Role of CaF2 on mechanochemically synthesized leucite as dental veneering glass ceramics

Leucite based glass ceramic is widely used in dental ceramics as porcelain fused to metals for veneering applications. Main properties considered here are high coefficient of thermal expansion and good mechanical properties. Owing to these requirements, high expansion phase such as leucite is incorporated in these glass ceramics. The present work was aimed to synthesise leucite using its stoichiometric batch compositions and subsequent high energy ball milling. CaF₂ was also added in another mix to study its role on leucite formation. Further prepared, leucite phase was added in separately prepared low temperature glass frit powders to control amount of glass and leucite content. X-ray diffraction results displayed that high energy ball milling and additive promoted the formation of leucite as a major crystalline phase. Furthermore, CaF₂ also suppressed the subsidiary crystallisation of kalsilite phase. Evaluated average coefficient of thermal expansion in the temperature range of 20–500°C was very close to the theoretical value of pure leucite.     

Sol–gel based fabrication and characterization of new bioactive glass–ceramic composites for dental applications

The fabrication of a new composite glass–ceramic with potential application in dental restoration was investigated. The developed material aims to modify the surface of dental ceramics creating bioactive surfaces able to improve material–cell interaction enhancing the bonding of the marginal gap between restoration and tooth. The application of the sol–gel method led to a microporous homogeneous glass–ceramic which can be applied as coating on commercial dental ceramic substrates. The microstructural, thermal, mechanical and biological properties of the fabricated coatings were studied and compared to the respective results of a previously investigated glass–ceramic composite. The material–cell interaction on these two sol–gel dental composites was studied in detail. The attachment and proliferation of both periodontal ligament and gingival fibroblast cells confirmed the bioactive behavior of the new materials and their ability to be potentially applied in dental restorations for soft tissue regeneration and sealing of the marginal gap.     

Coatings on Glass–Ceramic Granules for Dental Restorative Biomaterials

Two methods for coating glass–ceramic granules have been tested: self-assembly in aqueous solutions and the fluidized-bed technique. Both methods have been successfully used to produce nanocoatings on leucite–fluoroapatite glass–ceramic granules. Two chemical substances were developed for the experiments: (a) a combination of inorganic and organic compounds, (b) organic compounds alone, preferably comb polymers. Both groups of materials positively influenced the sintering properties of the glass–ceramics, because they suppressed tearing. The glass–ceramics were used to produce powder compacts for veneering materials, which would be used on metal frameworks (primarily multiunit dental bridges). With regard to the processing of the aqueous slurry, coatings with inorganic and organic substances are preferred.     

Fabrication of a Glass‐Ceramic‐to‐Metal Seal Between Ti–6Al–4V and a Strontium Boroaluminate Glass

Glass‐ceramics are widely utilized in the electronics industry to provide electrical insulation and to form leak‐tight joints with a range of metals. The coefficient of thermal expansion (CTE) of the glass‐ceramic can be controlled by the extent of crystallization to reduce detrimental tensile stresses in the joint. In recent years, there has been interest in using titanium alloys, in place of stainless steels, due to their lower density and superior specific strength. In this study, the heat treatment of a strontium boroaluminate glass has been tailored to create glass‐ceramics with mean CTEs ranging from 5.7 ± 0.1 × 10^?6/K to 9.7 ± 0.1 × 10^?6/K over the temperature range 303–693 K. The resultant glass‐ceramic consists of three crystalline phases and residual glass. A glass‐ceramic with a mean CTE of 6.9 ± 0.1 × 10^?6/K was subsequently fabricated to form a compression seal with a Ti–6Al–4V housing and a preoxidized Kovar pin. Single pin assemblies were shown to be reproducible in terms of microstructure and all passed a standard helium leak test, indicating that a successful seal had been produced.     

Design of LTCC with High Thermal Expansion

New applications of low-temperature co-fired ceramics (LTCC), such as pressure sensors or integrated functional layers, require materials that possess higher coefficients of thermal expansion (CTE). To fabricate LTCC with elevated CTE, two methods of material design are examined: firstly, glass ceramic composites (GCC), which consist of >50 vol% glass in the starting powder, and, secondly, glass-bonded ceramics (GBC), where glass is added as a sintering aid only. The CTE of GBC is mainly determined by the crystalline component. For GCC, the CTE can be well predicted, if CTE and elastic data of each phase in the microstructure are known. A nonlinear characteristic of the CTE versus phase composition was found with increasing E _crystals/ E _glass ratio and absolute CTE difference between the components. The glass composition and glass amount can be used to compensate the fixed properties of a crystalline material in a desired way. However, because the CTE and permittivity of a glass cannot be chosen independently, an optimum glass composition has to be found. For a given LTCC, it is possible to control the devitrification by shifting the glass composition. In this way, the resulting CTE values can be predicted more exactly and tailoring becomes possible. Different LTCC materials, based on the crystalline compounds Ba(La,Nd)₂Ti₄O₁₂, ZrO₂ (Y-TZP), SiO₂ (quartz), and specially developed glasses, possessing an elevated CTE of around 10 × 10⁻⁶ K⁻¹ while showing permittivity ɛ_r between 6 and 63, are introduced.     

玻璃陶瓷牙科材料研制中若干重要问题的探讨

锂硅酸盐玻璃陶瓷牙科材料包括用于CAD/CAM加工的主晶相为偏硅酸锂以及热压铸造的主晶相为二硅酸锂的材料,具有高强度、优异的生物相容性、良好的透光性和美学性能以及独特的修复通用性。通过分析国内外文献和专利上报导的锂硅酸盐玻璃陶瓷牙科材料采用的玻璃成分、晶化温度制度以及析出的主晶相,讨论如何根据牙科材料性能要求来确定玻璃陶瓷的主晶相,并以此来选择玻璃成分和相应的结晶温度制度。此外还指出了锂硅酸盐玻璃陶瓷牙科材料今后研制的发展方向。     

Flexural Strength and Open Porosity of Two Different Veneering Ceramics for Zirconia Framework

The studies were designed to compare the biaxial flexural test (BFT) and 3‐point bending test (3PBT) of two kinds of veneering ceramics for zirconia framework, and then to measure the open porosity of the veneering ceramics by Archimedes method and to characterize their micro‐structures using secondary electron imaging under scan electric microscope (SEM). Flexural strength of IPS Zirpress veneering ceramic was significantly higher than IPS e.max Ceram (IPSC), no significant difference was found between BFT and 3PBT when used to investigate the same type of veneering ceramic. SEM indicated that the open porosity of IPS e.max Ceram is higher than IPS Zirpress.     

Nanoscaled Interface Between Microgold Particles and Biphase Glass‐Ceramic Matrix

This article presents a detailed study on the nanoscaled interface between microelongated gold particles (GP) and biphase leucite/feldspar glass‐ceramic matrix. The glass‐ceramic composite with a nonuniform GP distribution was processed through hot‐pressing under vacuum using a commercial dental ceramic furnace for glass‐ceramic dental crown manufacturing. Heat treatments at 900°C, 1100°C, and 1300°C were conducted, and microstructural features along the interface were used to verify the chemical reactions between GP and glass‐ceramic matrix. It was observed that the amorphous glass‐ceramic matrix had nanoscaled biphase structures, and the distributed nanoscaled amorphous leucite phase was attracted to GP during hot‐pressing, and was more reactive with GP than the feldspar phase. The thickness of the interfacial phase formed through chemical reactions between GP and glass‐ceramic matrix is around 30 nm. The chemically bonded interface has contributed significantly toward the substantial improvements in both strength and toughness of the GP‐reinforced glass‐ceramic matrix composite. Characterization techniques, including X‐ray diffraction and field‐emission scanning electron Microscopy, incorporating X‐ray microanalysis using energy dispersive spectrometry, have been employed in this study.     

Evaluation of the high temperature performance of HfB2 UHTC particulate filled Cf/C composites

Room and high temperature flexural strength and coefficient of thermal expansion (CTE) of HfB₂ ultra‐high temperature ceramic (UHTC) particulate filled C_f/C composites are determined along with UHT oxidation behavior. Both room and high temperature strength of the composites were found to be broadly comparable to those of other thermal protection system materials currently being investigated. The CTE of the composites was measured both along and perpendicular to the fiber direction up to 1700°C and the values were found to depend on fiber orientation by approximately a factor of 3. Arc‐jet testing of the UHTC composites highlighted the excellent ultra‐high temperature oxidation performance of these materials.     

SCC mixes with poorly graded aggregate and high volume of limestone filler

Nonpozzolanic fillers are frequently used to optimise the particle packing and flow behaviour of cementitious paste in self-compacting concrete (SCC) mixes. This paper deals with the influence of finely ground limestone and crushed limestone dust on the properties of SCC mixes in the fresh and hardened state. Mixes were prepared using poorly graded crushed limestone aggregate. To compensate the lack of fine material in the crushed sand, a viscosity agent (VA) was added to the mixtures. The results obtained indicate that finer and better-graded limestone dust significantly increases the deformability of the paste. When a high volume of this filler was added to the SCC mix, the required self-compacting properties were achieved at a lower water/(cement+filler) ratio, and it also appeared that the addition of filler improves the 28-day compressive strength of concrete mixes due to the filler effect and improved fine-particle packing.     

Bioceramics and their application for dental restoration

Abstract

This review article covers the historical development of ceramics, from the beginnings to the present. Feldspar based ceramic biomaterials for veneering metal frameworks, which are based on the jacket porcelain crown, have firmly established themselves in restorative dentistry since the 1970s. Currently, the development of restorative dental materials that can be used to replace metal represents a major challenge. As a result, this review will focus on the latest materials in this field. These materials include glass ceramics as well as high performance sintered ceramics. Glass ceramics exhibit more favourable optical properties, such as translucency and colour, compared with high performance ceramics, while the latter demonstrate high flexural strength and toughness. Both groups of materials have specialised applications in restorative dentistry and are capable of covering all the indications of dental restorations. The two types of materials, that is, glass ceramics and ceramics, have to be processed in accordance with their properties. As a result, the processing techniques, such as moulding, sintering and machining, will be discussed in detail in addition to the properties of the materials. Additional development possibilities for the materials will be presented on the basis of customer/patient needs and the successful long term use of glass ceramics and ceramics. In this context, it is clear that high performance ceramics and layered composites (consisting of high performance ceramics veneered with glass ceramics) offer the best possible solution for indications in the posterior region of the mouth. In contrast, glass ceramics are used to fabricate inlays and onlays for all parts of the jaw. In addition, glass ceramics can be used to fabricate crowns and small bridges to replace anterior dentition.     

High-alumina composite concretes and coatings based on modified phosphate binder suspensions

Coatings and materials based on aluminum phosphate suspensions are studied that have high strength in a raw state and are suitable for carrying out combined firing of a refractory and coating. An aluminum phosphate binder provides preparation of high physicomechanical properties in the firing temperature range. Use of traditional mixes with a protective aluminum phosphate coatings, and also ceramic concrete based on this binder provide increased material corrosion resistance, and consequently an increase in object operating life and period between repairs for metallurgical and glass melting units.     

Preparation and characterization of covalently functionalized graphene using vinyl‐terminated benzoxazine monomer and associated nanocomposites with low coefficient of thermal expansion

We describe the preparation of vinyl‐terminated benzoxazine‐functionalized graphene using free radical grafting. The resulting functionalized graphene (f‐graphene) was incorporated into bis(3‐allyl‐3,4‐dihydro‐2H‐1,3‐benzoxazinyl)methane (V‐BF‐a) monomer in order that nanocomposites could be prepared. Results of scanning electron microscopy and transmission electron microscopy revealed that the sheets of f‐graphene were well dispersed throughout the matrix, and there was a strong interfacial interaction between the f‐graphene and polyV‐BF‐a. The inclusion of f‐graphene into the nanocomposites resulted in a material with a high thermal stability and a low coefficient of thermal expansion (CTE); increasing the content of f‐graphene reduced the CTE significantly more. A reduction in the CTE of up to 48% was produced by adding just 1 wt% of f‐graphene; this corresponded to an increase of 12 °C in the glass transition temperature. These results suggest that f‐graphene nanocomposites can be ‘tuned’ to give materials with both a low CTE and a high thermal stability, and that graphene composites of this type can thus be manufactured to withstand a wider range of temperatures.     

陶瓷材料应力腐蚀特性的测试与分析

建立了玻璃与陶瓷材料应力腐蚀（ＳＣＣ）特性的有效测试方法，对Ａｌ２Ｏ３陶瓷和普通钠钙玻璃两种材料在水、甲醇和甲酰胺三种介质中的ＳＣＣ特征进行了试验研究。结果表明，Ａｌ２Ｏ３陶瓷在上述三种介质中的ｄａ／ｄｔ－ＫＩ曲线均存在平台区，而钠钙玻璃在上述介质中均不存在平台区。Ａｌ２Ｏ３陶瓷和钠钙玻璃在三种介质中的ＳＣＣ敏感性大小依次为：蒸馏水〉甲醇〉甲酰胺。理论分析认为，ＳＣＣ敏感性与介质未共享电子轨道的非     

高温烟气过滤用多孔陶瓷材料的研制

以氧化铝纤维和玻璃粉为主体材料,活性炭粉为造孔剂,通过半干压成型工艺制备了高温烟气过滤陶瓷.详细研究了玻璃粉含量、造孔剂含量以及烧成温度对材料过滤阻力、抗折强度、显气孔率等性能的影响,并对原料配方和烧成制度进行了优化,最优配方为:氧化铝70wt%、玻璃粉30wt%、外加造孔剂25wt%、羧甲基纤维素钠8wt%,最佳的烧成温度为1100 ℃,制得的高温烟气过滤陶瓷抗折强度8.9 MPa,过滤阻力95 Pa,显气孔率达59%.     

Sealing Glass‐Ceramics with Near Linear Thermal Strain,Part I: Process Development and Phase Identification

A widely adopted approach to form matched seals in metals having high coefficient of thermal expansion (CTE), e.g. stainless steel, is the use of high CTE glass‐ceramics. With the nucleation and growth of Cristobalite as the main high‐expansion crystalline phase, the CTE of recrystallizable lithium silicate Li₂O–SiO₂–Al₂O₃–K₂O–B₂O₃–P₂O₅–ZnO glass‐ceramic can approach 18 ppm/°C, matching closely to the 18 ppm/°C–20 ppm/°C CTE of 304L stainless steel. However, a large volume change induced by the α‐β inversion between the low‐ and high‐ Cristobalite, a 1^st order displacive phase transition, results in a nonlinear step‐like change in the thermal strain of glass‐ceramics. The sudden change in the thermal strain causes a substantial transient mismatch between the glass‐ceramic and stainless steel. In this study, we developed new thermal profiles based on the SiO₂ phase diagram to crystallize both Quartz and Cristobalite as high expansion crystalline phases in the glass‐ceramics. A key step in the thermal profile is the rapid cooling of glass‐ceramic from the peak sealing temperature to suppress crystallization of Cristobalite. The rapid cooling of the glass‐ceramic to an initial lower hold temperature is conducive to Quartz crystallization. After Quartz formation, a subsequent crystallization of Cristobalite is performed at a higher hold temperature. Quantitative X‐ray diffraction analysis of a series of quenched glass‐ceramic samples clearly revealed the sequence of crystallization in the new thermal profile. The coexistence of two significantly reduced volume changes, one at ~220°C from Cristobalite inversion and the other at ~470°C from Quartz inversion, greatly improves the linearity of the thermal strains of the glass‐ceramics, and is expected to improve the thermal strain match between glass‐ceramics and stainless steel over the sealing cycle.     

Flexural Strength and Viscosity of Glass Ceramic Sealants for Solid Oxide Fuel Cell Stacks

The structural integrity of the sealing material is critical for the reliability of solid oxide fuel cell stacks. In this respect failure and deformation are aspects which need to be assessed in particular for glass ceramic sealant materials. Bending tests were carried at room temperature and typical stack operation temperature for glass ceramic sealants with different crystallization levels. Elastic moduli, fracture stresses, and viscosity values are reported. In addition to sintered bars some bending testing were carried out for steel specimens that were head‐to‐head joined with the glass ceramics similar as in a stack application. The ceramic particle reinforced sealant material was screen printed onto the steel. The results reveal a decrease of the strength for the partially crystallized sealant at operation relevant temperatures that can be associated with the viscous deformation of the material.     

Investigation of nanostructure and properties of aromatic–aliphatic polyamide‐based nanocomposites with clay additives

Aromatic–aliphatic polyamide/clay nanocomposites were produced using solution intercalation technique. Surface modification of the clay was performed with ammonium salt of aromatic diamine and the polyamide chains were produced by condensation of 4‐aminophenyl sulfone with sebacoyl chloride (SCC) in dimethyl acetamide. Carbonyl chloride endcapped polymer chains were prepared by adding extra SCC near the end of polymerization reaction. The nanocomposites were investigated for organoclay dispersion, water absorption, mechanical, and thermal properties. Formation of delaminated and intercalated nanostructures was confirmed by X‐ray diffraction and TEM studies. Tensile strength and modulus improved for nanocomposites with optimum organoclay content (8 wt %). Thermal stability and glass transition temperatures of nanocomposites increased relative to pristine polyamide with augmenting organoclay content. The amount of water uptake for these materials decreased as compared with the neat polyamide. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Fabrication and characterization of bioactive glass (45S5)/titania biocomposites

Bioactive glass (BG) (45S5) has been used successfully as bone-filling material in orthopedic and dental surgery but its lean mechanical strength limits its applications in load-bearing positions. Approaches to strengthen these materials decreased their bioactivity. In order to realize the optimal matching between mechanical and bioactivity properties, bioactive glass (45S5) was reinforced by introducing titania (TiO₂) in anatase form and treated at 1000 °C to form new bioactive glass/titania biocomposites. The prepared biocomposites were assessed by XRD, FT-IR, mechanical properties and SEM. The results verified that the increase of titania percentage to BG powder enhanced gradually the mechanical data of the prepared biocomposites. SEM and FT-IRRS confirmed the presence of a rich bone-like apatite layer post-immersion on the composite surface. It has been found that the new BG/titania biocomposite materials especially those containing high content of titania have high bioactivity properties and compressive strength values comparable to cortical bone. Therefore, these biocomposite materials are promising for medical applications such as bone substitutes especially in load-bearing sites.     

Press-Molding of High-Alumina Ceramic Castables. 1. Compaction and Properties of Matrix Systems Based on Mixed HCBS of Composition: Bauxite,Quartz Glass,and Commercial Alumina

Preparation of powders based on bauxite HCBS (Highly Concentrated Ceramic Binding Suspensions) containing high-disperse quartz glass and plasticized with refractory clay is described. The powders, molded at moderate pressure (50 – 100 MPa), give a high-density material with an initial porosity of 20 – 25%. Mixes containing 10 to 43% commercial alumina are tested. The materials can be used as matrices for press-molded high-alumina ceramic castables which, when sintered, show a high mechanical strength and volume constancy. __________ Translated from Novye Ogneupory, No. 5, pp. 39 – 44, May, 2005.