To improve the mechanical properties and performances of water-atomized powder metallurgy steels, it is necessary to enhance the density. Consolidating water-atomized steel powders via conventional pressing and sintering to a relative density level > 95 pct involves processing challenges. Consolidation of gas-atomized powders to full density by hot isostatic pressing (HIP) is an established process route but utilizing water-atomized powders in HIP involves challenges that result in the formation of prior particle boundaries due to higher oxygen content. In this study, the effect of density and processing conditions on the oxide transformations and mechanical properties from conventional press and sintering, and HIP are evaluated. Hence, water-atomized Cr–Mo-alloyed powder is used and consolidated into different density levels between 6.8 and 7.3 g cm−3 by conventional die pressing and sintering. Fully dense material produced through HIP is evaluated not only of mechanical properties but also for microstructural and fractographic analysis. An empirical model based on power law is fitted to the sintered material properties to estimate and predict the properties up to full density at different sintering conditions. A model describing the mechanism of oxide transformation during sintering and HIP is proposed. The challenges when it comes to the HIP of water-atomized powder are addressed and the requirements for successful HIP processing are discussed.
Construction mortars contain a broad variety of both inorganic and organic additives beside the cement powder. Here we present a study of tile mortar systems based on portland cement, quartz, methyl cellulose and different latex additives. As known, the methyl cellulose stabilizes the freshly prepared cement paste, the latex additive enhances final hydrophobicity, flexibility and adhesion. Measurements were performed by solid state nuclear magnetic resonance (NMR) and low voltage scanning electron microscopy (LVSEM) to probe the influence of the latex additives on the hydration, hardening and the final tile mortar properties. While solid state NMR enables monitoring of the bulk composition, scanning electron microscopy affords visualization of particles and textures with respect to their shape and the distribution of the different phases.
Within the alkaline cement paste, the poly(vinyl acetate) (VAc)-based latex dispersions stabilized by poly(vinyl alcohol) (PVA) were found to be relatively stable against hydrolysis. The influence of the combined organic additives methyl cellulose, poly(vinyl alcohol) and latexes stabilized by poly(vinyl alcohol) on the final silicate structure of the cement hydration products is small. But even small amounts of additives result in an increased ratio of ettringite to monosulfate within the final hydrated tile mortar as monitored by 27Al NMR. The latex was found to be adsorbed to the inorganic surfaces, acting as glue to the inorganic components. For similar latex water interfaces built up by poly(vinyl alcohol), a variation in the latex polymer composition results in modified organic textures. In addition to the networks of the inorganic cement and of the latex, there is a weak network build up by thin polymer fibers, most probably originating from poly(vinyl alcohol). Besides the weak network, polymer fibers form well-ordered textures covering inorganic crystals such as portlandite. 相似文献
Synthetic α - and β -Hopeite, two polymorphs of zinc phosphate tetrahydrates (ZPT) have been synthesized by hydrothermal crystallization
from aqueous solution at 20 ∘C and 90 ∘C respectively. Aside from their subtitle crystallographic differences originating from a unique hydrogen bonding pattern,
their thermodynamic interrelation has been thoroughfully investigated by means of X-Ray diffraction (XRD) and differential
scanning calorimetry (DSC), combined with thermogravimetry (TGA-MS). Using a new heterogeneous step-reaction approach, the
kinetics of dehydration of the two forms of ZPT was studied and their corresponding transition temperature determined. Low
temperature DRIFT, FT-Raman and 1H, 31P MAS-NMR reveal an oriented distortion of the zinc phosphate tetrahedra, due to a characteristic hydrogen bonding pattern
and in accordance with the molecular tetrahedral linkage scheme of the phosphate groups. Biogenic Hydroxyapatite (HAP) and
one of its metastable precursors, a calcium dihydrogen phosphate dihydrate (DCPD) or Brushite were also obtained and used
to underline the resulting variations of chemical reactivity in zinc phosphates. 相似文献
Summary: In construction, polymer fibers are commonly applied beside steel, glass and mineral fibers to improve material's flexibility to shear stress. As in other composite systems, there are compatibility problems present between the fibers and the cement due to the different chemical natures and the different thermal expansion coefficients of the cement and the polymers. Within this study the interactions between two Portland cements and polymer fibers were investigated by SEM and solid‐state NMR spectroscopy. To improve the wetting ability of the polymer fibers by the cement matrix, redispersible latex powders were successfully applied to improve the adhesion between the cement matrix and the fibers. Within this study, several solid‐state nuclear magnetic resonance (NMR) spectroscopy methods, detecting 1H, 13C, 27Al and 29Si nuclei, and scanning electron microscopy (SEM) were applied. Thus, cement pastes, inorganic additives and organic admixtures could be monitored individually.
SEM images of the interface between poly(propylene) fibers and Portland cement, hardened and hydrated in the presence of a 2 wt.‐% poly[(vinyl acetate)‐co‐ethylene] latex. 相似文献