Search

A novel type of mechanical presses design driven by three servo motors in parallel is proposed to develop high stamping capacity based on the theory of generalized function sets, and then is improved to enhance its practicability in engineering practice. The inverse and forward kinematics analyses are conducted later. Based on the fuzzy probability theory, the computing method of fuzzy reliability of kinematic accuracy of slider is then obtained. Finally, with a set of optimized length parameters of the presses given, the fuzzy reliability is calculated, on the basis of which a typical deep-drawing process is simulated.

This article presents the results of research on one case of employment for an individual with autism in a Chinese city. The research investigated the context of this employment, including family and local leaders' and coworkers' responses to it. Interviews and participant observation were conducted and almost daily work notes collected. Two major themes resulted. First, community efforts led to employment for an individual with autism providing a situation that was seen by all as mutually beneficial, to both the individual and her family as well as to the Community and local government, the Street Office. Second, despite this opportunity provided by the government, the mother's role continued to be essential and primary, including a main goal of making sure her daughter with autism could adapt herself to the coworkers' expectations and job requirements.

The effect of Al2O3 and Nb2O5 additions on the microstructure and thermal expansion behavior of tantalum (V) oxide has been studied. Both singly doped and co-doped compositions were examined. Compositions with 3 wt% Al2O3 (or greater), contained AlTaO4 as a second phase. Complete solubility was observed for Nb2O5 contents in the range 1–5 wt%. It was found that doping with Al2O3 results in a decrease of the coefficient of thermal expansion (CTE), such that increasing amounts of Al2O3 (1–5 wt%) cause a successive decrease in the CTE value. In the case of Nb2O5 additions, the result is an increase in the CTE of the tantalum (V) oxide. However, in the range 1–5 wt% Nb2O5, the CTE value is relatively insensitive to the amount of Nb2O5 added. Due to the countervailing influences of these two additive oxides, it was demonstrated that co-doping with Al2O3 and Nb2O5 is an effective strategy for tailoring the CTE of tantalum (V) oxide.

Tantalum (V) oxide (Ta2O5) has potential applications as part of an environmental barrier coating system for Si3N4-based turbine components. However, at elevated temperatures, Ta2O5 undergoes a phase transformation from the orthorhombic (β) phase to the tetragonal phase (α), which is undesirable because of the associated volume change. The purpose of the present work was to study the effect of alumina additions (0–5 wt%) on the β to α transformation temperature, and associated modifications to the Ta2O5 microstructure. Sintered microstructures were characterized using SEM (scanning electron microscopy), and XRD (X-ray diffraction) was used to identify the phases present at room temperature. It was found that for undoped Ta2O5, transformation of the low-temperature β-phase begins at ∼1300°C, and leads to extensive microcracking of the sintered sample. For samples containing alumina, an increase in the transformation temperature was observed. The solubility limit of alumina in Ta2O5 was between 1 and 3 wt%; for samples in which this was exceeded, the AlTaO4 second and phase particles were seen to be highly effective at inhibiting grain growth.

Mullite and mullite/ZrO2 ceramics were fabricated starting from Si/Al2O3 and Si/Al2O3/ZrO2 powder mixtures, which were mixed and attrition milled with TZP balls in water. Isopressed powder compacts were subjected to a heat treatment in air, during which the Si was oxidized to SiO2. At } 1410°C, reaction between Al2O3 and SiO2 occurred, resulting in mullite (3Al2O3·2SiO2). Depending on the composition of the starting powders, the end product was either single-phase mullite or a mullite composite. The reaction process was monitored by thermogravimetry and dilatometry. It was found that the microstructure and mechanical properties of the reaction-formed mullite ceramics were significantly improved by ZrO2 additions.

Fused deposition of ceramics (FDC) is one of the developing solid freeform fabrication (SFF) techniques. The successful production of high performance ceramics by the FDC process requires that no defects exist in the green parts. However, build defects, such as missing roads, poorly bonded layers or sub-perimeter voids can be encountered in improperly built FDC parts. In this study, a method known as WIP'ing (warm isostatic pressing) was evaluated for its ability to eliminate existing defects in GS44 Si 3 N 4 green FDC parts. Analogous to CIP'ing (cold isostatic pressing), the green FDC parts were rubber bagged and loaded into a pressure chamber filled with water soluble oil at different temperatures, ranging from 30 to 90°C, at pressures of up to 35 MPa. X-Ray radiography results indicated that at temperatures above 70°C, WIP'ing was effective in closing the gaps of the intentionally placed void defects in FDC parts. However, WIP'ing above 70°C was not effective in healing the defects completely. The fracture strengths of FDC parts with intentional added defects, WIP'ed above 70°C were substantially lower than control samples.

The RBAO (Reaction Bonded Aluminum Oxide) process and water-based gel casting were combined to produce alumina platelet reinforced RBAO composites. In the RBAO process, a precursor powder mixture of Al metal and Al2O3 is reacted to form 100% ceramic. To obtain composites in which the platelets are highly aligned, tapes consisting of 10 vol.% homogeneously distributed alumina platelets and 90 vol.% RBAO precursor powder were made by water-based gel casting using an extended doctor blade arrangement. Pellets of textured composites were subsequently prepared by stacking sheets cut from the green tapes, and filter pressing. Randomly oriented platelet composites were made using the same starting powders and gelling procedure, but without the tape-casting step. Reaction bonding was carried out in a box furnace in air. Microstructures of the composites were characterized using x-ray diffraction (XRD) and scanning electron microscopy (SEM). The effect of platelet orientation on contact damage behavior was studied using a Hertzian indentation method. It was found that the textured composite exhibited improved damage resistance compared to the random composite.

The technology of reaction bonding Al,O, (RBAO) can be modtjied by the use of St’-containing additives to yield low-to-zero shrinkage mullite composites. In the present work, SC particles were added to the Al/Al,Oj precursor mixture. During air heat-treatment, first Al oxidizes to Al,O, at 300-900°C thereafter Sic converts to SiO, (900-1200°C). Both phases form mullite (3Al,O,.SiO,) at temperatures >14OO”C. Depending on the hold time at 900612OO”C, the extent of Sic oxidation, hence the ratio of mullite to dispersed Sic can be controlled. Since both oxidation reactions and the mullitization are associated with volume expansions, the sintering shrinkage can either be fully or partially compensated for. The process parameters amount of Al and Sic, green density and degree of Sic oxidation can be utilized to fabricate low-to-zero shrinkage mullite composites.

Based on the RBAO technology, low-shrinkage mullite/SiC/Al[sub 2]O[sub 3]/ZrO[sub 2] composite were fabricated. A powder mixture of 40 vol% Al, 30 vol% Al[sub 2]O[sub 3], and 30 vol% SiC was attrition milled in acetone with TZP balls which introduced a substantial ZrO[sub 2] wear debris into the mixture. The precursor powder was isopressed at 300--900 MPa and heat-treated in air by two different cycles resulting in various phase ratios in the final products. During heating, Al oxidizes to Al[sub 2]O[sub 3] completely, while SiC oxidizes to SiO[sub 2] only on its surface. Fast densification (at > 1300 C) and mullite formation (at 1400 C) prevent further oxidation of the SiC particles. Because of the volume expansion associated with the oxidation of Al (28%), SiC (108%), and the mullitization (4.2%), sintering shrinkage is effectively compensated. The reaction-bonded composites exhibit low linear shrinkages and high strengths: shrinkages of 7.2%, 4.8%, and 3%, and strengths of 610, 580, and 490 MPa, corresponding to compaction pressure of 300, 600, and 900 MPa, respectively, were achieved in samples containing 49--55 vol% mullite. HIPing improved significantly the mechanical properties: a fracture strength of 490 MPa and a toughness of 4.1 MPa[center dot]m[sup 1/2] increased to 890more » MPa and 6 MPa[center dot]m[sup 1/2], respectively.« less

Important parameters controlling the reaction-bonding of Al2O3 (RBAO) process are Al content, particle size, and green density. Successful fabrication of high-strength bodies requires fine and homogeneous powders. Low milling intensity does not lead to the required particle fineness, whereas overmilling causes extensive oxidation and hydrolyzation of Al. Oxidation of Al during milling is enhanced by ZrO2 additions. As much of the Al should be oxidized by solid/gas reaction as possible. Milled RBAO precursor powder contains physically-adsorbed and chemically-bonded (aluminum hydroxide) water. Decomposition of chemically-bonded water promotes cracking in the temperature range 450–550°C. Bloating of RBAO occurs when the surface region becomes dense before complete decomposition of aluminum hydroxide in the interior of the body.

As an alternative to conventional Al 2 O 3 manufacturing, a novel technique, reaction bonding of aluminum oxide (RBAO), has been successfully developed. Although the technique is in its infancy, it has already demonstrated interesting characteristics, such as high green strength without organic additives, low-to-zero shrinkage tailorability, superior mechanical properties, superplastic transformability and broad microstructural versatility. One of the important technical potentials is to use RBAO as a matrix for large-scale second-phase particles, e.g. platelets and fibers, without causing the harmful residual stresses normally encountered with shrinking matrix materials.

Mullite ceramics were fabricated according to the recently developed reaction-bonded Al2O3 (RBAO) technology. Green compacts consisting of mechanically alloyed Al, SiC, and Al2O3 were heat-treated in two steps. During the first hold at 1200°C, Al and SiC were oxidized to form Al2O3 and SiO2. On further heating, mullite was formed which then sintered during the second hold at 1550°C. All reactions involved in the process were associated with volume expansions that almost compensated for the shrinkage on sintering. Processing details and microstructure development are discussed. Reaction-bonded mullite ceramics exhibit high fracture strength, e.g., 290 MPa at a density of 97% of theoretical density.

Mullite ceramics were fabricated according to the recently developed reaction-bonded Al2O3 (RBAO) technology. Green compacts consisting of mechanically alloyed Al, SiC, and Al2O3 were heat-treated in two steps. During the first hold at 1200°C, Al and SiC were oxidized to form Al2O3 and SiO2. On further heating, mullite was formed which then sintered during the second hold at 1550°C. All reactions involved in the process were associated with volume expansions that almost compensated for the shrinkage on sintering. Processing details and microstructure development are discussed. Reaction-bonded mullite ceramics exhibit high fracture strength, e.g., 290 MPa at a density of 97% of theoretical density.

The reaction-bonded Al 2 O 3 (RBAO) process is applied to fabricate open porosity Al 2 O 3 -based composites with SiC and Al 2 O 3 particulate inclusions. These are then gas-pressure infiltrated with liquid aluminum. The Al-infiltrated composites exhibit strongly improved mechanical properties, e.g. fracture toughness and bond strength of samples containing 30 vol.% 13 μm diameter Al 2 O 3 platelets are enhanced 1·6 to 5·8 MPa√m and from 85 to 760 MPa, respectively. In all cases, crack bridging by ductile Al ligaments is the main toughening mechanism. Filling of void defects, caused by particulate agglomeration, with Al is especially effective in reducing the strength-controlling flaw size.

SrTiO3 was investigated as an alternate seed material to grow Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) ferroelectric single crystals by seeded polycrystal conversion. Fully dense polycrystalline samples of PMN–32 mol% PT doped with 3 vol% excess PbO were top-seeded with (111) SrTiO3 substrates. Annealing for 10 h at 1150 °C resulted in growth of PMN–32PT single crystals with sizes on the order of several millimeters. Orientation imaging microscopy confirmed that the grown crystal exhibited the same crystallographic orientation as that of the SrTiO3 seed. Elemental distributions analyzed using energy dispersive spectroscopy indicated that interdiffusi on of the relevant elements was negligible.

A novel method of manufacturing strong Al{sub 2}O{sub 3} bodies with low-shrinkage is presented. Isopressed compacts of attrition-milled Al/Al{sub 2}O{sub 3} mixtures with 35 and 50 vol pct Al are heat-treated in two steps at 900-1150 C and 1200-1450 C, respectively. Fully reaction-bonded (oxidized and sintered) Al{sub 2}O{sub 3} (RBAO) ceramics exhibit a homogeneous distribution of fine (greater than 1 micron) closed pores. The respective strengths are relatively high, e.g., 350 MPa at 93 pct of theoretical density. Possible mechanisms operating during the reaction-bonding are discussed.

A novel reaction forming technique, reaction bonding of aluminum oxide (RBAO), has been successfully developed at TUHH. By adjusting processing parameters, such as composition, compaction pressure, and reaction bonding cycle, conditions for obtaining zero-shrinkage can be selected. RBAO therefore serves as an ideal matrix for the incorporation of second phase particles, e. g., platelets and fibers. The present paper focuses on net-shape forming of RBAO composites. Their microstructure and mechanical properties will be presented.

A novel method of manufacturing strong Al2O3 bodies with low (

Densification mechanisms during the sintering of CaO-stabilized ZrO2 have been studied using the technique of hot pressing. The dependence of the densification rate on applied pressure has been found to be linear suggesting that the kinetics are controlled by a diffusion process. The densification rate undergoes a maximum value at a composition of approximately 12 m/o CaO; this is close to the value at which the ionic conductivity maximum also occurs. Since this composition coincides with a phase boundary in this system, parallel experiments have been carried out using the CeO2/Y2O3 system in which no corresponding phase boundary occurs. The similar dopant dependence for the conduction and densification processes in these oxides suggests that the fast carrier (responsible for conduction) and the slow carrier (responsible for densification) depend in a similar way on the additive content. This in turn suggests that densification is controlled by the diffusion of cation interstitials and further that the maximum is caused by increasing degrees of defect association at high dopant contents.

A series of ceramic-metal composites was prepared by reaction bonding SiC and Al{sub 2}O{sub 3} particulate fillers with an Al{sub 2}O{sub 3} matrix and pressure infiltrating the residual porosity with Al metal. The reaction-bonding process was monitored by thermogravimetric and dilatometric methods. Composition and microstructure, as well as physical and mechanical properties of the composites, were evaluated for both as-reacted and as-infiltrated materials. 5 refs.