Two-step polarization reversal in biased ferroelectrics

Polarization reversal in polycrystalline ferroelectrics is shown to occur via two distinct and sequential domain reorientation steps. This reorientation sequence, which cannot be readily discriminated in the overall sample polarization, is made apparent using time-resolved high-energy x-ray diffraction. Upon application of electric fields opposite to the initial poling direction, two unique and significantly different time constants are observed. The first (faster time constant) is shown to be derived by the release of a residual stress due to initial electrical biasing and the second (slower time constant) due to the redevelopment of residual stress during further domain wall motion. A modified domain reorientation model is given that accurately describes the domain volume fraction evolution during the reversal process.open1

Predictive Model of Setting Times and Compressive Strengths for Low-Alkali, Ambient-Cured, Fly Ash/Slag-Based Geopolymers

The effects of curing temperature, blast furnace slag content, and Ms on the initial and final setting times, and compressive strengths of geopolymer paste and mortars are examined. The present work demonstrates that ambient-cured geopolymer pastes and mortars can be fabricated without requiring high alkalinity activators or thermal curing, provided that the ratios of Class F fly ash (40–90 wt%), blast furnace slag (10–60 wt%), and low alkalinity sodium silicate (Ms = 1.5, 1.7, 2.0) are appropriately balanced. Eighteen mix designs were assessed against the criteria for setting time and compressive strength according to ASTM C150 and AS 3972. Using these data, flexible and reproducible mix designs in terms of the fly ash/slag ratio and Ms were mapped and categorised. The optimal mix designs are 30–40 wt% slag with silicate modulus (Ms) = 1.5–1.7. These data were used to generate predictive models for initial and final setting times and for ultimate curing times and ultimate compressive strengths. These projected data indicate that compressive strengths >100 MPa can be achieved after ambient curing for >56 days of mixes of ≥40 wt% slag

Development of Ambient-Cured Low-Alkali High-Strength Geopolymers

The present work demonstrates that ambient-cured geopolymer pastes and mortars can be fabricated without the need for high alkalinity activators or heat curing, provided the ratio of Class F fly ash (40-90 wt%), blast furnace slag (10-60 wt%), and low-alkalinity sodium silicate (Ms = 1.5, 1.7, 2.0) is appropriately balanced. These compositions were characterised in terms of the setting time and compressive strength. These data were used to generate predictive models for the initial and final setting times as well as for the ultimate curing time and ultimate compressive strength. These projections indicate that compressive strengths in excess of 100 MPa can be achieved with geopolymer compositions with &#8805;40 wt% slag after ambient curing of 56 days.The effects of curing temperature (25°C, 40°C, 60°C for 24 h), Ms value (1.5, 1.7, 2.0), and slag content (10, 20, 30, 40 wt%) on the compressive strength development (1, 7, 14, 28 days) of the geopolymer mortars were analysed. These data were used to generate predictive models for 28-day compressive strength as a function of curing temperature and slag content at fixed Ms value. While the 1-day compressive strength was dependent largely on the curing temperature, the slag content dominated the 28-day strength. The ratio of the 1-day and 28-day compressive strengths as a function of curing temperature, Ms value, and slag content allows prediction of the maximal possible curing temperature of 65°C, above which no further advantage is to be gained. This predictive model also revealed that cold-weather casting of <10°C would extend excessively the setting time of the geopolymers. These data also allow prediction of the 28-day compressive strength based on only the 1-day compressive strength.The preceding data were interpreted in terms of the structural aspects of the amorphous geopolymer pastes produced from blends of blast furnace slag (BFS) and fly ash (FA) using X-ray diffraction (XRD) and 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR). The compressive strength development and underlying reaction kinetics of mortars with equivalent paste compositions were dependent principally on the formation of an amorphous analogue of anorthite (CAS2), which forms the (C)-N-A-S-H network. Phase equilibria and microstructural analyses of the crystalline phases of the C2AS-CAS2-NAS2-CS subsolidus phase assemblage elucidated that the N-A-S-H and (C)-N-A-S-H networks caused slow and fast mechanisms, respectively in forming the amorphous anorthite analogue. Data for 29Si MAS NMR demonstrated that the compressive strength correlates directly with the amount of amorphous anorthite analogue, which is attributed to its low Gibbs standard free energy. Data for 27Al MAS NMR support the view that ordering of the amorphous anorthite analogue contributed to the compressive strength development.The preceding was supplemented with equivalent data for geopolymers produced from blast furnace slag (BFS) or fly ash (FA) alone. The results showed that the geopolymerisation reaction rate was dependent on the structural complexity of the silicate species present initially in the raw materials. The anhydrous BFS consisted mainly of low-complexity silicate species, with the major species being an amorphous analogue of gehlenite (C2AS; Q1(1Al)), while the anhydrous FA contained silicate species of higher complexity (Q3 + Q4(1Al) + Q4), which represent an amorphous aluminosilicate analogue. The geopolymer based on BFS (BFS-G) revealed the formation of an amorphous analogue of anorthite (CAS2), which is the basis for the C-N-A-S-H network. Further, an amorphous analogue of wollastonite (CS) was detected by NMR. On the other hand, the geopolymer based on FA (FA-G) formed an amorphous analogue of nephelite (NAS2), which is the basis for the N-A-S-H network. Compositional mapping of the geopolymer gel matrix showed that geopolymerisation with BFS-G was more compositionally homogeneous than that with FA-G developments, which is a reflection of the fast and slow reaction mechanisms, respectively