Controlling the microstructure and properties of lithium disilicate glass-ceramics by adjusting the content of MgO

In order to obtain lithium disilicate glass-ceramics for dental restoration with both high strength and high translucency, lithium disilicate glass-ceramics with different MgO contents were prepared by melt-casting and heat treatment method. The effects of MgO content on the crystallization temperature, microstructure and flexural strength of lithium disilicate glass-ceramics were investigated. The results indicate that Mg²⁺ exists in the form of [MgO₄] in the network of lithium disilicate glass-ceramics when the MgO content is 0.56 mol% (M0.56), which is beneficial to increasing the homogeneity and thermal stability of the glass system, and short rod-like lithium disilicate crystals can be formed after heat treatment at 840°C. Thus, the obtained lithium disilicate glass-ceramics exhibit excellent comprehensive performance, with the flexural strength being 312 ± 23 MPa, and the average transmittance of visible light being 37.3% (d = 1.62 mm). Especially, the glass-ceramic sample shows better translucency than the commercially available products. The research results are of great significance for developing high performance lithium disilicate glass ceramics and promoting its broad application in the field of dental restoration.     

Investigation of the promotion effect on the sulfide sulfur mobility and on the catalytic activity of hydrotreating Co/Mo/Al2O3 catalysts using35S

Alkali nitrate promoted Raney Ru catalysts were prepared by hydrogenolysis of alkali nitrates (CsNO₃, RbNO₃, KNO₃, and NaNO₃) over Raney Ru. These catalysts were even more active in ammonia synthesis than Raney Ru promoted with metallic potassium. The promotion behaviour was different from that for the supported Ru catalysts, where metallic potassium was more effective than CsNO₃. Per-weight activity of CsNO₃ promoted Raney Ru was higher than that for any catalysts so far reported in ammonia synthesis under 80 kPa and at 573 K.     

Biaxial flexural strength and Weilbull characteristics of adhesively luted hybrid and reinforced CAD/CAM materials to dentin: effect of self-etching ceramic primer versus hydrofluoric acid etching

Abstract

This study evaluated the influence of the surface treatment and aging on the biaxial flexural strength of ceramic materials cemented to a dentin analogue. One hundred twenty disc-shaped specimens were allocated into 12 groups considering three study factors: ceramic material (lithium disilicate, leucite-based ceramic and hybrid ceramic), surface treatment (10% hydrofluoric acid etching?+?silane or self-etching glass-ceramic primer) and Aging (with 10,000 thermocycles of 5–37–55?°C or without). A tri-layer assembly was designed to mimic a cemented restoration (Variolink N) into a dentin analogue. All samples were submitted to the biaxial flexural strength assay. The flexural strength in MPa was calculated using the finite element method for each sample considering thickness, material properties, and the load to fracture during the in vitro test. Fractographic analysis was also performed. The data was evaluated using three-way ANOVA and Tukey test (α?=?5%). ANOVA showed influence for the Material*Treatment*Aging interaction on the flexural strength (p?=?0.011). The highest strength was calculated for lithium disilicate ceramic?+?self-etching ceramic primer without aging (499?±?17?MPa)^A and the lowest value for hybrid ceramic material?+?acid etching with aging (424?±?48?MPa)^E. According to the Weibull modulus, the most predictable strength was calculated for lithium disilicate?+?acid etching after aging. Acid etching or self-etching ceramic primer promotes similar immediate biaxial flexural strength for each evaluated ceramic. In the long-term, superior strength was observed using acid etching for lithium disilicate and the self-etching ceramic primer for the hybrid ceramic while no difference was observed for leucite-based ceramic.

Clinical implications: Some protocols combining the CAD/CAM ceramic material and the surface treatment could present suitable and stable flexural strength.     

A novel high-strength lithium disilicate glass-ceramic featuring a highly intertwined microstructure

To achieve long-term clinical performance and wider application of glass-ceramic dental restorations, it is urged to enhance the mechanical properties of glass-ceramic materials. In this study, a high-strength lithium disilicate glass-ceramic was developed in a SiO₂–Li₂O–Al₂O₃–MgO–P₂O₅–ZrO₂ related glass system, which demonstrates a high flexural strength of 562 ± 107 MPa. In this high-strength glass-ceramic, the microstructure features highly intertwined colonies of lithium disilicate. This novel microstructure effectively contributes to the improvement of flexural strength. The minor crystalline phases (β-quartz, MgAl₂Si₄O₁₂, and Li₃PO₄) embedded within the Li₂Si₂O₅ (LS₂) crystal colonies and residual glass matrix could further strengthen the glass-ceramic. The development process of such a novel microstructure and its possible formation mechanism are proposed. This material could be an excellent candidate for restorative dental applications up to three-unit posterior bridges.     

Strengthening and toughening of a multi-component lithium disilicate glass-ceramic by ion-exchange

A multi-component lithium disilicate (LD) glass-ceramic with interlocking microstructure consisting of rod-like LD crystals and glassy matrix was ion-exchanged over wide temperature and time ranges in pure NaNO₃ or mixed NaNO₃ and KNO₃ baths below the glass transition temperature. Treatment temperature, time and salt bath dependences of surface characteristics and mechanical properties for the ion-exchanged glass-ceramic were investigated. It was found that the glass-ceramic with limited glassy matrix could be remarkably strengthened and toughened in NaNO₃ bath by adjusting the treatment temperature to a moderate level, at which Li⁺/Na⁺ exchange between the glassy matrix and the salt bath could form an ion-exchanged layer with larger depth and less stress relaxation. Furthermore, by using the mixed salt bath, the undesirable exchange of K⁺/Na⁺ in pure NaNO₃ bath could be limited; further enhanced strengthening effect was achieved. The results might renew the interest on strengthening LD glass-ceramics by traditional ion-exchange process.     

硫酸铵法中低品位锂瓷土矿提锂尾液铷铯提取技术

研究了硫酸铵法利用宜丰中低品位[w（氧化锂）<2.0%]锂瓷土矿制备碳酸锂后的提锂母液进一步提取铷铯等贵金属的技术路线。一次碳化提锂后的母液中还含有若干溶解的铷、铯等金属的硫酸盐类,采用分步结晶法,结晶母液降温析出过程中,在温度高的前段步骤析出的是铷铯等溶解度较低的矾盐,而温度低的后段步骤析出的是钾铵等溶解度高的矾盐,从而达到铷铯矾与铵钾矾等初步分离的目的。将成矾除铝阶段硫酸铝钾、硫酸铝铵等矾盐,以焙烧分解的形式继续分出硫酸钾和氧化铝。     

Multi-component ion exchange kinetics with PAN-KCoFC composite ion exchanger

An ion exchange kinetic study was performed using PAN-KCoFC for removal of cesium ion from mixed solution of Cs, Sr, Ni and Ba ions. Uptake curves were obtained for a multi-component ion exchange system as well as binary system. A PAN-KCoFC composite ion exchanger showed higher selectivity for Cs ion over the Sr, Ba, Ni ions. A homogeneous model could predict accurately the uptake curve for both the binary and multi-component systems. Solid phase effective pore diffusivities obtained by modeling ranged between 10⁻⁵ cm₂/min and 10⁻⁶ cm₂/min.     

Nucleation inhibition despite lower glass viscosities? - Effect of the B2O3, Na2O and K2O addition on the crystallization behavior of lithium disilicate glasses

Stoichiometric lithium disilicate glasses were doped with up to 2 mol% B₂O₃, Na₂O or K₂O and the crystallization kinetics were determined as a function of the temperature using in situ hot-stage microscopy. The additives lead to a decrease of the glass viscosities in the studied temperature range with the exception of the boron-containing samples which show slightly higher viscosities in the nucleation range. While in the boron-containing samples, lithium disilicate solid solutions precipitate during thermal treatment, in the other samples a mixture of lithium disilicate, lithium metasilicate and cristobalite was detected. The steady-state nucleation rates decrease by up to one order of magnitude and especially in the case of the additive B₂O₃, the induction times for nucleation are remarkably longer. While the additives B₂O₃ and K₂O lead to a decrease of the crystal growth velocities, in case of Na₂O, the contrary effect can be noticed. Possible reasons for the observed effects are discussed.     

Stereochemistry of Electrophilic Reactions of 4-t-Butyl-1-phenylcyclohexyllithium, -sodium, -potassium and -cesium

The first three title compounds were prepared by treating the diastereomeric 4-t-butyl-1-phenyl-1-phenylthiocyclohexanes 8 or 9 (Scheme 3) with lithium, sodium, or potassium naphthalenide, respectively. The potassium derivative was also obtained by treatment of 9 with sodium/potassium alloy and the cesium derivative was similarly obtained by reaction with cesium/potassium/sodium alloy. Quenching of any of these salts with D₂O gave cis- and trans-4-t-butyl-1-phenylcyclohexane-1-d(cis- and trans- 7 -d) in an approximately 1:1 ratio. The reaction was stereoconvergent and the product ratio independent of the metal; the result is explained in terms of a diffusion controlled reaction of a planar or near-planar carbanion. In contrast, methylation with methyl iodide gave the product of axial methylation 10 in an approximately 2:1 predominance over diastereomer 11 . This result is explained in terms of Curtin-Hammett kinetics involving a stereoelectronically preferred axial approach to the electrophile. ¹³C NMR spectral studies of the lithium, potassium, and cesium salts indicate extensive delocalization of the anionic charge into the ring—somewhat more so with the K and Cs than with the Li salt. This finding supports the hypothesis that at least the K and Cs salts involve a planar carbanion moiety, whereas the Li salt may be slightly nonplanar. Both spectrally and stereochemically, the title compounds differ substantially from the earlier studied 2-phenyl-1,3-dithianes.     

Fundamental Chemistry of Cesium Extraction from Acidic Media by HCCD in FS‐13

Abstract

We previously published a model for cesium extraction from acidic media by the protonated form of the hexachlorinated derivative of the chloro‐protected cobalt bis(dicarbollide), HCCD, dissolved in trifluoromethylphenyl sulfone, FS‐13. The model indicated that Cs extraction proceeds through a series of ion‐paired and/or dissociated extraction equilibria. Additional Cs distribution ratio data has been obtained and the model refined and simplified. It is demonstrated that the equilibrium exclusively involving the exchange of proton for cesium by formation of ion‐paired CsCCD models the Cs distribution data very well, particularly for the concentrations of HCCD greater than ～0.0005 M (0.5 mM). Finally, activity corrections for the aqueous phase to the Cs distribution data results in good agreement to the theoretical value of ?1 for slope (log‐log) analysis of the data over a wide range of HNO₃ and HCCD concentrations.     

Transport numbers and ionic mobilities in nitrate melts by EMF-measurements on concentration cells with transference

The transport numbers of the cation constituents, referred to the common anion constituent, have been measured for the molten systems KNO₃ + AgNO₃, RbNO₃ + AgNO₃, and CsNO₃ + AgNO₃ over the entire range of compositions at temperatures varying from 230° to 350°C by the use of concentration cells with transference. The ionic mobilities have been calculated from the existing data of the densities and electric conductivities for these systems. For all three systems, the mobility of Ag⁺ is higher than that of the other cation in the region rich in silver nitrate while the opposite is true for the region rich in alkali nitrate. This is equivalent to the statement that the curve representing the transport numbers as a function of composition crosses the diagonal (“inversion point”). This inversion varies systematically with the temperature and the ion diameter of the alkali ion.     

影响硫酸生产催化剂低温活性的几个因素

研究了用碳化母液制备的低温硫酸催化剂中铯、铷、锂等碱金属元素的助催化作用和熔盐用量对催化剂低温活性的影响。结果表明,铯的低温助催化效果最佳,其用量宜控制在Cs／V＝0.3～0.6。当Cs／V＝0.6时,410℃的ＳＯ_２转化率达42.5%,比无铯催化剂的转化率高9.5%。铷的低温助催化作用比铯低,宜采取措施,适当提高母液的Cs／Rb。在多种碱金属元素混合使用的情况下,锂元素有较好的助催化作用,其最佳用量为Li／Ｖ＝0.4,此时410℃的ＳＯ_２转化率达46%。锂盐的添加顺序对催化剂低温活性亦有一定影响。发现活性组份含量对催化剂低温活性和高温活性影响较大,M／V宜小于4.5。     

A Review of Information on Ferrocyanide Solids for Removal of Cesium from Solutions

Abstract

Ferrocyanide solids have important applications to the removal of radioactive cesium from nuclear waste solutions. These materials are prepared by mixing soluble ferrocyanides and salts of divalent transition metals or other divalent cations. The simple precipitations most commonly give very fine particles or slimes of variable compositions. Special preparation procedures have been developed to control the compositions or to prepare granular solids suitable for column operation. The removal of cesium from solutions has been measured for many different ferrocyanide solids. Some of these solids show an exchange of K⁺, Na⁺, or NH₄ ⁺ for cesium, but many show sorptions of cesium without a true ion exchange. The performance for cesium removal is described by measurements of the distribution coefficients for cesium with large excesses of ferrocyanides, the capacity for cesium with excess cesium in solution, and the rates of cesium removal. The chemical and physical stability, the solubility, and the elution or recovery requirements for ferrocyanide solids are important to practical applications. These properties are reviewed along with several of the proposed applications.     

ESR study of the chromium oxide-alkali metal oxide systems

An ESR investigation of chromium oxide samples containing lithium, sodium, potassium, rubidium and cesium oxides has been carried out. Two kinds of defect structures attributed to Cr³⁺ vacancies connected with occluded alkali metal ions and Cr⁶⁺ ions in the α-Cr₂O₃ lattice are discussed. The observed fine structure is interpreted by means of axially symmetric spin Hamiltonian and two different values for parameter D. The possibility for ACr₃O₈ structure formation is also considered. The connection between these defect structures and the catalytic properties of the chromium catalyst is considered.     

Equilibrium Distribution Coefficients of Some Nitrate Impurities in Sodium Nitrate from Zone Refining

Abstract

The equilibrium distribution coefficients of 10 impurities (Li, K, Rb, Cs, Ag, TI, Mg, Ca, Sr, and Ba) in sodium nitrate were determined from the measured values of the effective distribution coefficients obtained by zone refining. The equilibrium distribution coefficients obtained are as follows: LiNO₃, 9.2 × 10^?2; KNO₃, 3.0 × 10^?1; RbNO₃, 2.2 × 10^?2; CsNO₃, 1.3 × 10^?3; AgNO₃, 7.8 × 10^?1; TlNO₃, 5.2 × 10^?1; Mg(NO₃)₂, 7.4 × 10^?2; Ca(NO₃)₂, 1.5 × 10^?2; Sr(NO₃)₂, 3.0 × 10^?2; and Ba(NO₃)₂, 2.1 × 10^?2. These values are in favorable agreement with those estimated from the phase diagrams and/or those calculated from the heat of solid solution.     

Influence of heat treatment temperature on the properties of the lithium disilicate-fluorcanasite glass-ceramics

This study aims to investigate the influence of heat treatment temperatures on the mechanical properties and chemical solubility (CS) of lithium disilicate-fluorcanasite glass-ceramics and to develop new dental materials. The glasses and glass-ceramics were prepared using CaF₂-SiO₂-CaO-K₂O-Na₂O-Li₂O-Al₂O₃-P₂O₅-based glass system using a conventional melt quenching method followed by a two-stage crystallization process. This two-stage method involves two heating temperature steps: first at a constant temperature (T_S1) of 600°C and second step at varying temperatures (T_S2) of 650, 700, 750, and 800°C. The crystallization behavior, phase formation, microstructure, translucency characteristic, density, hardness, fracture strength, and CS were investigated. It was found that the lithium disilicate crystal acted as the main crystalline phase, and the crystalline phase of fluorcanasite occurred at the heat treatment temperatures of 750 and 800°C. In addition, it was found that density, hardness, fracture strength, and CS increased while the translucency values decreased with increasing heat treatment temperatures. Furthermore, the CS increased dramatically when the fluorcanasite phases occurred in the glass-ceramic samples. The maximum density values, Vickers hardness, fracture toughness, and flexural strength are 2.56 g/cm³, 6.73 GPa, 3.38 MPa.m^1/2, and 259 MPa, respectively. These results may offer a possibility to design a new material for dental applications based on lithium disilicate-fluorcanasite glass-ceramics.     

Lithium Disilicate Glass‐Ceramics by Heat Treatment of Lithium Metasilicate Glass‐Ceramics Obtained by Hot Pressing

Lithium disilicate glass‐ceramics are widely used as dental ceramics due to their machinability and translucency. In this study, lithium disilicate glass‐ceramic was fabricated through heat treatment of lithium metasilicate glass‐ceramics obtained by hot pressing of glass powder composed of SiO₂–Li₂O–P₂O₅–ZrO₂–Al₂O₃–K₂O–CeO₂ at low temperature. The crystalline phase, microstructure, and mechanical properties were investigated. The results indicated that lithium metasilicate glass‐ceramic with a relative density of higher than 99% was obtained after hot pressing, and glass‐ceramic with interlocked rod‐like Li₂Si₂O₅ crystals and good flexural strength (338 ± 20 MPa) was successfully obtained through heat treatment. The two‐step method was believed to be feasible in tailoring the microstructure and mechanical properties of lithium disilicate glass‐ceramics.     

Rubidium and Cesium Ion Adsorption by a Potassium Titanium Silicate-Calcium Alginate Composite Adsorbent

In this work, a composite spherical adsorbent, which employs potassium titanium silicate as an adsorption active component, and calcium alginate as a carrier, was successfully prepared. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to characterize the adsorbent. The kinetics and thermodynamics of rubidium and cesium ions adsorption were investigated comprehensively, by considering the effects of initial concentration, temperature, solution pH, and coexisting NaCl. According to the determination coefficients, the pseudo second-order kinetic model provided an impressive and comparable correlation, and the second-order rate constant and the initial adsorption rate increase with increasing temperature. In general, the equilibrium adsorption amount increases with the increasing initial metal ion concentration, but decreases with increasing coexisting NaCl. The adsorption capacity keeps constant in the pH value range 3-12 and slightly fades when the temperature increases from 25 to 55°C. Under similar conditions, rubidium and cesium show the similar adsorption amount. The adsorbent has a fast adsorption rate and an adsorption capacity of about 1.55 mmol g^?1 for rubidium and 1.47 mmol g^?1 for cesium when the initial metal ion concentration is 0.10 mol L^?1. The composite adsorbent is effective for the adsorption of rubidium or cesium ions from simulated brines.     

Potassium hexacyanocobalt ferrate and ammonium molybdophosphate sorption bags for the removal of 137Cs from aqueous solutions and a simulated waste

In the present study, the removal and immobilization of ¹³⁷Cs from aqueous waste solutions and a simulated waste was investigated. Two inorganic ion‐exchange complexes: di‐potassium hexacyanocobalt(II)‐ferrate(II), K₂CoFe(CN)_6·xH₂O (KCFC), and ammonium‐12‐molybdophosphate [(NH₄)₃PMo₁₂O_40·aq] (AMP), were charged separately into porous nylon bags (sorption bags) of 5 µm pore diameter. The first complex (KCFC) was prepared in our laboratory. The second (AMP) was used for comparison. Easy handling and increased potential for reuse characterize the sorption bag technique. The KCFC complex was investigated by thermal gravimetric analysis (TGA) and differential thermal analysis (DTA), porosity, infra‐red (IR) and X‐ray powder diffraction (XRD). The chemical and structural investigations revealed that the KCFC complex has adequate ion‐exchange capacity and high affinity for ¹³⁷Cs. The sorption bag technique showed promising results for the removal of ¹³⁷Cs from aqueous waste solutions. The KCFC bags showed the highest ¹³⁷Cs removal (～0.91 g g^?1), at pH 8.5; AMP bags gave the highest ¹³⁷Cs removal (～0.97 g g^?1) in 0.1 N HNO₃ and 1.5 N HNO₃, both with a waste: sorbent ratio of 80 cm³ g^?1. Sorption data for both KCFC and AMP revealed a good fit to the Freundlich sorption isotherm. To assess the potential of sorption bags, the used bags were regenerated in different leachants and reused in further sorption investigations. ¹³⁷Cs recovery from the used sorbents was ～0.46 g g^?1 using 6 N HCl as leachant for AMP, compared with ～0.253 g g^?1 for KCFC using 6 N NaCl. These results indicated stronger cesium immobilization in the KCFC complex. Copyright © 2003 Society of Chemical Industry     

Strengthening of a lithium disilicate glass-ceramic by rapid cooling

In order to improve the mechanical properties, a traditional physical strengthening process was applied to a lithium disilicate (LD) glass-ceramic with a dual-phase microstructure consisting of a glassy matrix and LD crystals. The strengthening process was based on the transformation behavior of the glass-ceramic. The process was conducted by heating the glass-ceramic to a temperature below the dynamic softening point, and then rapid cooling in silicon oil (quenching). Residual stresses and mechanical properties of the glass-ceramic were investigated after the quenching. It was found that residual compressive macro-stresses could be induced in the surface layer of the LD glass-ceramic by the quenching. The residual stresses remarkably increased with increasing the quenching temperature to near the dynamic softening temperature. Compared with the corresponding annealing state, the LD glass-ceramic could be effectively strengthened and toughened by the quenching at a suitable temperature. The results displayed the strengthening possibility and potential of the LD glass-ceramic by the traditional physical process.