Experimental study and numerical simulation of the dissolution of blast furnace slag in alkaline solution

Geopolymers are formed by the reaction between an alkaline activator and an aluminosilicate precursor. As any cement-based materials, the reaction starts from the dissolution of aluminosilicate precursor in the alkaline solution. The dissolution kinetics of aluminosilicate precursor determines the reaction kinetics and strength development of geopolymers. In this study, the dissolution of blast furnace slag in alkaline solution will be investigated experimentally and numerically. In the experimental program, 0.1 gram of blast furnace slag is dissolved in 200 mL of alkaline solution. During the dissolution experiments, the solution is sampled at set time intervals up to 2 hours. The sampled solution is subject to the analysis by the means of inductively coupled plasma-optical emission spectrometry (ICP-OES), by which the concentrations of Ca, Si and Al are determined as functions of time. By studying the concentrations of Ca, Si and Al with time, the influence of the alkalinity of alkaline activator on the dissolution of blast furnace slag are investigated and the dissolution rates of Ca, Si and Al are determined. Furthermore, the relationship between the alkalinity of alkaline solution and the dissolution rates are studied, and the dissolution rate constants are determined. In the numerical simulation, the dissolution of blast furnace slag in alkaline solution is simulated using real-shape particles of slag [1]. The irregular shape of blast furnace slag is characterized by spherical harmonic series [2]. The Lattice Boltzmann method is used to simulate the transport of aqueous ions and the dissolution rate constants obtained by the experiments are used as input to model the dissolution of blast furnace slag. The numerical model is first applied to simulate the dissolution of blast furnace slag carried out in the experimental program. After validation with the experimental results in terms of elemental concentrations with time, this numerical model is used to study the influence of slag chemistry and temperature on the dissolution of blast furnace slag in alkaline solution. Reference [1] Y. Zuo, Z Qian, E.J. Garboczi, G Ye, Numerical simulation of the initial particle parking structure of cement/geopolymer paste and the dissolution of amorphous silica using real-shape particles, submitted to Cement Concrete Res. [2] E.J. Garboczi, Three-dimensional mathematical analysis of particle shape using X-ray tomography and spherical harmonics: Application to aggregates used in concrete, Cement Concrete Res, 32 (2002) 1621-1638

Direct Measurement of Piezoelectric Response around Ferroelectric Domain Walls in Crystals with Engineered Domain Configuration

We report the first investigation of the piezoelectric response on a nanoscale in the poled ferroelectric crystals with engineered configuration of domains. Piezoresponse force microscopy of tetragonal 0.63PMN-0.37PT relaxor-based ferroelectric crystals reviled that the d33 piezoelectric coefficient is significantly reduced within the distance of about 1 um from the uncharged engineered domain wall. This finding is essential for understanding the mechanisms of the giant piezoresponse in relaxor-based crystals and for designing new piezoelectric materials

Optically Isotropic and Monoclinic Ferroelectric Phases in PZT Single Crystals near Morphotropic Phase Boundary

We report the finding of unusual scale-dependent symmetry below the ferroelectric Curie temperature in the perovskite Pb(Zr1-xTix)O3 single crystals of morphotropic phase boundary compositions. The crystals of tetragonal symmetry (from x-ray diffraction experiments) on sub-micrometer scale exhibit a macroscopic (optically determined) cubic symmetry. This peculiar optical isotropy is explained by the anomalously small size of tetragonal ferroelectric domains. Upon further cooling the crystals transform to the phase consisting of micrometer-sized domains of monoclinic Cm symmetry.Comment: 7 pages, 3 figure

Hysteresis in acoustic properties of ferroelectric relaxor Pb[(Zn1/3Nb2/3)0.955Ti0.045]O3 single crystals studied by Brillouin and dielectric spectroscopies

Acoustic and dielectric properties of Pb[(Zn1/3Nb2/3)1–xTix]O3 (PZN-xPT) single crystals with x=0.045 have been studied by the high-resolution micro-Brillouin scattering and dielectric spectroscopy in a wide temperature range. The softening of the Brillouin shift and the increase of dielectric relaxation time upon cooling indicated the formation of polar nanoregions (PNRs) and the slowing down of their dynamics. In contrast to the acoustic properties of typical model relaxors such as lead magnesioniobate, the change in the Brillouin shift near its minimum became sharper on heating compared to the change on cooling, pointing to the clear existence of hysteresis in the dynamics of the diffuse phase transition in PZN-4.5%PT. Since the number of PNRs will increase upon cooling, it may be expected that the kinetics of the phase transition would become slower, the lower the transition temperature resulting in the more sluggish, broad feature of the Brillouin shift observed during cooling. This result may indicate that the number and size of polar nanoregions, which are dependent on temperature, play an important role in the development of the mesoscopic ferroelectric order in PZN-4.5%PT

Polar domain structural evolution under electric field and temperature in the (Bi0.5 Na0.5)TiO3 -0.06BaTiO3 piezoceramics

Lead‐free bismuth sodium titanate and related compounds are of great interest as promising candidates for piezoelectric applications. However, the full understanding of this family of materials is still a challenge partly because of their structural complexity and different behaviors with or without the application of an external electric field. Here, piezoresponse force microscopy is used to gain insight into the mesoscopic‐scale domain structure of the morphotropic phase boundary (MPB) composition of (1‐x)Bi0.5Na0.5TiO3‐xBaTiO3 solid solution at x = 0.06 (abbreviated as BNT‐6BT). The evolution of the domains with the changes of the electric field and temperature has been thoroughly examined in conjunction with the crystal structure analysis and dielectric studies. It is found that ferroelectric domains with size of hundreds of nanometers are embedded in a relaxor state without visible domains on a mesoscopic scale, which are considered to contribute to the tetragonal and cubic phases in the material, respectively. Temperature‐independent domain configuration is observed in the unpoled sample from room temperature to 200°C. While, temperature‐dependent domain configuration is observed in the poled sample. The homogenously poled state breaks into the mixed domain configuration containing polydomain structure and invisible state around the so‐called depoling temperature. The structural changes on different length scales are also discussed. This work provides an in‐depth understanding of the structural and domain changes under an electric field and the temperature‐dependent domain evolution in both unpoled and poled states in the BNT‐BT solid solution of the MPB composition

Impact of quenched random fields on the ferroelectric-to-relaxor crossover in the solid solution (1−x)BaTiO3−xDyFeO3

Lead-based perovskite relaxor ferroelectrics are widely used as materials for numerous applications due to their extraordinary dielectric, piezoelectric, and electrostrictive properties. While the mechanisms of relaxor behavior are disputable, the importance of quenched (static) random electric fields created at nanoscale by the disordered heterovalent cations has been well recognized. Meanwhile, an increasing amount of scientific and technological efforts has been concentrated on lead-free perovskites, in particular, solid solutions of classical ferroelectric BaTiO 3 (BT), which better meet ecological requirements. Among BT-based solutions the homovalent systems are elaborately studied where strong random electric fields are absent, while the solubility limit of heterovalent solutions is typically too low to fully reveal the peculiarities of relaxor behavior. In this paper, we prepare a perovskite solid solution system (1 − x )Ba 2 + Ti 4 + O 3 − x Dy 3 + Fe 3 + O 3 (0 x 0 . 3) and study it as a model heterovalent lead-free system. We determine crystal structure, ferroelectric, and dielectric properties of ceramics in a wide range of temperatures and concentrations, construct a phase diagram, and find and analyze the concentration-induced crossover from normal ferroelectric to relaxor behavior. We demonstrate that quenched random electric fields of moderate strength promote the ferroelectric-to-relaxor crossover, but do not change qualitatively the peculiarities of relaxor behavior, while strong enough fields destroy the relaxor state, so that the material becomes an ordinary linear dielectric. The experimental results are compared with the predictions of known theories of relaxor ferroelectricity

Structure of Pb(Fe2/3W1/3)O3 Single Crystals with Partial Cation Order

Despite intensive studies on the complex perovskite Pb(Fe2/3W1/3)O3 (PFWO) relaxor, understanding the exact nature of its multifunctional properties has remained a challenge for decades. In this work we report a comprehensive structural study of the PFWO single crystals using a combination of synchrotron X-ray diffraction and high-resolution electron microscopy. The set of {h + ½, k + ½, l + ½} superlattice reflections was observed for the first time based on single-crystal synchrotron X-ray experiments (100–450 K) and transmission electron microscopy investigations, which indicates some kind of B-cation ordering in PFWO which had been thought to be totally disordered. It was found that (1) the crystal structure of PFWO should be described by a partly ordered cubic perovskite (i.e. Fm − 3m), (2) the weak ferromagnetic properties and excess magnetic moment of PFWO can be understood based on non-random distribution of Fe cations between the 4a and 4b sites, and (3) the Pb displacement disorder is present in this material and the cations are probably displaced along the <100> directions. The X-ray diffraction results of this investigation show that partial cation ordering indeed exists in PFWO, which makes it necessary to revisit the generally accepted interpretations of the results obtained up to date. In agreement with X-ray diffraction study the main results of TEM study include: (1) a long range order that can be described with the Fm − 3m symmetry is reliably detected, (2) the coherence length of that long range order is in the order of 1–2 nm and (3) no remarkable chemical inhomogeneity is found in the tested PFWO crystal, excluding the possibility of a compositional ordering arising from substitutional defects in the perovskite structure