Hydrostatic, uniaxial, and triaxial compression tests on unpoled "Chem-prep" PZT 95/5-2Nb ceramic within temperature range of -55 to 75 degrees C.

Sandia is currently developing a lead-zirconate-titanate ceramic 95/5-2Nb (or PNZT) from chemically prepared ('chem-prep') precursor powders. Previous PNZT ceramic was fabricated from the powders prepared using a 'mixed-oxide' process. The specimens of unpoled PNZT ceramic from batch HF803 were tested under hydrostatic, uniaxial, and constant stress difference loading conditions within the temperature range of -55 to 75 C and pressures to 500 MPa. The objective of this experimental study was to obtain mechanical properties and phase relationships so that the grain-scale modeling effort can develop and test its models and codes using realistic parameters. The stress-strain behavior of 'chem-prep' PNZT under different loading paths was found to be similar to that of 'mixed-oxide' PNZT. The phase transformation from ferroelectric to antiferroelectric occurs in unpoled ceramic with abrupt increase in volumetric strain of about 0.7 % when the maximum compressive stress, regardless of loading paths, equals the hydrostatic pressure at which the transformation otherwise takes place. The stress-volumetric strain relationship of the ceramic undergoing a phase transformation was analyzed quantitatively using a linear regression analysis. The pressure (P{sub T1}{sup H}) required for the onset of phase transformation with respect to temperature is represented by the best-fit line, P{sub T1}{sup H} (MPa) = 227 + 0.76 T (C). We also confirmed that increasing shear stress lowers the mean stress and the volumetric strain required to trigger phase transformation. At the lower bound (-55 C) of the tested temperature range, the phase transformation is permanent and irreversible. However, at the upper bound (75 C), the phase transformation is completely reversible as the stress causing phase transformation is removed

Experimental deformation of an «anhydrous » synthetic dunite

A series of constant strain-rate experiments has been conducted on a synthetic dunite at temperatures of 1100-1300° C and 15 kbar confining pressure. The specimens are stronger than in all previous studies, except those of Post ; however, one carefully dried specimen agrees approximately with the trend of Post's data. Examination by transmission electron microscopy discloses that the dislocation substructure is very similar to that of naturally deformed olivine. The substructure is dominated by tilt boundaries in (100), while tilt and twist boundaries in (001) and (010), respectively, are also present. Free dislocations are primarily in the screw orientation, and screws with b = [001] are remarkably straight suggesting that they are extended. The dominance of free screw dislocations over edges appears to be in conflict with climb-controlled creep theories. Plots of dislocation densities and recrystallized grain-sizes versus stress agree well with the results of earlier investigators.Une série d'expériences à vitesse de déformation constante a été conduite sur une dunite synthétique pour des températures de 1100° à 1300° C et à 15 kbar de pression hydrostatique. Les échantillons sont ici plus résistants que ceux de toutes les expériences antérieures, sauf celles de Post. Cependant, l'un de nos échantillons, soigneusement desséché avant expérience, donne des résultats approximativement comparables à ceux de Post. L'examen de ces échantillons en microscopie électronique à transmission montre que leur sous-structure de dislocations est très proche de celle de l'olivine naturellement déformée : elle est dominée par des sous-joints de flexion (100), mais les sous-joints de flexion (001) et de torsion (010) sont aussi présents. Les dislocations libres ont surtout une orientation vis, et les dislocations vis avec b = [001] sont remarquablement rectilignes, ce qui suggère qu'elles sont dissociées. Parmi les dislocations libres, la prédominance des vis sur les coins semble contredire les théories du fluage contrôlé par la montée. Les courbes reliant les densités de dislocations et les dimensions des néoblastes avec la contrainte sont conformes aux résultats déjà publiés par d'autres auteurs.Zeuch David H., Green Harry W. Experimental deformation of an «anhydrous » synthetic dunite. In: Bulletin de Minéralogie, volume 102, 2-3, 1979. Mécanismes de déformation des minéraux et des roches

In-situ emplacement and densification of geomaterials for sealing applications

A method and apparatus are described for forming improved seals in boreholes formed in host rock by using the apparatus to introduce a feedstock into the borehole and simultaneously subject the introduced feedstock to both compressive and shear stresses until the borehole becomes filled and sealed. These seals are primarily intended to restrict the movement of radioactive or hazardous materials into the accessible environment

Mechanical properties and shear failure surfaces of two alumina powders in triaxial compression

In the manufacture of ceramic components, near-net-shape parts are commonly formed by uniaxially pressing granulated powders in rigid dies. Density gradients that are introduced into a powder compact during press-forming often increase the cost of manufacturing, and can degrade the performance and reliability of the finished part. Finite element method (FEM) modeling can be used to predict powder compaction response, and can provide insight into the causes of density gradients in green powder compacts; however, accurate numerical simulations require accurate material properties and realistic constitutive laws. To support an effort to implement an advanced cap plasticity model within the finite element framework to realistically simulate powder compaction, the authors have undertaken a project to directly measure as many of the requisite powder properties for modeling as possible. A soil mechanics approach has been refined and used to measure the pressure dependent properties of ceramic powders up to 68.9 MPa (10,000 psi). Due to the large strains associated with compacting low bulk density ceramic powders, a two-stage process was developed to accurately determine the pressure-density relationship of a ceramic powder in hydrostatic compression, and the properties of that same powder compact under deviatoric loading at the same specific pressures. Using this approach, the seven parameters that are required for application of a modified Drucker-Prager cap plasticity model were determined directly. The details of the experimental techniques used to obtain the modeling parameters and the results for two different granulated alumina powders are presented

Continuum-based FEM modeling of ceramic powder compaction using a cap-plasticity constitutive model

Common ceramic component manufacturing typically involves the processing of the raw materials in powder form. Granulated powder is formed into a green body of the desired size and shape by consolidation, often by simply pressing nominally dry powder. Ceramic powders are commonly pressed in steel dies or rubber bags with the aim of producing a near-net-shape green body for subsequent sintering. Density gradients in these compacts, introduced during the pressing operation, are often severe enough to cause distortions in the shape of the part during sintering due to nonuniform shrinkage. In such cases, green machining or diamond grinding operations may be needed to obtain the desired final shape and size part. In severe cases, nonuniform shrinkage may even cause fracture in the parts during sintering. Likewise, density gradients can result in green bodies that break during ejection from the die or that are too fragile to be handled during subsequent processing. Empirical relationships currently exist to describe powder compaction but provide little understanding of how to control die design or compaction parameters to minimize density gradients thereby forcing the designer to use expensive and time consuming trial and error procedures. For this reason, interest has grown in developing computational tools to address this problem (Aydin et al., 1996 and Coube, 1998). The goal of the present work was to develop a general continuum-based finite element model for ceramic powder compaction that can be used to aid and guide the design and pressing of ceramic powders. Such a model can be used to improve both part and die/bag pressing design, resulting in more efficient and cost effective ways to make better parts

Raman Study of Lead Zirconate Titanate Under Uniaxial Stress

The authors used micro-Raman spectroscopy to monitor the ferroelectric (FE) to antiferroelectric (AFE) phase transition in PZT ceramic bars during the application of uniaxial stress. They designed and constructed a simple loading device, which can apply sufficient uniaxial force to transform reasonably large ceramic bars while being small enough to fit on the mechanical stage of the microscope used for Raman analysis. Raman spectra of individual grains in ceramic PZT bars were obtained as the stress on the bar was increased in increments. At the same time gauges attached to the PZT bar recorded axial and lateral strains induced by the applied stress. The Raman spectra were used to calculate an FE coordinate, which is related to the fraction of FE phase present. The authors present data showing changes in the FE coordinates of individual PZT grains and correlate these changes to stress-strain data, which plot the macroscopic evolution of the FE-to-AFE transformation. Their data indicates that the FE-to-AFE transformation does not occur simultaneously for all PZT grains but that grains react individually to local conditions