Constrained Sintering of Low-Temperature Co-Fired Ceramics

This paper discusses the effect of uniaxial compressive stress and pressureless constraint on the microstructure, density, and shrinkage anisotropy during the sintering of two commercial low-temperature co-fired ceramic (LTCC) systems, i.e., Heraeus CT2000 (CT) and DuPont 951Tape (DU). Under uniaxial compression, the ratio of axial to transverse shrinkage of DU is significantly higher than that of CT. A simple linear viscous theory was used to estimate the change in the strain rates produced by the external stress and the stress required to produce zero shrinkage. The theory was found to overestimate the measured stress-induced strain rates. The uniaxial compressive stress required for zero overall shrinkage was estimated to be ∼60 kPa for DU and 80 kPa for CT. The estimate for the DU materials was in good agreement with the experimental data, but there was significant deviation for the CT material. Higher viscosity and higher constraining stresses led to lower densities in pressure-less constrained CT specimens compared with DU.     

Bending Creep Test to Measure the Viscosity of Porous Materials during Sintering

A bending creep test is proposed for measuring the change in viscosity of a porous material during densification. Equations, based on simple beam deflection theory, were derived to obtain the viscosity from a series of loading experiments using rectangular samples of different densities. By measuring the deflection in the center of the specimen between the spans, the viscosity of a porous material during densification can be measured. Experiments with porous Y₂O₃-stabilized ZrO₂ beams were used to illustrate the bending creep test. Consistent with theory the viscosity increased from 50 to 400 GPa·s as the sample densified from 87% to 98% density. The rapid increase in viscosity was considered to be a result of both densification and grain growth.     

Axonal and nonneuronal cell responses to spinal cord injury in mice lacking glial fibrillary acidic protein

We have examined the regeneration of corticospinal tract fibers and expression of various extracellular matrix (ECM) molecules and intermediate filaments [vimentin and glial fibrillary acidic protein (GFAP)] after dorsal hemisection of the spinal cord of adult GFAP-null and wild-type littermate control mice. The expression of these molecules was also examined in the uninjured spinal cord. There was no increase in axon sprouting or long distance regeneration in GFAP-/- mice compared to the wild type. In the uninjured spinal cord (i) GFAP was expressed in the wild type but not the mutant mice, while vimentin was expressed in astrocytes in the white matter of both types of mice; (ii) laminin and fibronectin immunoreactivity was localized to blood vessels and meninges; (iii) tenascin and chondroitin sulfate proteoglycan (CSPG) labeling was detected in astrocytes and the nodes of Ranvier in the white matter; and (iv) in addition, CSPG labeling which was generally less intense in the gray matter of mutant mice. Ten days after hemisection there was a large increase in vimentin+ cells at the lesion site in both groups of mice. These include astrocytes as well as meningeal cells that migrate into the wound. The center of these lesions was filled by laminin+/fibronectin+ cells. Discrete strands of tenascin-like immunoreactivity were seen in the core of the lesion and lining its walls. Marked increases in CSPG labeling was observed in the CNS parenchyma on either side of the lesion. These results indicate that the absence of GFAP in reactive astrocytes does not alter axonal sprouting or regeneration. In addition, except for CSPG, the expression of various ECM molecules appears unaltered in GFAP-/- mice.     

Effects of rho-chloroamphetamine on locomotor activity and brain 5-hydroxyindoles

The conversion of 14C-maltose into glucose, lactate and 14 CO2 was studied in perfused livers from fed and fasted rats and in isolated hepatocytes. Maximal glucose production was 30 mM x g-1 x h-1; half-maximal rates were found with 3 mM maltose. About 0.01 % of the radioactivity infused was recovered as 14CO2. The addition of maltose had no effect on rates of oxygen consumption, lactate production or ketogenesis. The data suggest that maltose did not serve as a major substrate for biosynthetic or energy producing processes under the conditions of the perfused rat liver.     

Aqueous slip casting of transparent yttrium aluminum garnet (YAG) ceramics

Transparent YAG ceramics were prepared by slip casting an aqueous dispersed mixture of commercial Al₂O₃ and Y₂O₃ powders. The powders were co-dispersed with poly(acrylic acid) and citric acid. Polyethylene glycol of 0.5 wt.% (PEG 4000) and 0.5 wt.% tetraethyl orthosilicate were added as binder and a sintering aid, respectively. Dried samples were vacuum sintered at 1800 °C for 16 h. In general, YAG ceramics cast from Newtonian suspensions were optically transparent and had optical transmittances >80% from 340 to 840 nm. Slightly flocculated dispersions, as evidenced by higher viscosity and non-Newtonian rheology, resulted in translucent samples with large pores and lower optical transmittances.     

Effective Density Characterization of Soot Agglomerates from Various Sources and Comparison to Aggregation Theory

Soot particle (black carbon) morphology is of dual interest, both from a health perspective and due to the influence of soot on the global climate. In this study, the mass-mobility relationships, and thus effective densities, of soot agglomerates from three types of soot emitting sources were determined in situ by combining a differential mobility analyzer (DMA) and an aerosol particle mass analyzer (APM). High-resolution transmission electron microscopy was also used. The soot sources were diesel engines, diffusion flame soot generators, and tapered candles, operated under varying conditions. The soot microstructure was found to be similar for all sources and settings tested, with a distance between the graphene layers of 3.7–3.8 Å. The particle specific surface area was found to vary from 100 to 260 m²/g. The particle mass-mobility relationship could be described by a power law function with an average exponent of 2.3 (±0.1) for sources with a volatile mass fraction <10% and primary particle sizes of 11–29 nm. The diesel exhaust from a heavy duty engine at idling had a substantially higher volatile mass fraction and a higher mass-mobility exponent of 2.6. The mass-mobility exponent was essentially independent of the number of primary particles in the range covered (N_pp = 10–1000). Despite the similar exponents, the effective density varied substantially from source to source. Two parameters were found to alter the effective density: primary particle size and coating mass fraction. A correlation was found between primary particle size and mass-mobility relationship/effective density and an empirical expression relating these parameters is presented. The effects on the DMA-APM results of doubly charged particles and DMA agglomerate alignment were investigated and quantified. Finally, the dataset was compared to three theoretical approaches describing agglomerate particles’ mass-mobility relationship.

Copyright 2013 American Association for Aerosol Research     

Tailoring particle alignment and grain orientation during tape casting and templated grain growth

The quality of crystallographic alignment in textured ceramics produced by tape casting and templated grain growth (TGG) has been little studied despite its demonstrated impact on magnetic, piezoelectric, and optical properties. Physical and crystallographic alignment of anisotropic template particles is shown to be directly linked to the casting rate, gap height, and casting viscosity during tape casting. These parameters are shown to affect the shape and magnitude of the shear rate profile under the doctor blade during casting which in turn causes a gradient in the torque acting on anisotropic particles. The magnitude of the torque, the time the slurry is exposed to torque during casting, and the ratio of casting height to template diameter are demonstrated to enable the particle alignment process to be tailored to produce well-aligned template particles. Crystallographic alignment of the textured ceramic was quantified by grain misalignment angle (full width at half maximum, FWHM) and degree of orientation (r) and is directly correlated with the degree of torque during casting. High-quality alignment (FWHM = 4.5°; r = 0.13) was demonstrated in the model TGG system consisting of submicrometer alumina and 5 vol% 11 μm diameter template platelet particles.     

Development of Textured Mullite by Templated Grain Growth

Highly textured mullite was obtained by enhancing anisotropic grain growth by TiO₂ doping and by templating grain growth on oriented acicular mullite seed particles in a mullite precursor. Upon heating, the mullite precursor crystallized and densified to an equiaxed microstructure of 1-2 µm mullite grains at which time the mullite seed particles grew rapidly in the length direction ( c -axis) to produce a highly textured microstructure. By changing the seed (template) particle concentration, a range of oriented microstructures and anisotropic grains could be produced.