bluuebunny/crossref_metadata_2025 huggingface dataset:
Cross‐presentation: underlying mechanisms and role in immune surveillance
Authors: Rock Kenneth L. , Shen Lianjun
Published: September 2005

Summary:  It was originally thought that a cell's major histocompatibility complex (MHC) class I molecules presented peptides derived exclusively from proteins synthesized by the cell itself. However, in some circumstances, antigens from the extracellular environment can be presented on MHC class I molecules and stimulate CD8+ T‐cell immunity, a process termed cross‐presentation. Cross‐presentation was originally discovered as an obscure phenomenon in transplantation immunity. However, it is now clear that it is a major mechanism by which the immune system monitors tissues and phagocytes for the presence of foreign antigen. Cross‐presentation is the only pathway by which the immune system can detect and respond to viral infections or mutations that exclusively occur in parenchymal cells rather than in bone marrow‐derived antigen‐presenting cells (APCs). Professional APCs, such as dendritic cells, are the principal cells endowed with the capacity to cross‐present antigens. In this process, the APCs acquire proteins from other tissue cells through endocytic mechanisms, especially phagocytosis or macropinocytosis. The internalized antigen can then be processed through at least two different mechanisms. In one pathway, the antigen is transferred from the phagosome into the cytosol, where it is hydrolyzed by proteasomes into oligopeptides that are then transported by the transporter associated with antigen processing to MHC class I molecules in the endoplasmic reticulum or phagosomes. In a second pathway, the antigen is cleaved into peptides by endosomal proteases, particularly cathepsin S, and bound by class I molecules probably in the endocytic compartment itself. Depending on the nature of the antigen, one or both of these pathways can contribute to cross‐presentation in vivo. The outcome of cross‐presentation can be either tolerance or immunity. Which of these outcomes occurs is thought to depend on whether antigens are acquired by themselves alone, leading to tolerance, or with immunostimulatory signals, leading to immunity. One source of such signals is from dying cells that release immunostimulatory ‘danger’ signals that promote the generation of immunity to their cellular antigens. In addition to the critical role of cross‐presentation in normal immune physiology, this pathway has considerable potential for being exploited for developing subunit vaccines that elicit both CD4+ and CD8+ T‐cell immunity.
Right place, right time, right peptide: DO keeps DM focused
Authors: Denzin Lisa K. , Fallas Jennifer L. , Prendes Maria , Yi Woelsung
Published: September 2005

Summary:  Peptide loading of major histocompatibility class II molecules is catalyzed in late endosomal and lysosomal compartments of cells by the catalytic action of human leukocyte antigen (HLA)‐DM (H‐2M in mice). In B cells, dendritic cells and thymic epithelial cells, the peptide loading of class II molecules is modified by the expression of the non‐classical class II molecule, HLA‐DO (H‐2O in mice). Collectively, studies to date support that DO/H‐2O expression inhibits the presentation of antigens acquired by cells via fluid phase endocytosis. However, in B cells, the expression of H‐2O promotes the presentation of antigens internalized by the B‐cell receptor. In this review, we summarize the literature pertaining to DO assembly, transport, and function, with an emphasis on the function of DO/H‐2O.
Accessory molecules in the assembly of major histocompatibility complex class I/peptide complexes: how essential are they for CD8+ T‐cell immune responses?
Authors: Garbi Natalio , Tanaka Satoshi , Van Den Broek Maries , Momburg Frank , Hämmerling Günter J.
Published: September 2005

Summary:  Assembly of major histocompatibility complex (MHC) class I molecules in the endoplasmic reticulum is a highly coordinated process that results in abundant class I/peptide complexes at the cell surface for recognition by CD8+ T cells and natural killer cells. During the assembly process, a number of chaperones and accessory molecules, such as transporter associated with antigen processing, tapasin, ER60, and calreticulin, assist newly synthesized class I molecules to facilitate loading of antigenic peptides and to optimize the repertoire of surface class I/peptide complexes. This review focuses on the relative importance of these accessory molecules for CD8+ T‐cell responses in vivo and discusses reasons that may help explain why some CD8+ T‐cell responses develop normally in mice deficient in components of class I assembly, despite impaired antigen presentation.
MHC class I alleles and their exploration of the antigen‐processing machinery
Authors: Groothuis Tom A. M. , Griekspoor Alexander C. , Neijssen Joost J. , Herberts Carla A. , Neefjes Jacques J.
Published: September 2005

Summary:  At the cell surface, major histocompatibility complex (MHC) class I molecules present fragments of intracellular antigens to the immune system. This is the end result of a cascade of events initiated by multiple steps of proteolysis. Only a small part of the fragments escapes degradation by interacting with the peptide transporter associated with antigen presentation and is translocated into the endoplasmic reticulum lumen for binding to MHC class I molecules. Subsequently, these newly formed complexes can be transported to the plasma membrane for presentation. Every step in this process confers specificity and determines the ultimate result: presentation of only few fragments from a given antigen. Here, we introduce the players in the antigen processing and presentation cascade and describe their specificity and allelic variation. We highlight MHC class I alleles, which are not only different in sequence but also use different aspects of the antigen presentation pathway to their advantage: peptide acquaintance.
Interferon‐γ, the functional plasticity of the ubiquitin–proteasome system, and MHC class I antigen processing
Authors: Strehl Britta , Seifert Ulrike , Krüger Elke , Heink Sylvia , Kuckelkorn Ulrike , Kloetzel Peter‐M.
Published: September 2005

Summary:  The proteasome system is a central component of a cascade of proteolytic processing steps required to generate antigenic peptides presented at the cell surface to cytotoxic T lymphocytes by major histocompatibility complex (MHC) class I molecules. The nascent protein pool or DRiPs (defective ribosomal products) appear to represent an important source for MHC class I epitopes. Owing to the destructive activities of aminopeptidases in the cytosol, at most 1% of the peptides generated by the ubiquitin–proteasome system seems to be made available to the immune system. Interferon‐γ (IFN‐γ) helps to override these limitations by the formation of immunoproteasomes, the activator complex PA28, and the induction of several aminopeptidases. Both immunoproteasomes and PA28 use cleavage sites already used by constitutive proteasomes but with altered and in some cases dramatically enhanced frequency. Therefore, two proteolytic cascades appear to have evolved to provide MHC class I epitopes. The ‘constitutive proteolytic cascade’ is designed to efficiently degrade proteins to single amino acid residues, allowing only a small percentage of peptides to be presented at the cell surface. In contrast, the IFN‐γ‐controlled proteolytic cascade generates larger amounts of appropriate antigenic peptides, assuring more peptides to overcome the proteolytic restrictions of the constitutive system, thereby enhancing MHC class I antigen presentation.
Complexity, contradictions, and conundrums: studying post‐proteasomal proteolysis in HLA class I antigen presentation
Authors: Saveanu Loredana , Carroll Oliver , Hassainya Yousra , Van Endert Peter
Published: September 2005

Summary:  The vast majority of the peptides produced during protein degradation by the cytosolic proteasome‐ubiquitin system are consecutively hydrolyzed to single amino acids by multiple cytosolic peptidases preferring intermediate length or short substrates. The small fraction of peptides surviving the aggressive cytosolic environment can be recruited for presentation by major histocompatibility complex (MHC) class I molecules. However, such peptides may frequently have to be adapted to the strict MHC class I‐binding requirements by one or several N–terminal‐trimming steps. A recent model proposes that an initial step, in which peptides of 15 or more residues are shortened by cytosolic tripeptidylpeptidase II, is followed by additional trimming by cytosolic or endoplasmic reticulum (ER) aminopeptidases. In humans, at least two ER resident aminopeptidases, ERAP1 and ERAP2, contribute to trimming of human leukocyte antigen class I ligands. These interferon‐γ‐regulated metallopeptidases show distinct substrate preferences and may have to act in a concerted fashion to remove some complex or longer N‐terminal extensions and to trim the full spectrum of precursor peptides. This task is likely facilitated by the formation of presumably heterodimeric ERAP1‐2 complexes. RNA interference experiments suggest that both enzymes are important for normal antigen presentation, but precise determination of the extent and the cellular context of their requirement will be left to future experimentation.
Downregulation of cell surface receptors by the K3 family of viral and cellular ubiquitin E3 ligases
Authors: Lehner Paul J. , Hoer Simon , Dodd Roger , Duncan Lidia M.
Published: September 2005

Summary:  The mK3, K3, and K5 gene products from the γ2 group of γ‐herpesviruses are the founding members of a family of membrane‐associated ubiquitin E3 ligases. As part of the viral immunoevasion strategy, expression of these proteins results in a decrease in cell‐surface major histocompatibility complex class I molecules and other immunoreceptors including intercellular adhesion molecule‐1, CD86, and CD1d. These viral gene products all possess a characteristic cytosolic N‐terminal RING‐CH domain, responsible for ubiquitination of the target protein, and two membrane‐spanning segments required for substrate specificity. For the majority of substrates, ubiquitination at the cell surface leads to rapid internalization and endolysosomal degradation, while mK3 ubiquitinates class I molecules associated with the peptide‐loading complex resulting in proteasome‐mediated degradation. Related viral genes with similar functions have been found in poxviruses, suggesting appropriation of these genes from the eukaryotic host. Ten membrane‐associated RING‐CH (MARCH) human genes with a similar organization have now been identified, and their overexpression leads to ubiquitination and downregulation of a variety of cell‐surface immunoreceptors. While all the MARCH proteins are predicted to act as ubiquitin E3 ligases, their physiological role and substrates remain to be defined.
Recognition of open conformers of classical MHC by chaperones and monoclonal antibodies
Authors: Hansen Ted H. , Lybarger Lonnie , Yu Lawrence , Mitaksov Vesselin , Fremont Daved H.
Published: September 2005

Summary:  There is considerable evidence that the conformation and stability of class I and class II major histocompatibility complex (MHC) proteins is dependent upon high‐affinity peptide ligation, but structural data for an empty MHC protein unfortunately is lacking. However, several monoclonal antibodies (mAbs) that specifically detect open MHC conformers have been characterized, and they provide insights into the changes associated with peptide loading and unloading. Here, the structural changes make the argument that certain of these open conformer‐specific mAbs recognize analogous MHC segments as the molecular chaperones tapasin and DM. MHC residues located in regions flanking the peptide‐terminal anchoring pockets have been implicated in both chaperone and monoclonal antibody binding. Indeed, we propose these regions serve as peptide‐binding hinges that are uniquely accessible in open MHC.
Viral modulation of antigen presentation: manipulation of cellular targets in the ER and beyond
Authors: Lilley Brendan N. , Ploegh Hidde L.
Published: September 2005

Summary:  Viruses that establish long‐term infections in their hosts have evolved a number of methods to interfere with the activities of the innate and adaptive immune systems. Control of viral infections is achieved in part through the action of cytotoxic T lymphocytes (CTLs) that recognize cytosolically derived antigenic peptides in the context of class I major histocompatibility complex (MHC) molecules. Viral replication within host cells produces abundant proteinaceous fodder for proteasomal digestion and display by class I MHC products. Tactics that disrupt antigen‐presentation pathways and prevent the display of peptides to CD8+ CTLs have been favored during the course of host‐virus co‐evolution. Viral immunoevasins exploit diverse cellular processes to interfere with host antiviral functions. The study of such viral factors has uncovered novel host proteins that assist these viral factors in their task and that themselves perform important cellular functions. Here, we focus on viral immunoevasins that, together with their cellular targets, interfere with antigen‐presentation pathways. In particular, we emphasize the intersection of the cellular quality‐control machinery in the endoplasmic reticulum with the herpesvirus proteins that have co‐opted it.
All the peptides that fit: the beginning, the middle, and the end of the MHC class I antigen‐processing pathway
Authors: Shastri Nilabh , Cardinaud Sylvain , Schwab Susan R. , Serwold Thomas , Kunisawa Jun
Published: September 2005

Summary:  The end result of the antigen‐processing pathway is the display of peptide‐bound major histocompatibility complex I (pMHC I) molecules. The pMHC I molecules are expressed on the cell surface where they can be surveyed by CD8+ T cells for abnormal proteins. MHC I molecules present a large repertoire of peptides that fit perfectly in their binding grooves and represent the otherwise hidden intracellular contents. Many peptides originate as defective ribosomal products in the cytoplasm. In a stepwise manner, the antigen‐processing pathway generates and protects the proteolytic intermediates until they yield the final peptides that can fit the MHC I in the endoplasmic reticulum.
Lysosomal membrane permeabilization during apoptosis ‐ involvement of Bax?
Authors: Kågedal Katarina , Johansson Ann‐Charlotte , Johansson Uno , Heimlich Gerd , Roberg Karin , Wang Nancy S. , Jürgensmeier Juliane M. , Öllinger Karin
Published: September 2005

Bcl‐2 family members have long been known to control permeabilization of the mitochondrial membrane during apoptosis, but involvement of these proteins in lysosomal membrane permeabilization (LMP) was not considered until recently. The aim of this study was to investigate the mechanism underlying the release of lysosomal proteases to the cytosol seen during apoptosis, with special emphasis on the role of Bax. In human fibroblasts, exposed to the apoptosis‐inducing drug staurosporine (STS), the release of the lysosomal protease cathepsin D to the cytosol was observed by immunocytochemistry. In response to STS treatment, there was a shift in Bax immunostaining from a diffuse to a punctate pattern. Confocal microscopy showed co‐localization of Bax with both lysosomes and mitochondria in dying cells. Presence of Bax at the lysosomal membrane was confirmed by immuno‐electron microscopy. Furthermore, when recombinant Bax was incubated with pure lysosomal fractions, Bax inserted into the lysosomal membrane and induced the release of lysosomal enzymes. Thus, we suggest that Bax is a mediator of LMP, possibly promoting the release of lysosomal enzymes to the cytosol during apoptosis.
Dengue virus inoculation to human skin explants: an effective approach to assess in situ the early infection and the effects on cutaneous dendritic cells
Authors: Limon‐Flores Alberto Yairh , Perez‐Tapia Mayra , Estrada‐Garcia Iris , Vaughan Gilberto , Escobar‐Gutierrez Alejandro , Calderon‐Amador Juana , Herrera‐Rodriguez Sara Elisa , Brizuela‐Garcia Adriana , Heras‐Chavarria Monica , Flores‐Langarica Adriana , Cedillo‐Barron Leticia , Flores‐Romo Leopoldo
Published: September 2005

Although dengue virus (DV) enters through skin while mosquitoes feed, early contacts remain unexplored regarding the cutaneous viral fate and in situ immune responses. We addressed this by exposing healthy, non‐cadaveric, freshly obtained human skin explants to a human DV2 isolate. We demonstrated negative‐strand DV‐RNA and non‐structural protein‐1, both suggestive of viral replication in skin. Although control, mock‐infected and DV‐infected explants showed less (MHC‐CII+/CD1a+/Langerin+) Langerhans cells, deranged morphology and decreased frequency were more apparent in DV‐infected explants. Whereas DV+ cells were infrequent in epidermis and completely absent in dermis, some areas of basal epidermis were clearly DV+, presumably keratinocytes, cells where TUNEL positivity revealed apoptosis. Unlike fresh, control and mock‐infected skin, DV‐infected explants expressed CD80 and CD83, indicative of dendritic cell (DC) activation and maturation, respectively. However, sequential sections indicated that these cells were not DV+, suggesting that activated/mature DCs capable of priming T cells, probably, were not infected. Alternatively, the occasionally infected epidermal DC might not have reached maturation. Interestingly, skin DV infection apparently uncouples the DC activation/maturation process from another crucial DC function, the subsequent migration into dermis. This was suggested, because upon cutaneous DV infection, the few emerging CD83+ (mature) DCs remained within the outer epidermis, while no dermal CD83+ DCs were observed. These paradoxical effects might represent unknown DV subversion strategies. This approach is relatively easy, quick (results in 48 h), economical for developing countries where dengue is re‐emerging and advantageous to evaluate in situ viral biology, immunity and immunopathology and potential antiviral strategies.
Arterial endothelial function in a porcine model of early stage atherosclerotic vascular disease
Authors: Turk James R. , Henderson Kyle K. , Vanvickle Gregory D. , Watkins Justin , Laughlin M. Harold
Published: September 2005

Cardiovascular disease (CVD) is the leading cause of morbidity and mortality in the United States and is projected to become the leading cause of mortality in the world. Atherosclerosis is the most important single factor contributing to this disease burden. In this study, we characterize relationships between endothelial dysfunction and vascular disease in an animal model of diet‐induced, early‐stage atherosclerotic vascular disease. We tested the hypothesis that hypercholesterolaemia induces vascular disease and impairs endothelium‐dependent relaxation (EDR) in conduit arteries of adult male Yucatan pigs. Pigs were fed a normal fat (NF) or high fat cholesterol (HFC) diet for 20–24 weeks. Results indicate that, while the HFC diet did not alter EDR in femoral or brachial arteries, EDR was significantly decreased in both carotid and coronary arteries. Sudanophilic fatty streaks were significantly present in the abdominal aorta and common carotid artery. Histopathology revealed increased intima‐media thickness (IMT) and foam cell accumulation in Stary Stage I–III lesions in the abdominal aorta, common carotid artery and femoral arteries. In the coronary arteries, the accumulation of foam cells in Stary Stage I and II lesions resulted in a trend for increased IMT. There was no evidence of vascular disease in the brachial arteries. These results indicate that early stages of CVD (Stary Stage I–III) precede decreases in EDR induced by HFC diet, because femoral arteries exhibited foam cell accumulation and an increased IMT but no change in endothelial function.
Wetting in an Electronic Packaging Ceramic System: 1, Wetting of Tungsten by Glass in Controlled Oxygen Partial Pressure Atmospheres
Authors: Lazaroff Jerold E. , Ownby P. Darrell , Weirauch Douglas A.
Published: September 2005

The wetting of tungsten by a Cr2O3‐colored, CaO‐MgO‐Al2O3‐SiO2 glass was found to be independent of temperature between 1300° and 1500°C, but strongly dependent on furnace atmosphere. Similar results using two gas buffer systems (CO/CO2 and H2/H2O) established oxygen partial pressure, p02 as the critical parameter. The contact angle decreased over a narrow p02 range as the p02 increased, with a stable contact angle existing in both lower and higher p02 ranges. The solid‐liquid interfacial energy, γSL, controlled the wetting behavior. An increase in the adsorbed oxygen layer at the solid‐liquid interface resulted in a lower γSL and a lower contact angle. The equilibrium contact angles, established after 8‐h isothermal holds, ranged from 50‐55° at a p02= 10−15.5 atm to 30° at a p02= 10−10.9 atm. Two different drop formation techniques were used to show that the temperature and atmospheric conditions at the time of solid‐liquid interface formation affect the stable contact angles. The contact angle was higher when the solid‐liquid interface was established at the test temperature (doser tube technique) than when the drop was formed in situ from a piece of glass placed on the substrate at room temperature (nondoser method). This contact angle difference was again attributed to a higher γSL from the doser method due to the presence of less adsorbed oxygen at the time of the creation of the solid‐liquid interface.
Reaction Densification of α′‐SiAlON: 1, Wetting Behavior and Acid‐Base Reactions
Authors: Menon Mohan , Chen I‐Wei
Published: September 2005

Phase development during reaction hot pressing of α‐Si3N4, AlN, Al2O3, and M2OZ powder mixtures (M = Li, Mg, Ca, Y, Nd, Sm, Gd, Dy, Er, and Yb) forming α′‐SiAlON has been studied. The wetting behavior of the ternary eutectic melt of M2Oz‐Al2O3‐SiO2 was found to control the reaction sequence during hot pressing. Li, Ca, Mg, Nd, Sm, and Gd were found to preferentially wet and react with Si3N4 first, whereas Dy, Er, and Yb preferentially wetted and reacted with AlN first. The intermediate phases are Si rich in the former and Al rich in the latter case. Pearson's principle of acid‐base theory, which predicts decreasing basicity of oxides of Li, Ca, Mg, Nd, Gd, Sm, Dy, Er, and Yb and decreasing acidity of oxides of Si and Al, is used to understand these reactions.
Crack Healing and Stress Relaxation in Al2O3 SiC “Nanocomposites”
Authors: Thompson A. Mark , Chan Helen M. , Harmer Martin P. , Cook Robert E
Published: September 2005

The crack‐healing behavior of A12O3 and Al2O3‐SiC nanocomposite was studied using Vickers indentations to generate precracks. After annealing in argon for 2 h at 1300°C, radial cracks in the nanocomposite healed: The cracks closed and there was a small degree of rebonding in the vicinity of the crack tip. In contrast, radial cracks in alumina grew when exposed to the same annealing treatment. The different responses are attributed to the fracture mode and toughening mechanism in each material: In the nanocomposite, the cracks close as the residual stresses surrounding the indentations relax. Radial cracks open and grow in A12O3 because microstructural toughening is diminished during heating to the annealing temperature. An implication is that strength‐limiting machining flaws in these materials behave similarly, thereby accounting for the strengthening effect of annealing in this “nanocomposite“ system.
Spectral Absorption Coefficient of Molten Aluminum Oxide from.0.385 to 0.780 μm
Authors: Weber J. K. Richard , Krishnan Shankar , Anderson Collin D. , Nordine Paul C.
Published: September 2005

The spectral extinction coefficient, kλ, and the absorption coefficient, αλ, of liquid aluminum oxide were determined from emission measurements on partially molten single‐crystal sapphire filaments. Measurements were made at wavelengths from 0.385 to 0.780 μm in both argon and oxygen atmospheres and at 0.650 μm in a 10% H2‐90% N2 mixture. The values of αλ were found to vary with the time for which the specimen had been molten and achieved different steady‐state values in the three processing atmospheres. A decrease in αλ with wavelength (Urbach tail) was observed from 0.385 μm to about 0.420 μm in argon and 0.490 μm in oxygen, and approximately constant values were obtained at longer wavelengths. Steady‐state values of αλ at the optical pyrometer wavelength of 0.650 μm were 9 ± 2 cm−1 in argon, 41 ± 8 cm−1 in oxygen, and 14 ± 2 cm−1 in the 10% hydrogen‐90% nitrogen gas mixture.
Deformation Bands in Ceria‐Stabilized Tetragonal Zirconia/Alumina : II, Stress‐Induced Aging at Room Temperature
Authors: Sergo Valter , Clarke David R.
Published: September 2005

A stress‐induced aging phenomenon is observed to occur at room temperature in deformation bands introduced into a 8.5 mol% ceria‐stabilized tetragonal zirconia/alumina (CeTZP/A12O3) composite by flexural loading. The aging occurs with time after unloading and in laboratory air. Over a period of 100 days, the concentration of monoclinic zirconia within a deformation band increases and, in addition, the wedge‐shaped deformation band grows with time. Accompanying these two changes are an increase in the tensile stress in the remaining tetragonal zirconia within the deformation band and a consequential increase in the overall compressive stress within the band. The average value of the monoclinic concentration within the deformation band is found to increase parabolically with time, suggesting the mechanism responsible for the observed aging is diffusion limited. Away from the deformation bands, no aging is observed to occur, suggesting aging is stress dependent. Although a water‐vapor‐mediated mechanism cannot be ruled out, it is proposed that the observed aging is in fact due to a tensile stress assisted chemical reduction of Ce4+to Ce3+whose rate is controlled by the indiffusion of oxygen vacancies driven by the tensile stress gradient. It is further proposed that the deformation band grows with time, since the region ahead of the band is under tension and hence subject to an enhanced rate of reduction
Microstructural Development of Coherent Tetragonal Precipitates in Magnesium‐Partially‐Stabilized Zirconia: A Computer Simulation
Authors: Wang Yunzhi , Wang Hong‐Ying , Chen Long‐Qing , Khachaturyan Armen G.
Published: September 2005

A new computer simulation technique for modeling morphological pattern formation during nucleation, growth, and coarsening of coherent misfitting particles is developed. Microstructure evolution during precipitation of tetragonal phase from Mg‐partially‐stabilized cubic zirconia is investigated. Our computer simulation shows that during the initial stage of precipitation, the tetragonal particles formed by homogeneous nucleation display strong alignment along certain crystallographic directions, forming the so‐called “tweed” pattern. During subsequent growth of the spatially correlated nuclei, an alternating band structure is observed, with each individual band consisting of lens‐like‐shaped tetragonal phase particles of the same orientation variant dispersed in the cubic matrix. The particles in the neighboring bands are twin‐related. The microstructures obtained in our computer simulation seem to agree well with the experimental observations.
Ternary Phase Equilibria in the Zr‐Si‐C System
Authors: Wang Yaozu , Carim Altaf H.
Published: September 2005

Phase equilibria in the ternary Zr‐Si‐C system at 1200°C are investigated in this work. A specific effort was made tofabricate a stable or metastable Zr3SiC2 phase, analogous to Ti3SiC2, but no evidence of such a compound was observed.The existence of the Zr5Si4 and Zr3Si phases, which were previously described in analyses of the Zr‐Si binary, isconfirmed in this study and their phase relationships withinthe ternary Zr‐Si‐C system are established by electronmicroprobe analysis and transmission electron microscopy.
Synthesis of Titanium Silicon Carbide
Authors: Arunajatesan Sowmya , Carim Altaf H.
Published: September 2005

Synthesis of bulk titanium silicon carbide (Ti3SiC2) from the elemental Ti, Si, and C powders has been accomplished for the first time, using the arc‐melting and annealing route. The effects of various parameters on the phase purity of the Ti3SiC2 have been examined, including the starting composition of the powders, compaction technique, arc‐melting of the samples, and temperature and time of anneal. The best bulk samples, containing about 2 vol% TiC as the second phase, were made from Si‐deficient and C‐rich starting compositions. Based on electron probe microanalysis data from a number of bulk samples, it appears that Ti3SiC2 exists over a range of compositions; the Ti‐Si‐C ternary section has been modified to reflect this. The purest samples of the ternary phase were obtained by leaching powders of silicide‐containing samples in diluted HF, and contained over99vol%Ti3SiC2.
Formation of Boron Nitride Inclusions in Hot Isostatically PressedSilicon Nitride‐Silicon Carbide Composites
Authors: Turan Servet , Knowles Kevin M.
Published: September 2005

Small graphite‐like boron nitride inclusions have been detected, using transmission electron microscopy, in silicon nitride particulate‐reinforced silicon carbide composites hot isostatically pressed in tantalum cans. The inclusions are thought to arise indirectly from fine particles of boron nitride sprayed onto the internal surface of the tantalum can to prevent a chemical reaction between the tantalum and the silicon carbide. Boron oxide present on the surface of these particles is able to evaporate and penetrate the powder compacts during hot isostatic pressing and then react with silicon nitride and silicon carbide to form the observed inclusions.
Chemical Equilibria in the Titanium(ll) Hydride‐Boron Nitride System
Authors: Benko Ewa
Published: September 2005

Calculations of chemical equilibria for the TiH2‐BN system using the Villars, Cruise, Smith (VCS) algorithm were carried out for the 27–2027°C temperature range and the 1.3 × 10−3‐1 × 107 Pa pressure range. Two BN:TiH2 molar ratios were selected, 1:1 and 2:1. It follows from these studies that TiH2 can react with BN in a very wide range of pressures and temperatures, giving new phases, namely TiB2, TiB, and TiN. The formation of these phases is strongly influenced by pressure‐temperature parameters. In order to verify theoretical calculations by experiment, equimolar and 2:1 BN:TiH2 batches were prepared and heated at 1427°C under vacuum, in an argon atmosphere, and under high pressure. Phase identification carried out by X‐ray diffraction fully corroborated the computational results.
Crack Front Stability and Hackle Formation in High Velocity Glass Fracture
Authors: Beauchamp Edwin K.
Published: September 2005

Optical and scanning electron microscopy of “hackle streaks” and interference microscopy on fracture surfaces showed that major velocity hackle features can be produced at substantially lower K values than that associated with the hackle boundary in glass fracture. A novel method was used to determine crack propagation velocity along the streaks. Hackle generation was associated with distortion along the crack front from prior fracture events or from stress pulses generated at other hackle features. Explanations are presented for changes in velocity hackle at mist and hackle boundaries and for the low experimental values of the terminal velocity for fracture in glass.
Indentation Behavior of Soda‐Lime Silica Glass, Fused Silica, and Single‐Crystal Quartz at Liquid Nitrogen Temperature
Authors: Kurkjian Charles R. , Kammlott Gunther W. , Chaudhri M. Munawar
Published: September 2005

In an attempt to elucidate the processes involved in the formation of indentation impressions, Vickers hardness measurements have been made on soda‐lime silica glass, fused silica, and crystalline quartz indented at room temperature and 77 K. The hardness of all three materials increases by a factor of ∼2.5 on cooling to liquid nitrogen temperature. High‐magnification SEM photographs revealed that the deformation and cracking patterns of the glasses changed strikingly: no shear lines were observed within the indentations, and ring cracking occurred instead of radial/median cracking. In addition, cracking occurs at much higher loads than at room temperature. The hardness results have been explained in terms of volume flow (densification) rather than shear flow (viscous or plastic) for the glasses at low temperature. The quartz crystal, on the other hand, deformed plastically at both room temperature and 77 K. Cracking differences result from changes in both flow and water activity
Multiple Cracking of Unidirectional and Cross‐PlyCeramic Matrix Composites
Authors: Kuo Wen‐Shyong , Chou Tsu‐Wei
Published: September 2005

This paper examines the multiple cracking behavior of unidirectional and cross‐ply ceramic matrix composites. For unidirectional composites, a model of concentric cylinders with finite crack spacing and debonding length is introduced. Stresses in the fiber and matrix are found and then applied to predict the composite moduli. Using an energy balance method, critical stresses for matrix cracking initiation are predicted. Effects of interfacial shear stress, debonding length and bonding energy on the critical stress are studied. All the three composite systems examined show that the critical stress for the completely debonded case is lower than that for the perfectly bonded case. For cross‐ply composites, an extensive study has been made for the transverse cracking in 90° plies and the matrix cracking in 0° plies. One transverse cracking and four matrix cracking modes are studied, and closed‐form solutions of the critical stresses are obtained. The results indicate that the case of combined matrix and transverse crackings with associated fiber/matrix interfacial sliding in the 0° plies gives the lowest critical stress for matrix cracking. The theoretical predictions are compared with experimental data of SiC/CAS cross‐ply composites; both results demonstrated that an increase in the transverse ply thickness reduces the critical stress for matrix cracking in the longitudinal plies. The effects of fiber volume fraction and fiber modulus on the critical stress have been quantified. Thermal residual stresses are included in the analysis.
Hot‐Pressed MoSi2‐Particulate‐Reinforced α‐SiAlON Composites
Authors: Nordberg Lars O. , Ekström Thommy
Published: September 2005

MoSi2‐particulate‐reinforced α‐SiAlON ceramic composites containing 10, 20, 25, and 30 vol% were prepared by hot pressing at 1750°‐1800°C. The α‐SiAlON matrix was of the composition (Y0.48Si10.00A12.30O1.17N15.29). The hardness for the fully dense samples changed from HV10 = 22.5 to 15.3 GPa and the toughness from 3.2 to around 5.2 MPa.m1/2 when up to 30 vol% MoSi2 was present. Two interesting microstructural features have been found. First, with an increasing amount of MoSi2 a pronounced coalescence of MoSi2 particles formed a “dual phase” material. The second effect was the growth of elongated α‐SiAlON grains in the matrix with 10 vol% MoSi2 added. The oxidation resistance has been determined to be unaffected by the addition of 2hd vol % MoSi2 at 1250°C in oxygen gas of l atm pressure.
Carbon‐Fiber‐Reinforced Concrete as an Intrinsically Smart Concrete for Damage Assessment during Dynamic Loading
Authors: Chen Pu‐Woei , Chung D. D. L.
Published: September 2005

Concrete containing short carbon fibers (0.2–0.5 vol%) wasfound to be an intrinsically smart concrete that can sense elastic and inelastic deformation, as well as fracture. The signal provided is the change in electrical resistance, which is reversible for elastic deformation and irreversible for inelastic deformation and fracture. The presence of electrically conducting short fibers is necessary for the concrete to sense elastic or inelastic deformation, but the sensing of fracture does not require fibers. The fibers serve to bridge the cracks and provide a conduction path. The resistance increase is due to conducting fiber pullout in the elastic regime, conducting fiber breakage in the inelastic regime, and crack propagation at fracture.
Carbothermal Synthesis of TiC Whiskers via a Vapor‐Liquid‐Solid Growth Mechanism
Authors: Ahlén Niklas , Johnsson Mats , Nygren Mats
Published: September 2005

TiC whiskers were synthesized via a vapor‐liquid‐solid (VLS) growth mechanism in the temperature region 1200°‐1600°C in a protecting argon atmosphere. The starting materials consisted of TiO2, carbon, nickel, and an alkali‐metal chloride, such as NaCl, used as a precursor for chlorine. The overall chemical reaction was based on a straightforward carbothermal reduction process, but the reactions that actually took place were more complex and involved formation of gaseous TiClx species, which were transported to the nickel catalyst. The formation of TiC whiskers depended on the processing conditions used, mixing procedure of the precursor materials applied, and properties of precursor materials chosen, e.g., their particle sizes. A route for synthesizing TiC whiskers at a yield of 80‐90 vol% was established. The whiskers were ∼1 μm in diameter and 10–30 μm in length; they were straight and had smooth surfaces. The main impurities were TiC particles, minor amounts of unreacted carbon, and remnants of the nickel catalyst.
Kinetics of Thermal Oxidation of Silicon Nitride Powders
Authors: Butt Darryl P. , Albert Diane , Taylor Thomas N.
Published: September 2005

The kinetics of thermal oxidation of H. C. Starck M‐11 and Ube SN‐E10 Si3N4 powders was evaluated in the temperature range 650°‐1200°C using isothermal and nonisothermal thermogravimetric analysis, high‐resolution transmission electron microscopy, and X‐ray photoelectron spectroscopy. Between 700° and 1200°C, the isothermal kinetics was modeled equally well using the Ginstling‐Brounshtein and Zuravlev‐Lesokhin‐Tempeľman equations. Despite their similar particle sizes and surface areas, the two powders exhibited different oxidation kinetics. The activation energies for oxidation of the M‐11 and SN‐E10 powders were determined to be 400 and 540 kJ/mol, respectively, between 1000° and 1200°C, and 230 and 260 kJ/mol, respectively, between 700° and 1000°C. The parabolic rate constants for oxidation of the two powders were comparable to those reported in the literature for monolithic, chemically‐vapor‐deposited Si3N4 at the higher temperatures. At lower temperatures, the oxidation kinetics of the M‐11 powder was nearly linear, whereas the kinetics of the SN‐E10 powder remained power‐law dependent.
Brittle Failure Prediction of Ceramics Using a Multiscale Approach
Authors: Berdin Clotilde , Cailletaud Georges , Jeulin Dominique
Published: September 2005

Brittle failure is classically modeled by the Weibull distribution, based on a phenomenological approach. The procedure fails if the distribution of the failure‐initiating defects varies within material batches and may lead to a wrong conclusion about the predictive power of the model. The purpose of this paper is to highlight this problem, which can occur with brittle materials such as ceramics. This study, based on the results of tensile and bending tests made on silicon nitride specimens, consists of three steps: (i) micrographic and fractographic studies are performed to determine the defect‐size density in each type of specimen and the mechanical behavior of the defects, (ii) the classical modeling with a Weibull distribution is shown to fail to predict the results, and (iii) a new approach for the identification of a fracture model is developed, based on step (i). The fracture prediction is computed from the actual defectsize distribution in each material batch, so that the procedure can be adapted to changes in the defect‐size distribution caused by the manufacturing process.
Short‐Range and Medium‐Range Order in Lithium Silicate Glasses, Part II: Simulation of the Structure by the Reverse Monte Carlo Method
Authors: Uhlig Harald , Hoffmann Michael J. , Lamparter Hannes‐Peter , Aldinger Fritz , Bellissent Robert , Steeb Siegfried
Published: September 2005

The results of X‐ray and neutron diffraction experiments with (Li2O)x(SiO2)100_x glasses (x = 0, 20, 33.3, and 40) are discussed using the Reverse Monte Carlo method to produce atomic clusters, which are analyzed with respect to the O‐O distribution, the angular correlations within the first and higher coordination spheres, and the bond‐orientational multipole moments. Within Li2O‐containing silicate glasses, each lithium atom is surrounded by a ring of six SiO4 tetrahedra in such a way that four oxygen atoms lie at the joints of a tetragonal cell centered by lithium. Thus, the lithium atoms increase the order of the glass.
Passive‐Oxidation Kinetics of High‐Purity Silicon Carbide from 800° to 1100°C
Authors: Ramberg C. Eric , Cruciani Gary , Spear Karl E. , Tressler Richard E. , Ramberg Charles F.
Published: September 2005

Highly textured chemically vapor‐deposited silicon carbide (CVD‐SiC) thick films were oxidized and compared to single‐crystal SiC and single‐crystal silicon. The oxidation rates of the (111) face of the cubic CVD‐SiC were the same as those of the (0001) face of the single‐crystal SiC. Similarly, the opposite faces of the two materials, () and (000), also oxidized at nominally the same rates. The () and (000) faces oxidized much faster than their opposite (lll)/(0001) faces. Ellipsometry measurements and kinetic results implied that differences existed between the oxides that grew on the opposite faces. A regression method was developed to analyze the oxide thickness versus time versus temperature behavior of the specimens simultaneously. This technique was compared to typical methods for analyzing temperature‐dependent processes and estimated temperature‐ dependent parameters (e.g., activation energy) and their errors more accurately.
Improved Aqueous Dispersion of Silicon Nitride with Aminosilanes
Authors: Buchta Michael A. , Shih Wei‐Heng
Published: September 2005

The addition of a standard fiberglass surfactant, 3‐aminopropyltriethoxysilane (APS), improves whisker and powder dispersion of silicon nitride aqueous suspensions. Aqueous suspensions of APS‐coated silicon nitride have lower viscosities, increased consolidation, and higher dried green‐body densities compared to uncoated silicon nitride in suspensions with pH values ≤8. The APS coating shifts the isoelectric point (IEP) of silicon nitride to a more basic value, dependent on the concentration of APS coating. Suspension pH measurements indicate that APS extracts one hydrogen ion for each APS molecule either chemisorbed on the particle surface or dissolved in the solution. Optical microscopy reveals that dilute suspensions coated with APS at pH 10 are qualitatively more dispersed than uncoated silicon nitride at pH 7. Our results show increased dispersion of APS‐coated silicon nitride in acidic environments, with a 12% increase in green density under identical wetpressing conditions.
Kinetics of Silicon Monoxide Ammonolysis for Nanophase Silicon Nitride Synthesis
Authors: Lin Dahcheng , Kimura Shoichi
Published: September 2005

Silicon monoxide vapor generated from Si/SiO2 mixed‐powder compacts was used with NH3 to synthesize silicon nitride in a tubular flow reactor operated at temperatures in the range of 1300°‐1400°C. The ammonolysis of SiO with excess NH3 was very rapid, yielding three different types of silicon nitride at different longitudinal locations in the reactor: amorphous nanophase powder of an average size of about 20 nm, amorphous whiskers of a few micrometers in diameter, and α‐polycrystals. The amorphous products were heat‐treated for crystallization at temperatures between 1300° and 1560°C in a stream of dissociated NH3, N2, or N2/H2 mixture gas. When dissociated NH3 was used, nanophase powder was crystallized at 1300°C. The yield of nanophase silicon nitride from SiO varied from 13% to 43%, depending on operating conditions.
Formation of New Compounds Ln4Al2O9 in the Ln2O3.Al2O3 System (Ln = La, Pr, Tb)
Authors: Dohrup Jesper , Høyvald Annette , Mogensen Gurli , Jacobsen Claus J. H. , Villadsen Jørgen
Published: September 2005

The new compounds La4Al2O9, Pr4Al2O9, and Tb4Al2O9 were identified in the system 2Ln2O3.Al2O3. X‐ray powder diffraction showed that they were isomorphous with the previously characterized Ln4Al2O9 (Ln = Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Y) and Ln4Ga2O9 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd). La4Al2O9: P21/c, a = 7.97 Å, b = 11.09 Å, c = 11.41 Å, β= 108.8°, Z = 4; Pr4Al2O9: P21/c, a = 7.80 Å, b = 11.03 Å, c = 11.31 Å, β= 109.6, Z = 4; and Tb4Al2O9: P21c, a= 7.40 Å, b= 10.58 Å, c = 11.16 Å, β= 107.2°, Z = 4. The lanthanum and praseodymium compounds were metastable phases in the Ln4Al2O9 systems and transformed into LaAlO3 and La2O3, and PrAlO3 and PrO1.83 upon prolonged heating at 1200°C, respectively. Ln4Al209 (Ln = La, Pr) were synthesized by precipitation of A1(OH)4−with Ln3+(aq) and subsequent heating at 900°C for 10 h. Tb4AI2O9 required calcination at 1200°C for 10 h. The formation of La4Al2O9 and Pr4Al2O9 was followed by TGA‐DSC, which was also used to assess their thermal stability. Several other members of the above family of compounds were similarly synthesized by the new synthesis route. Particularly, for Nd4Al2O9 this represented a significantly improved method.
Influence of Coarse Aggregate Content on the Thermal Conductivity of Alumina–Spinel Castables
Authors: Chan Chen‐Feng , Ko Yung‐Chao
Published: September 2005

The thermal conductivity of three commercial‐quality alumina‐spinel castables containing 90–96 wt% A12O3 was measured in the range room temperature to 1000°C by a hot wire method. Experimental results indicate that the thermal conductivity increases with an increase in coarse aggregate content; higher corundum content and purer alumina aggregates are more important than coarse aggregate content for higher thermal conductivity of alumina‐spinel castables.
Phase Diagram for the SiF4 Join in the System SiO2‐Al2O3‐SiF4
Authors: Moyer John R.
Published: September 2005

Alumina reacts with 1 atm of SiF4 below 660°± 7°C to form A1F3 and SiO2. At higher temperatures the product is a mixture of fluorotopaz and AIF3. Mixtures of fluorotopaz and AIF3 decompose in 1 atm of SiF4 at 973°± 8°C and form tabular α‐alumina. The equilibrium vapor pressure of SiF4 above mixtures of fluorotopaz and AlF3 is log p(atm) = 9.198 – 11460/T(K). Fluorotopaz itself decomposes at 1056°± 5°C in 1 atm of SiF4 to give acicular mullite, 2Al2O3.1.07SiO2. Alumina and mullite are stable in the presence of 1 atm of SiF4 above 973° and 1056°C, respectively. The phase diagram of the system SiO2‐Al2O3‐SiF4 shows only gas‐solid equilibria.
Crystallinity and Stoichiometry of Nano‐Structured Sol‐Gel‐Derived BaTiO3 Monolithic Gels
Authors: Shimooka Hirokazu , Kuwabara Makoto
Published: September 2005

The crystallization behavior and stoichiometric changes of barium titanium alkoxide‐derived monolithic gels prepared by the sol‐gel process using a high‐concentration Ba2Ti precursor solution (0.8 mol/L) were investigated during aging at room temperature. Crystallization of the gels (which were amorphous, per X‐ray diffraction analysis immediately after gelation) into the BaTiO3 perovskite phase increased during aging and was associated with significant shrinkage of the gels. Crystallization reached a value of ∼82% by the final stage of shrinkage, assuming the degree of crystallization of a gel treated at 600°C to be 100%. The stoichiometry of the gels (Ba/Ti molar ratio) also changed considerably during aging, as estimated by the concentrations of Ba and Ti that remained in the expelled liquid resulting from syneresis at any time during the aging process. Deviation in the Ba/Ti ratio of the precursor solution ranged from 0.015 at the initial stage of shrinkage to 0.003 at the final stage, a value determined by inductively coupled plasma atomic emission spectroscopy. The present study demonstrates the great advantage of using high‐concentration precursor solutions of barium titanium alkoxides, rather than low‐concentration solutions, to obtain BaTiO3 gel monoliths with high density and crystallinity and little stoichiometric deviation, by sol‐gel processing at room temperature.