v-K-data for silica from interrupted lifetime measurements

Different methods were applied so far in order to determine subcritical crack growth for silica. Mostly, fracture mechanics standard tests with macro cracks were used for this purpose. In this report, we evaluated the subcritical crack growth curves from interrupted lifetime tests on silica bending specimens containing small natural flaws. The resulting v-K-curve showed crack growth rates down to 10$^{-14}$ m/s indicating a threshold for subcritical crack growth at K$_{th}$$\approxeq$0.31 MPa$\sqrt{m}$ In the plot of v=f(K/K$_{Ic}$) slight material differences could be eliminated and suitable agreement with macro-crack results by Wiederhorn and Bolz [1] on DCB-specimens and Michalske et al. [2] on DCDC-specimens could be stated

Electric field-assisted ion exchange strengthening of borosilicate and soda lime silicate glass

In this study, we investigate the effects of electric field-assisted ion exchange (EF-IE) on potassium for sodium ion exchanges of soda borosilicate and soda lime silicate glasses. The results show that applying an electric field (E-field) with the intensity of 1000 V cm-1 for few minutes produces an exchanged layer with a thickness comparable to the conventional chemical strengthening for 4 hours. There is a critical E-field that increases the mobility and, therefore, the diffusion coefficient of the potassium ions in the glasses. The increase is, perhaps, related to the evolution of the glass structure due to the penetration of potassium ions under an E-field. Vickers indentations showed that strong compression is generated in the glass by EF-IE; however, the bending strength improvement is limited because of the presence of large surface defects and the stress distribution inhomogeneit

Thermal behaviour of geopolymer materials: Influence of temperature on mineralogical composition and microstructure of sodium-potassium polysialates

DSC, TGA, XRD, SEM and mechanical tests were used to characterize geopolymer materials based on Na2O-K2O-Al 2O3-SiO2 system up to 1300°C. For all compositions, sodium silicate was used as binder. The samples of geopolymer which were cured at room temperature demonstrated dimensional stability up to 1200°C, with exception of samples with K which was stable up to 1300°C, retaining their predominant x-ray-amorphous tetrahedral Al and Si network (polysialates). The four point flexural and compressive strengths test demonstrated a slight decrease of mechanical properties up to 700°C followed by an exponential increase up to the melting temperature

Bi-axial four points flexural and compressive strength of geopolymer materials based Na2O-K2O-Al2O 3-SiO2 systems

Bi-axial four point flexural and compressive strength tests were used for the assessment of mechanical properties of geopolymer material-based, calcined kaolin and kaolinitic clays. Various activating solutions which consist in the mixture of potassium and sodium hydroxide, water and sodium silicate were designed and tested. Six specimens, over a wide number of investigated compositions, were selected with SiO2/Al2O3 varying from 1:1 to 3:1, The compositions 1:1 and 2:1 were obtained by using two different grades of kaolin as raw materials (standard and sand-rich), while the 3:1 was obtained by adding required amounts of silica to either kaolin or kaolinitic clay, All the samples were prepared by slip casting, using density (≈1,5 g/cm3) as indicator of the optimum viscosity for shaping geopolymer pastes, and cured at room temperature for different periods. The bi-axial four points flexural strength values vary from 13 to 21 MPa while the compressive strength vary from 45 to 67 MPa, being essentially influenced by curing time, SiO2/Al2O3 and K 2O/Na2O/H2O ratios, Increasing the SiO 2/Al2O3 ratio from 1:1 to 3:1, the mechanical properties increase but longer setting and curing times were required, Low SiO2/Al2O3 ratio results the appearance of micro cracks and deformations during curing. The flexural and compressive behaviour of the specimens studied were directly correlated to the porosity, density and the final product microstructure. The chemical behaviour of the six compositions is discussed with respect to basic dissolution-hydrolysis- polycondensation processes that occur in Na2O-K2O-Al 2O3-SiO2 systems

Correlation between microstructural evolution and mechanical properties of a-quartz and alumina reinforced K-geopolymers during high temperature treatments

Geopolymer material based K 2O-Al 2O 3-SiO 2 systems were investigated for high temperature applications. High viscous geopolymer paste was prepared using standard grade metakaolin dissolved in potassium hydroxide/potassium silicate solution (volume ratio, 1 : 1). The alkalinity of the solution was predetermined to optimise the dissolution of amorphous aluminosilicate. Quartz sand and alumina (25, 50 and 75 wt-%) were successively introduced as filler to form refractory concretes. RGPS4, RGPS2 and RGPS34 for sand and RGPA4, RGPA2 and RGPA34 for alumina were produced. The samples obtained were cured in sealed plastic containers for 7 days at room temperature before sintering at 200, 800, 1000, 1100 and 1200°C. The heating rate was 5°C min -1, and the dwelling time at peak temperature completes the sintering time to 4 h. Unvaried mechanical strength, up to 800°C, was observed: ~25 MPa for biaxial and ~20 MPa for uniaxial four-point bending strengths. This behaviour was ascribed to the maintenance of the structure of the geopolymer materials within the temperature interval considered. Above this temperature, all the samples presented significant densification; the most important increase in mechanical strength was ~65 and ~60 MPa respectively for biaxial and uniaxial four-point bending strengths. This was accompanied by a decrease in porosity. The dense structures observed at 1000-1100°C were affected by a further increase in temperature. Microcracks, vitrification and deformation appeared at 1200°C, which can be considered as detrimental to the mechanical properties. The mechanical properties and the microstructural evolution were found to be influenced by the amount of fillers added. Si and Al rich geopolymer concretes were found promising for the development of potential thermoresistant materials through the cold process. © 2012 Institute of Materials, Minerals and Mining

Conventional and field-assisted sintering of nanosized Gd-doped ceria synthesized by co-precipitation

Gadolinium-doped ceria is an attractive electrolyte for potential application in SOFCs operating at intermediate temperature; for such use, the fundamental compositions typically contain 10–20 mol% Gd2O3. In this work, we produced nanosized 10 mol% gadolinium-doped ceria powder by co-precipitation, starting from Ce and Gd nitrate solutions and using ammonia solution as precipitating agent. The co-precipitate was characterized by DTA-TG, TEM, XRD and nitrogen adsorption analyses. We studied the behavior of the nanopowder under both conventional and Flash sintering. Very different behavior was seen: the conventional sintering cycle produced a poorly densified material, while Flash sintering allowed production of a perfectly densified material, with uniform sub-micrometric grain size

Ion exchange process: History, evolution and applications

Aim of this paper is to present a review on some aspects and applications of ion exchange process in glasses, ferroelectric and polymers in the fields of optics, nanotechnology, gas sensors and chemical strengthening. The formation of nanoparticles in ion-exchanged glasses, as effect of ion or laser irradiation, is discussed. A discussion on the potentialities of ion exchange process in comparison to ion implantation in optical devices and nanotechnology is also introduced. Analytical techniques applied to the study of the ion exchange process are illustrated. The studies of ion exchange process in "Natural materials" constitute the content of a specific paragraph, for applications in water cleaning. Some initial considerations on the "old age" of this technique are introduced

Design of inorganic polymer cements: Effects of matrix strengthening on microstructure.

The effects of bulk composition and microstructure on strengthening mechanisms and fracture resistance were investigated for metakaolin based geopolymers. Crack initiation and propagation under mechanical and environmental stresses were correlated to pore size distribution and bulk composition. Based on IR spectra and porosity analysis, it was observed that capillary pores and the nature of alkali-aluminosilicate hydrated named polysialates (H\u2013M\u2013A\u2013S) determine cracks initiation site in the Al-rich geopolymer samples with low Si/Al molar ratio (range 1.23\u20131.5). These samples exhibited low flexural strength, Young modulus and Impact toughness as well as delayed failure under environmental stresses. Increasing the Si/Al molar ratio (range 1.79\u20132.42) improved the flexural strength and Impact toughness. Formation of more polymerized H\u2013M\u2013A\u2013S phases contributed to strengthening the matrices and hinder axial cracks; consequence of toughening mechanisms developed by the coarsening on molecular scale of H\u2013M\u2013A\u2013S