Very different materials are named “Glass,” with Young's modulus (E) and Poisson's ratio (ν) extending from 5 to 180 GPa and from 0.1 to 0.4, respectively, in the case of bulk inorganic glasses. Although glasses have in common the lack of long-range order in the atomic organization, they offer a wide range of structural features at the nanoscale and we show in this analysis that beside the essential role of elastic properties for materials selection in mechanical design, the elastic characteristics (E, ν) at the continuum scale allow to get insight into the short- and medium-range orders existing in glasses. In particular, ν, the atomic packing density (Cg) and the glass network dimensionality appear to be strongly correlated. Maximum values for ν and Cg are observed for metallic glasses (ν∼0.4 and Cg>0.7), which are based on cluster-like structural units. Atomic networks consisting primarily of chains and layers units (chalcogenides, low Si-content silicate, and phosphate glasses) correspond to ν>0.25 and Cg>0.56. On the contrary, ν<0.25 is associated with a highly cross-linked network, such as in a-SiO2, with a tri-dimensional organization resulting in a low packing density. Moreover, the temperature dependence of the elastic moduli brings a new light on the structural changes occurring above the glass transition temperature and on the depolymerization rate in the supercooled liquid. The softening rate depends on the level of cooperativity of atomic movements at the source of the deformation process, with an obvious correlation with the “fragility” of the liquid.

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Elastic Properties and Short-to Medium-Range Order in Glasses

In this paper, a summary of the development of high-temperature silicon nitride (T>1200°C) is provided. The high-temperature capacity of various advanced commercial silicon nitrides and materials under development was analyzed in comparison with a silicon nitride without sintering additives produced by hot isostatic pressing. Based on this model Si 3 N 4 composed of only crystalline Si 3 N 4 grains and amorphous silica in the grain boundaries the influence of various sintering additive systems will be evaluated with focus on the high-temperature potential of the resulting materials. The specific design of the amorphous grain-boundary films is the key factor determining the properties at elevated temperatures. Advanced Si 3 N 4 with Lu 2 O 3 or Sc 2 O 3 as sintering additive are characterized by a superior elevated temperature resistance caused by effective crystallization of the grain-boundary phase. Nearly clean amorphous films between the Si 3 N 4 grains comparable to that of Si 3 N 4 without sintering additives were found to be the reason of this behavior. Benefit in the long-term stability of Si 3 N 4 at elevated temperatures was observed in composites with SiC and M o Si 2 caused by a modified oxidation mechanism. The insufficient corrosion stability in hot gas environments at elevated temperatures was found to be the main problem of Si 3 N 4 for application in advanced gas turbines. Progress has been achieved in the development of potential material systems for environmental barrier coatings (EBC) for Si 3 N 4 ; however, the long-term stability of the whole system EBC-base Si 3 N 4 has to be subject of comprehensive future studies. Besides the superior high-temperature properties, the whole application process from cost-effective industrial production, reliability and failure probability, industrial handling up to specific conditions during the application have to be focused in order to bring advanced Si 3 N 4 currently available to industrial application.

Silicon Nitride for High‐Temperature Applications

The temperature dependence of the viscosity and stress-relaxation kinetics of sol–gel-derived SiOC glasses that contain up to 14 at.% carbon have been characterized in the temperature range of 1000°–1400°C. The viscosity, as determined from relaxation experiments, is in good agreement with the creep viscosity and is typically two orders of magnitude higher than the viscosity of vitreous silica. However, materials suffer from partial crystallization at >1150°C, and the precipitation of β-SiC nanocrystals induces a flow-hardening behavior and results in a dynamic increase in viscosity, especially at >1200°C and for glasses with a high carbon content.

Creep Viscosity and Stress Relaxation of Gel‐Derived Silicon Oxycarbide Glasses

Purpose: Purpose of this paper is the applying of new technology in injection moulding technique and investigation of reinforcement of PC as dispersed phase inside PP matrix (Table 1). Second aim of work is enrichment of those composites by nanoclay and analyzing mechanical behaviour of nanocomposites. Design/methodology/approach: According to design of experiments (DOE) specimens were injection moulded in the shape of rectangular bars. Additionally advanced technology of melt manipulation inside mold cavity after injection was used. To achieve this purpose Ferromatik Milacron injection moulding machine, equipped with externally controlled mold was used. Findings: Addition of nanoclay clearly presents highly reinforced system, especially for neat matrix. Evenly dispersed PC particles within PP majority show reinforcement as well. Inducement of shear rate in injection moulding radically improved absorption of energy in nanocomposite. Research limitations/implications: Different variation of material composition, such combination with other polymers and use of different reinforcements (flexible or either rigid) is required to be checked in the further work. Practical implications: Reinforcement obtained thanks to dispersed phase and nanofillers creates composites with improved mechanical properties. Originality/value: Morphology development reflects on mechanical behaviour. Its manipulation may affect and improve mechanical properties. Use of advanced technologies opens wide range of possibilities in processing of polymer based systems. At present there is limited number of research of processing-structure-properties relationships of polymer-polymer composites and nanocomposites.

Silicon nitride ceramics - review of structure, processing and properties

Thermal expansion coefficients ( α ), glass-transition ( T g ) and Littleton ( T L ) temperatures, hardness and density of the RE–Si–Mg–O–N glasses (RE=Sc, Y, La, Nd, Sm, Gd, Yb, and Lu) were investigated to separate the effects of rare-earth elements and nitrogen on the properties of bulk glasses. These properties depend approximately linearly on nitrogen content, density of the glass and ionic radius of the corresponding lanthanide. Lanthanides modify hardness of the studied oxynitride glasses with 20–24 eq.% N by 11.4–14.4%, α by ∼13%, T g by 31–35 °C, and T L by 33–43 °C. Deviations from linearity found in Nd-, Sm- and Yb-doped glasses were possibly due to valence change. Non-lanthanide dopants, Sc and Y, often do not fit the dependence but result in comparable properties. The effects of nitrogen and rare-earth dopants are independent and additive.

Thermal expansion and glass transition temperature of the rare-earth doped oxynitride glasses

The preparation details of bulk glasses in the Ln-Si-Al-O-N systems (Ln = Y, Ce, Nd, Sm, Eu, Dy, Ho and Er) containing 17 equivalent % nitrogen are reported. The properties of these oxynitride glasses are examined in detail. Changes observed in density, molar volume, hardness, thermal expansion and glass transition temperature are presented and are found to vary linearly with the cationic field strength or ionic radius of the rare earth modifier. The implications for changes in glass structure caused by the substitution of one rare earth cation for another are discussed in relation to the property changes observed.

Formation of LnSiAlON glasses and their properties

Silicon nitride-based ceramics contain oxynitride glass phases at the grain boundaries which can impair subsequent high temperature properties. Studies of bulk glasses in the Y-Si-Al-O-N system have been carried out and it has been shown that up to 10 atomic % N can be incorporated into these oxynitride glasses. Nitrogen increases the viscosity, hardness and glass transition temperature of the glasses. Heat treatments of Y-Si-Al-O-N glasses have been carried out and the crystalline phases formed are reported. Further improvements are possible if glass-ceramic processes using two-stage heat treatments are introduced. This paper reviews the development of oxynitride glasses, the effects of nitrogen on properties and reports on the glassceramic heat treatments.

Yttrium oxynitride glasses: Properties and potential for crystallisation to glass-ceramics

Thirteen glasses of the general formula (M1, M2)9.33Si14Al5.33O41.5N5.67 where M1=La or Nd and M2=Y or Er have been prepared with M1/(M1+M2) fractions of 1, 0.75, 0.5, 0.25, and 0. Data for molar volume (MV), glass compactness (C), Young's modulus (E), microhardness (H), glass transition temperatures (Tg), and dilatometric softening temperatures (Td) have been recorded. In addition, temperatures at which crystallization exotherms arise have also been determined as well as crystalline phases present after the glasses had been heat treated to 1300°C in nitrogen. The results clearly demonstrate that glass properties vary linearly with effective cation field strength (CFS) of the combined modifiers (M1, M2), which is calculated from the atomic fractions of M1 and M2 and their associated CFSs. Glass stability in both the La–Y and La–Er systems reaches a maximum at M1 and M2 fractions of 0.5 because of the relative stability of different oxynitride and disilicate phases with changes in ionic radius. Furthermore, La appears to stabilize the α polymorph of yttrium disilicate because of combined La–Y ionic radius effects.

Properties and Crystallization of Rare‐Earth Si–Al–O–N Glasses Containing Mixed Trivalent Modifiers

Nucleation and crystallization studies were conducted on a YSiAlON glass that contained 17 equiv.% nitrogen (7.5 at.%) by using a two-stage nucleation-and-growth treatment. Classical and differential thermal analysis (DTA) techniques were both used to study the crystallization process, to ensure that the optimum heat-treatment schedule that yielded a fine microstructure and minimum residual glass was applied. The optimum nucleation and crystallization temperatures were determined from DTA traces that were recorded from isothermal heat treatments at different nucleating temperatures, ranging between Tg - 40°C and Tg+ 100°C for 1 h and crystal-growth temperatures in the range of 1170°-1310°C for 0.5 h, respectively. The activation energy for the crystallization process was determined, based on the analysis of the variation of peak temperature at five different heating rates. Specimens heat treated in a tube furnace under nitrogen gas were subjected to microscopical investigation, and results showed variations in the volume fraction of crystalline phases and crystal size with nucleation temperature. The nucleation temperature, Tg+ 40°C (1025°C), which corresponded to the maximum volume fraction of crystalline phases and minimum crystal size, was consistent with the optimum nucleation temperature, Tg+ 35°C (1020°C), as determined from DTA. The time and temperature of nucleation and crystal growth dictated the nature and size of the crystalline phases. Properties such as hardness and density were assessed and correlated to the nucleation temperature. The influence of sample specific surface on the devitrification mechanisms was estimated, and bulk nucleation was observed to be the dominant nucleation mechanism.

Classical and Differential Thermal Analysis Studies of the Glass‐Ceramic Transformation in a YSiAlON Glass

The oxidation behaviour of oxynitride glasses in M-Si-Al-O-N system (M = Nd, Y) has been studied in air and under various oxygen pressures. The influence of glass composition on the oxidation resistance was studied at 1075 °C by varying Y Al or Si Al ratios and also by substituting Y by Nd and by varying the nitrogen content. The reactivities in air of the YSiAlON system is studied in the temperature range of 1000–1150 °C. The oxidation resistance of YSiAlON glasses is better than for glasses containing Nd. A pressure law is determined for the YSiAlON system based upon data from oxidation at 1050 and 1075 °C at oxygen pressures in the range of 15–100 kPa. The nature and morphology of oxidised samples were characterised by optical microscopy, XRD and SEM. These investigations reveal that the oxidation of the studied oxynitride glasses is governed by a reaction process where the progress of the internal interface is the limiting step.

E. Nestor

Papers

Formation of LnSiAlON glasses and their properties

Yttrium oxynitride glasses: Properties and potential for crystallisation to glass-ceramics

Properties and Crystallization of Rare‐Earth Si–Al–O–N Glasses Containing Mixed Trivalent Modifiers

Classical and Differential Thermal Analysis Studies of the Glass‐Ceramic Transformation in a YSiAlON Glass

Oxidation behaviour of yttrium and neodymium oxynitride glasses