolytes. Univariate and multivariate approaches are applied to determine both the COx (alumina content) and the CR (NaF/AlF3 molar ratio) by Raman spectroscopy without referring to an additional internal reference of intensity. The procedure was successfully tested and validated on industrial samples.

Thermodynamic considerations of the corrosion of nickel ferrite refractory by Na3AlF6–AlF3–CaF2–Al2O3 bath

Thermodynamic analysis was carried out to interpret the results of corrosion testing of nickel ferrite samples in cryolite-based baths. The equilibrium between cryolite-based baths and nickel ferrite was considered.Isopleths between cryolite-based baths and nickel ferrite confirmed that for the temperature range of interest (1223–1273 K) there was limited solubility of nickel ferrite in the bath.To better understand the formation of the metal from nickel ferrite, the effect of reducing potentials on nickel ferrite and nickel ferrite–cryolite-based bath systems were considered. The formation of a metal phase was predicted at relatively high pO2. The metal phase was nickel-rich at higher pO2, becoming enriched in iron as the pO2 decreased. The oxide phases seen in corroded nickel ferrite samples corresponded to the spinel phase in the thermodynamic calculations. Penetration of aluminium oxides into the spinel phase seen in the experimental samples occurred only under a reducing potential.

Electrical conductivity of NaF–AlF3–CaF2–Al2O3–ZrO2 molten salts

The electrical conductivity of NaF–AlF3–CaF2–Al2O3–ZrO2 system was studied by a tube-type cell with fixed cell constant. The results show that the electrical conductivity of NaF–AlF3–3%Al2O3–3%CaF2–ZrO2 molten salt system decreases with increase of ZrO2 content in an interval of 0–5%. The increase of 1% ZrO2 results in a corresponding electrical conductivity decrease of 0.02 S/cm, and the equivalent conductivity increases with the increase of molar ratio of NaF to AlF3. When the temperature increases by 1 °C, the electrical conductivity increases by 0.004 S/cm. At last, the regression equations of electrical conductivity relative to temperature and ZrO2 are obtained by quadratic regression analysis.

C K‐edge NEXAFS study of fluorocarbon formation on carbon anodes in molten NaF–AlF3–CaF2

Operational instability from processes occurring at the anode during the production of aluminum in the commercial Hall‐Héroult process may lead to an increase in undesirable fluorocarbon emissions, higher energy use, and shorter plant life. One contribution to this instability may be the possible formation of a fluorocarbon film at the electrode interface. Here, the surface composition of graphite anodes after electrolysis in molten NaF–AlF3–CaF2 at 990 °C is investigated for evidence of fluorocarbon formation using C K‐edge near edge X‐ray absorption fine structure. Fluorocarbon is identified on an anode surface after prolonged anode effect (very high overpotential with increased cell resistance) and also on an anode surface after normal electrolysis without anode effect. This provides evidence that fluorocarbon formation may occur prior to anode effect lowering the surface tension of the anode and therefore resulting in dewetting to contribute to the onset of the anode effect. Confirmation that such compounds form furthers our understanding of electrochemical reactions of graphite with fluoride and of the fundamental processes that occur in an aluminum smelter cell. Copyright © 2013 John Wiley & Sons, Ltd.

Corrosion of nickel ferrite refractory by Na3AlF6–AlF3–CaF2–Al2O3 bath

Abstract Based on research on cermet inert anodes for aluminium production, it has been suggested that nickel ferrite spinel might be suitable for use as a sidewall refractory in Hall-Heroult cells. A corrosion resistant sidewall would allow elimination of the frozen bath ledge, and has potentially huge benefits in terms of energy savings and increased productivity. However, little work has been done to assess nickel ferrite's suitability as a refractory. Dense nickel ferrite samples were prepared and characterized, and corrosion tests in cryolite based baths were conducted. Results confirm that the spinel does have good corrosion resistance. The corrosion mechanism is complex, involving grain boundary attack and formation of a Ni–Fe alloy. This alloy could pose a risk in terms of contamination of the aluminium. The use of additives to restrict penetration of grain boundaries may be the key to development of a successful spinel based refractory.

Photoluminescence properties of Sm3+-doped SFB glasses for efficient visible lasers

The photoluminescence properties of different concentrations of Sm3+ ions doped sodium fluoroborate (SFB) glasses of composition Na2O–LaF3–CaF2–AlF3–B2O3–SmF3 have been investigated. The energy level analysis is carried out using free-ion Hamiltonian model. The Judd–Ofelt intensity parameters are used to evaluate the laser characteristic parameters such as spontaneous transition probability, radiative lifetime and fluorescence branching ratio for 4G5/2→6HJ (J=5/2, 7/2, 9/2 and 11/2) emission transitions of Sm3+ ion. The principal photoluminescence transitions of interest are identified by recording the emission spectra for different Sm3+ ion concentrations and measuring their emission cross-sections, integrated absorption cross-sections and optical gain parameters. The decay profiles from the 4G7/2 excited manifold state to its lower lying energy levels have been recorded by monitoring the excitation and emission wavelengths at 402 and 600nm, respectively. The dependence of effective fluorescence lifetime on the Sm3+ concentration is also discussed.

An XPS study of the fluorination of carbon anodes in molten NaF–AlF3–CaF2

In the Hall–Heroult process for aluminum production electrolysis takes place in molten cryolite (NaF–AlF3–CaF2) with carbon electrodes. Dewetting of the anode leads to operational instability. A surface energy change due to surface fluorocarbon formation during electrolysis is suggested as a contributing factor to diminishing the wettability of the anode. In this work the surface composition of graphite anodes after electrolysis in molten NaF–AlF3–CaF2 is investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) for evidence of fluorination. Fluorocarbon is identified on an electropolished region of an anode surface resulting from anode effect. The discovery of surface fluorination provides an insight into high temperature electrochemical reactions of carbon with molten fluoride salts and informs aluminum smelter cell operation.

CaF2–AlF3–SiO2 glass-ceramic with low dielectric constant for LTCC application

Abstract Low temperature co-fired ceramic (LTCC) materials based on CaF2–AlF3–SiO2 oxyfluoride glass-ceramics were prepared. The sintering behaviors, phase transition, microstructures, and the dielectric properties of the glass-ceramics, as well as their compatibility with Ag electrode were investigated systematically. It was revealed that the oxyfluoride glass-ceramics could be sintered at 800–825 °C. Moreover, as-prepared glass-ceramics exhibited low dielectric constant (5.93–4.93), low dielectric loss ( 150 MPa), proper thermal expansion coefficient (8.4–6.0 ppm/°C) and good sintering compatibility with silver electrode, which provided promising candidates for LTCC application.

The Relationship Between 27 Al Quadrupolar Parameters and AlF 6 3– Octahedron Connectivity in Crystalline and Glassy Fluoroaluminates

27Al SATRAS and MQ-MAS spectra were recorded for eight crystalline compounds, from the CaF2–AlF3 and BaF2–AlF3 binary and BaF2–CaF2–AlF3 ternary systems, and four glass compositions with different CaF2/BaF2/AlF3 contents. For the crystalline phases, the reconstruction of the spectra leads to the precise determination of the NMR parameters. For the glassy phases, the 27Al SATRAS spectra have been reconstructed using quadrupolar parameter distributions. The main finding of this study is the dependence of the quadrupolar frequency on the type of AlF63– octahedron connectivity. For crystalline phases the experimental νQ values range between 75 kHz and 510 kHz for structures built up from isolated octahedra, and are between 560 kHz and 1250 kHz for structures built up from isolated chains of cis-connected octahedra, and are between 1530 kHz and 1580 kHz for structures built up from isolated chains of trans-connected octahedra. In the glassy phases the maximum of the quadrupolar frequency distribution shifts toward larger values and its width increases with increasing AlF3 content, and subsequently with the number of connected octahedra. The range of the νQ values seems to indicate that when the octahedra are connected, cis connections occur most frequently. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)