Early Stages Of Densification Research Articles

Fundamental mechanical properties of ground switchgrass for quality assessment of pellets

Pelletization is one of the promising technologies for biomass densification. Many studies have been conducted to determine the optimum conditions for pelletizing. However, no relationship between the mechanical properties of the ground biomass and pellet quality has been published. The goal of this research was to develop and validate a predictive relationship between mechanical properties of the ground biomass in early stages of densification (<100kPa) and the quality metrics of pellets. Switchgrass was ground with two different screen sizes (3.175mm and 6.35mm) and conditioned at two different levels of moisture content (17.5%, and 20%, w.b.). Hydrostatic triaxial compression (HTC) tests were performed using the Cubical Triaxial Tester (CTT) to determine bulk modulus, compression index, and spring back index below 100kPa. Pellets were formed with ground biomass using a farm-scale pellet mill and diametral tensile strength, axial compressive strength, pellet density, and durability were determined. Regression equations between pellet quality and mechanical property having an R2 value higher than 0.80 were selected for prediction. The validation against quality of pellets, which were formed at an alternate condition, confirms that the strength quality metrics of switchgrass pellets can be predicted using size reduced materials' bulk modulus and spring-back index.

A new heating route of spark plasma sintering and its effect on alumina ceramic densification

Alumina ceramics were prepared by spark plasma sintering (SPS). Effects of manipulating the SPS power output on densification, microstructure and flexural strength of the obtained alumina ceramics were investigated. The results showed that use of a few discrete high amplitude units as the heating power instead of the low continuous conventional power output for heating significantly improved compaction of the alumina particles in the early stage of densification, hence more homogeneous microstructures in the consolidated materials. The better flexural strength of the resultant alumina ceramics indicated the higher efficiency of the SPS heating power rearrangement.

Microstructural characterization of spark plasma sintered boron carbide ceramics

Fully dense boron carbide specimens were fabricated by the spark plasma sintering (SPS) technology in the absence of any sintering additives. Densification starts at 1500 °C and the highest densification rate is reached at about 1900 °C. The microstructure of the ceramic sintered at 2200 °C, with heating rates in the 50–400 °C/min range, displays abnormal grain growth, while for a 600 °C/min heating rate a homogeneous distribution of finely equiaxed grains with 4.05 ± 1.62 μm average size was obtained. TEM analysis revealed the presence of W-based amorphous and of crystalline boron-rich B 50N 2 secondary phases at triple-junctions. No grain-boundary films were detected by HRTEM. The formation of a transient liquid alumino-silicate phase stands apparently behind the early stage of densification.

Sintering of glass matrix composites with small rigid inclusions

We investigated the effect of dispersed crystalline particle volume content Φ on sintering of glass matrix composites (GMC) for low-temperature co-fired ceramics (LTCC) applications. Such composites typically consist of alumo-borosilicate glass and α-Al 2O 3 powders of similar average particle size ( D 50 ≈ 3 μm). Sintering shrinkage was observed by dilatometry and heating microscopy and was backed up by glass viscosity measurements. Microstructure analysis revealed that α-Al 2O 3 particles do neither show significant dissolution into the liquid phase nor detectable crystallization throughout LTCC firing schedules. Therefore, in this study α-Al 2O 3 particles were treated as small rigid inclusions. It was found that Φ lowers the shrinkage rate of GMC. While the lowering is small for small Φ and at the early stage of densification it progressively increases during sintering, and final shrinkage shifts up to 170 K to higher temperatures for Φ = 0.45. The behaviour observed could be explained assuming that sintering is controlled by the effective viscosity, which progressively increases non-linearly during densification due to the gradually wetting of the surface area of corundum particles. We could demonstrate that Al 2O 3 cluster can cause residual pores and reduce the attainable shrinkage. The reduction of attainable shrinkage is found to depend on Φ 3, reaching about 8% at Φ = 0.45.

Effects of Rare‐Earth (RE) Intergranular Adsorption on the Phase Transformation, Microstructure Evolution, and Mechanical Properties in Silicon Nitride with RE2O3+MgO Additives: RE=La, Gd, and Lu

Silicon nitride powders most often consist primarily of the α phase, which transforms into the β phase during the densification and microstructural evolution of Si3N4 ceramics. The temperature at which the transformation initiates in the presence of a combination of MgO and RE2O3 densification additives is found to decrease with increasing atomic number of the rare earth (RE). This trend coincides with the predicted and observed decrease in the affinity of the RE to segregate to and absorb on the prism planes of hexagonal prism‐shaped β grains with an increase in the atomic number of the RE. When RE adsorption is diminished, Si (and N) attachment on the smooth prism planes is enhanced, which increases diametrical growth rates, normally reaction‐rate limited by an attachment mechanism. Combined with the typically fast [0001] growth, it is this augmented grain growth that contributes toward the initiation of the α–β transformation at lower temperatures. With the enhanced transformation, observations reveal an increase in the number of β grains growing in the early stages of densification. On the other hand, increased RE adsorption leads to greater growth anisotropy, resulting in the formation of higher aspect ratio grains. Thus, Lu2O3 generates larger diameter, yet elongated, reinforcing grains, while La2O3 results in reinforcing grains of a higher aspect ratio. The Gd2O3 additive transformation and microstructural characteristics lie intermediate to those of the lanthanide end‐member elements. Despite these differences, a substantial fraction of large reinforcing grains were found for each additive composition. As a result, the mechanical properties of the resultant ceramics are similar with flexure strengths in excess of 1 GPa, fracture toughness values &gt;10 MPa·m1/2 at room temperature, and excellent strength retention (&gt;800 MPa) at 1200°C.

Densification Behavior of Calcium Phosphates on Spark Plasma Sintering

Ceramics of hydroxyapatite (Ca10(PO4)6(OH)2: HA) and β-tricalcium phosphate (β-Ca3(PO4)2: β-TCP), were prepared by spark plasma sintering (SPS) at the temperatures from 800 °C to 1000 °C for 10 min with a heating rate of 25 °C·min-1. The HA ceramics prepared at 900 °C and 1000 °C showed transparency. On the other hands, transparent β-TCP ceramics was obtained by SPS at 1000 °C. In analysis of the densification behavior during sintering of HA and β-TCP by SPS, dominant sintering mechanism was plastic flow in the early stage of densification. Transparent ceramics should be the most suitable materilas to investigate the interface between human cells and ceramics.

Variation of sintering parameters at an early stage of densification affecting β-Si 3N 4-microstructure

The influence of sintering parameters at an early stage of densification on the evolution of a bimodal microstructure in Si 3N 4 ceramics was investigated. Commonly two different methods are pursued to design a bimodal Si 3N 4 microstructure: (i) annealing at a later sintering stage ( T > 1850 °C) initiating β-Si 3N 4 grain growth via Ostwald ripening and (ii) seeding with β-Si 3N 4 nuclei, which abnormally grow during the liquid-phase sintering process. In this study, a third and novel method to design Si 3N 4 microstructures by affecting intrinsic nucleation phenomena at an early sintering stage is presented. In order to study the influence of sintering parameters on β-Si 3N 4 nuclei formation during the early stage of densification, temperature and pressure were systematically changed. Starting from identical green bodies (identical processing and doping), the variation of the sintering parameters affected intrinsic β-Si 3N 4 nucleation. This procedure allows variation in the fineness of the matrix as well as in the number and dimension of the large elongated β-Si 3N 4 grains embedded in the matrix. Since identical green bodies are used as starting material, the resulting microstructure can easily be tailored toward corresponding application needs.

Effect of resin impregnation methods at the early stage of densification on the impregnation efficiency, microstructure, and thermal stability of carbon–carbon composites

AbstractThe effect of impregnation methods and heat‐treatment temperature at the early stage of densification on the impregnation efficiency, microstructure, and thermal stability of carbon–carbon composites prepared from carbon–phenolic green composites was studied. The results suggest that simultaneous application of pressure and heat provides better impregnation performance during densification at the early stage of carbon–carbon composite fabrication than ultrasonic impregnation and vacuum infiltration impregnation. Also, optical microscopic examination strongly supports the result of the impregnation efficiency obtained using three different resin impregnation methods. An additional heat‐treatment at 2000 °C after carbonization results in better thermal stability and a denser microstructure of the fiber and matrix of the composite. For the carbonized composites, including a fully cured furfuryl alcohol resin impregnant, the weight loss measured by a thermogravimetric method quantitatively agrees with the weight loss occurred in a carbonization furnace at the corresponding temperature. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 183–192, 2002

Morphology and Sinterability of Thermally Treated Carbothermally Synthesized Silicon Nitride Powders

Carbothermally produced alpha‐Si3N4 powder is thermally treated (1400°‐1500°C for 4‐12 h) in nitrogen‐gas to determine the impact of thermal treatment on powder composition, particle size, morphology, and sinterability. Thermal treatment reduces residual carbon and oxygen content and reduces internal porosity (lower surface area), which results in a rounding of the particles. The lower oxygen content of the heat‐treated, carbothermally produced alpha‐Si3N4 powder allows Y2O3‐Al2O3‐SiO2 sintering formulation to be adjusted so that an improvement in the early stage of densification can be achieved.

The effect of density anisotropy on the yielding and flow behavior of partially consolidated powder compacts

The effect of density anisotropy on the flow behavior of partially consolidated powder compacts of an alpha-two titanium aluminide alloy (Ti-24A1-11Nb) was established by conducting hot compression tests on samples made by hot die pressing or hot isostatic pressing (“hipping”). For the former consolidation method, in which anisotropic density distributions were developed, the upset tests were conducted both parallel and perpendicular to the die pressing direction. The flow stress results from these experiments, as well as those from the tests on the “hipped” material, were interpreted in terms of calculations of the stress intensification factor (φ). When the stress intensification factors were plotted as a function of areal relative density, rather than volumetric relative density, a unique dependence of φ on density was obtained. This dependence showed a smooth transition from a behavior measured by others for the early stages of densification to a behavior theoretically predicted for the latter stages of densification, in which isolated pores are closed.

Constrained Densification of Alumina/Zirconia Hybrid Laminates, I: Experimental Observations of Processing Defects

Various forms of damage were observed in pressure‐less‐sintered Al2O3/ZrO2symmetric laminates and asymmetric laminates (bilayers) fabricated by tape casting and lamination. These defects included channel cracks in the ZrO2 layers, Al2O3 edge‐effect cracks parallel to the layers, delamination in the Al2O3layers, and debonding between the Al2O3and ZrO2layers. Based on detailed microscopic observations, the defects were attributed to sintering rate and thermal expansion mismatch between the layers. Cracks or cracklike defects were formed in the early stages of densification, and these cracks either opened during sintering or acted as preexisting flaws for thermal expansion mismatch cracks. Consequently, the extent of cracking could be reduced or even eliminated by decreasing mismatch stresses during the sintering and cooling stages. This can be accomplished by reducing the heating and/or cooling rates or by adding Al2O3in the ZrO2layers. The sintering mismatch stresses were estimated from the degree of curling in asymmetric laminates and from layer viscosities that were obtained by cyclic loading dilatometry. The measured curvature was an indication of the mismatch in sintering strain between Al2O3and ZrO2and were consistent with the dilatometric data that were obtained for the component layers.

97/00494 On the properties of washed straw

Pelletization is one of the promising technologies for biomass densification. Many studies have been conducted to determine the optimum conditions for pelletizing. However, no relationship between the mechanical properties of the ground biomass and pellet quality has been published. The goal of this research was to develop and validate a predictive relationship between mechanical properties of the ground biomass in early stages of densification (< 100 kPa) and the quality metrics of pellets. Switchgrass was ground with two different screen sizes (3.175 mm and 6.35 mm) and conditioned at two different levels of moisture content (17.5%, and 20%, w.b.). Hydrostatic triaxial compression (HTC) tests were performed using the Cubical Triaxial Tester (CTT) to determine bulk modulus, compression index, and spring back index below 100 kPa. Pellets were formed with ground biomass using a farm-scale pellet mill and diametral tensile strength, axial compressive strength, pellet density, and durability were determined. Regression equations between pellet quality and mechanical property having an R2 value higher than 0.80 were selected for prediction. The validation against quality of pellets, which were formed at an alternate condition, confirms that the strength quality metrics of switchgrass pellets can be predicted using size reduced materials' bulk modulus and spring-back index.

Pressureless Co-Sintering of Al2O/ZrO2 Multilayers and Bilayers

AbstractVarious types of damage were observed in pressureless-sintered Al2O3/ZrO2 symmetric laminates (multilayers) and asymmetric laminates (bilayers) fabricated by tape casting and lamination. These defects included channel defects in ZrO2-containing layers, Al2O3 surface defects parallel to the layers, decohesion between the layers, and transverse damage within the Al2O3 layer in the bilayers. Detailed microscopic observation attributed the defects to a combined effect of mismatch in both sintering rate and thermal expansion coefficient between the layers. Crack-like defects were formed in the early stages of densification, and these defects acted as pre-existing flaws for thermal expansion mismatch cracks. Curling of the bilayers during sintering was monitored and the measured rate of curvature change, along with the layer viscosities obtained by cyclic loading dilatometry, was used to estimate the sintering mismatch stresses. The extent of damage could be reduced or even eliminated by decreasing the difference in layer sintering rate. This was accomplished by reducing the heating rates or by adding Al2O3 in the ZrO2 layers.

Crack Growth and Damage in Constrained Sintering Films

The constrained sintering of films on substrates leads to a reduction in densification rate and may lead to processing flaws. This paper reports on a study of damage and cracking in sintering films, with particular emphasis on the growth of preexisting cracks. Experiments have been conducted with glass and polycrystalline Al2O3 films on various substrates. The effect of important variables (viz., film thickness, crack length, and friction with the substrate) on crack growth is reported. The experiments with glass films show that cracking occurs above a critical film thickness which is in good quantitative agreement with a recent analysis for this problem. In the case of Al2O3 films, we observe a diffuse damage zone ahead of cracks. Crack growth occurs by the coalescence of microcracks with each other and with the main crack. Some possible reasons for this difference between the glass and Al2O3 films are presented. As a model for diffuse damage, the stability of a sintering film under spatial variations in constitutive parameters is analyzed. It is shown that the film is unstable to small perturbations only in the early stages of densification, and that for viscous sintering the films are usually kinetically stable.

Microstructure and Densification of Sintered (B+C)-Doped β-Silicon Carbide

AbstractThe role of boron and carbon during densification of sintered β-SiC was investigated through the combined approach of in-situ dilatometry and CTEM/AEM inspection of TEM-foils referring to well-defined densification events. Preliminary data obtained indicate that in the early stages of densification, boron is not enriched in the continuous carbon-rich surface layer covering the β-SiC powder particles nor does it segregate to internal interfaces in high quantities. Small boron quantities are dissolved in the SiC grains- Simultaneously with the β- to α- phase transformation, decomposition of foam-like B4C aggregates releases small B4C particles, which are bound intragranularly to α-SiC.

Change of internal/pore structure in alumina green body during initial sintering

A novel characterization method was applied to study the morphological changes of large pores in a ceramic green body during the early stage of densification at 1100°C for 1–64 h. Large processing pores were present in the green body. They were preserved after densification up to a relative density of 83.9% with their shape and size unchanged significantly. The pore size distribution determined by mercury porosimetry showed the growth of matrix pores of small size, but failed to show the change of large processing pores with densification.

Sintering of Platelike Iron(III) Oxide Powders

Packing and sintering of platelike hematite powders were studied with a special emphasis on pore behavior, and the results were compared with those for acicular powders. Platy particles gave a relatively high green density of 65% of the theoretical value, but did not achieve high fired densities. This is in strong contrast to acicular powders, in which high fired densities were reached from a relatively low green density under the same firing conditions. Features observed with platy particles have been interpreted as being due to the wedgelike pores which tend to close at the early stages of densification.

Direct Evidence on the Elimination of Pore by the Applied Pressure in Hot Isostatic Pressing of Alumina

ABSTRACTRemoval of natural pore during capsule hot isostatic pressing (HIP) and normal sintering were examined directly with the novel liquid immersion technique, which can reveal details of micro-structure and characterize flaw-forming defects of microns-size in lightly densified specimens. The results were supplemented by the pore size measurement with the mercury porosimetry, microstructural examination with SEM as well as microstructural examination of specimens after final densification. The behaviors in both pore size and shape changes were different in HIP and normal sintering. Removal of large pores of microns-size started from the beginning of densification in HIP. Whereas large pores grow with densification in normal sintering. Large pores changes their shape drastically in HIP, but hardly changes in normal sintering. The behavior of pore in the early stage of densification is closely related to the microstructure of densified specimen; without HIP, large defects of processing origin are present in near-fully dense specimen. These observations were discussed in terms of current sintering theory.

Dynamics of Weakly Connected Solids: Sintering of Polymeric Aerogels

ABSTRACTA combination of small angle scattering (SAS) and neutron spin-echo (NSE) spectroscopy is used to characterize the structure and dynamics of polymeric silica aerogels during sintering. The SAS data indicate that densification at short length scales precedes the densification at longer scales (comparable to that of the pore structure). Interpreted within the fracton model, the NSE data are consistent with an initial decrease in connectivity during relatively early stages of densification.

Hot‐Pressing of MgO with NaF

The results of using NaF additives in hot‐pressing MgO are described and compared with the results when LiF is used. To obtain nearly theoretical density, the temperatures required for hot‐pressing with NaF are ∼100°C higher than those required with LiF. Specimens made with NaF were generally translucent after hot‐pressing or subsequent annealing. Sodium fluoride inhibits grain growth and can produce a duplex grain structure, whereas LiF enhances grain growth during annealing. Both NaF and LiF wet MgO and lubricate it well. The present study and other investigations of fluoride densification aids indicate that the formation of a liquid phase, with resultant wetting and lubrication, is important, especially in the early stages of densification. Possible solution of anions and cations is discussed.