Focused Ion Beam-Prepared Transmission Electron Microscopy Examination of Atmospheric Chemical Vapor-Infiltrated Silicon Carbide Morphology.

Silicon carbide (SiC)-based ceramic matrix composites (CMCs) are utilized for their refractory properties in the aerospace industry. The composition and structure of these materials are crucial to maintaining the strength, toughness, oxidation, and creep resistances that are desired of silicon carbide. This work analyzes the chemical composition of the matrix in batches of SiC/SiC (silicon carbide fiber-reinforced silicon carbide matrix) minicomposites that are processed by chemical vapor infiltration of the BN interphase and SiC matrix on single Hi-Nicalon Type S fiber tows using a range of processing parameters. The analysis was performed here to investigate the potential causes of variation in matrix tensile strength in the various batches of minicomposites. Six different morphologies present in the silicon carbide matrix were observed: smooth, nodular, rough nodular, bumpy, nucleated, and plate-like. It was found that high-matrix tensile strength minicomposite batches contained solely the smooth morphology, while low-matrix tensile strength minicomposite batches contained a variety of other morphologies. FIB/TEM was used to study the atomic and crystal character of each individual morphology. Smooth SiC is oriented by the (111) planes and is primarily SiC, while the other morphologies are randomly oriented and contain significant oxygen. These results match the tensile strength tests, which pointed to smooth SiC as the strongest matrix material.

Morphological Control of Silicon Carbide Deposited on Hi-Nicalon Type S Fiber Using Atmospheric Pressure Chemical Vapor Infiltration.

Silicon carbide coated onto Hi-Nicalon Type S fiber is of great interest to the aerospace industry. This work focuses on tuning the reaction parameters of atmospheric pressure SiC CVI using CH3SiCl3 to control the morphology of the coatings produced. Depth of CH3SiCl3 from 1 to 14 cm, temperature from 1000 to 1100 °C, and flow rate of H2 carrier gas from 5 to 30 SCCM were examined. Coating morphologies ranged from smooth to very nodular, where spherical growths were present along the entire deposition zone. The parameters that yielded a smooth deposition throughout the 20 cm deposition zone were 4-6 cm of CH3SiCl3(l) depth, 1100 °C, and 10 SCCM of H2 as a carrier gas. Tensile testing using acoustic emission sensors was performed on SiCf/BN/CVI-SiC minicomposites with different coating morphologies. The tensile tests revealed that smooth coatings have better mechanical performance than the nodular coatings; nodular coatings promote premature ultimate brittle failure, while smooth coatings exhibit toughening mechanisms. Smooth coatings had higher average matrix cracking strength (248 MPa) and ultimate tensile strength (541 MPa) than average nodular coating matrix cracking strength (147 MPa) and ultimate strength (226 MPa).

Rapid Chemical Vapor Infiltration of Silicon Carbide Minicomposites at Atmospheric Pressure.

The chemical vapor infiltration technique is one of the most popular for the fabrication of the matrix portion of a ceramic matrix composite. This work focuses on tailoring an atmospheric pressure deposition of silicon carbide onto carbon fiber tows using the methyltrichlorosilane (CH3SiCl3) and H2 deposition system at atmospheric pressure to create minicomposites faster than low pressure systems. Adjustment of the flow rate of H2 bubbled through CH3SiCl3 will improve the uniformity of the deposition as well as infiltrate the substrate more completely as the flow rate is decreased. Low pressure depositions conducted at 50 Torr deposit SiC at a rate of approximately 200 nm*h-1, while the atmospheric pressure system presented has a deposition rate ranging from 750 nm*h-1 to 3.88 µm*h-1. The minicomposites fabricated in this study had approximate total porosities of 3 and 6% for 10 and 25 SCCM infiltrations, respectively.

Identification and temporal expression of putative circadian clock transcripts in the amphipod crustacean Talitrus saltator.

BACKGROUND: Talitrus saltator is an amphipod crustacean that inhabits the supralittoral zone on sandy beaches in the Northeast Atlantic and Mediterranean. T. saltator exhibits endogenous locomotor activity rhythms and time-compensated sun and moon orientation, both of which necessitate at least one chronometric mechanism. Whilst their behaviour is well studied, currently there are no descriptions of the underlying molecular components of a biological clock in this animal, and very few in other crustacean species. METHODS: We harvested brain tissue from animals expressing robust circadian activity rhythms and used homology cloning and Illumina RNAseq approaches to sequence and identify the core circadian clock and clock-related genes in these samples. We assessed the temporal expression of these genes in time-course samples from rhythmic animals using RNAseq. RESULTS: We identified a comprehensive suite of circadian clock gene homologues in T. saltator including the 'core' clock genes period (Talper), cryptochrome 2 (Talcry2), timeless (Taltim), clock (Talclk), and bmal1 (Talbmal1). In addition we describe the sequence and putative structures of 23 clock-associated genes including two unusual, extended isoforms of pigment dispersing hormone (Talpdh). We examined time-course RNAseq expression data, derived from tissues harvested from behaviourally rhythmic animals, to reveal rhythmic expression of these genes with approximately circadian period in Talper and Talbmal1. Of the clock-related genes, casein kinase IIß (TalckIIß), ebony (Talebony), jetlag (Taljetlag), pigment dispensing hormone (Talpdh), protein phosphatase 1 (Talpp1), shaggy (Talshaggy), sirt1 (Talsirt1), sirt7 (Talsirt7) and supernumerary limbs (Talslimb) show temporal changes in expression. DISCUSSION: We report the sequences of principle genes that comprise the circadian clock of T. saltator and highlight the conserved structural and functional domains of their deduced cognate proteins. Our sequencing data contribute to the growing inventory of described comparative clocks. Expression profiling of the identified clock genes illuminates tantalising targets for experimental manipulation to elucidate the molecular and cellular control of clock-driven phenotypes in this crustacean.

Characterization, localization and temporal expression of crustacean hyperglycemic hormone (CHH) in the behaviorally rhythmic peracarid crustaceans, Eurydice pulchra (Leach) and Talitrus saltator (Montagu).

Crustacean hyperglycemic hormone (CHH) has been extensively studied in decapod crustaceans where it is known to exert pleiotropic effects, including regulation of blood glucose levels. Hyperglycemia in decapods seems to be temporally gated to coincide with periods of activity, under circadian clock control. Here, we used gene cloning, in situ hybridization and immunohistochemistry to describe the characterization and localization of CHH in two peracarid crustaceans, Eurydice pulchra and Talitrus saltator. We also exploited the robust behavioral rhythmicity of these species to test the hypothesis that CHH mRNA expression would resonate with their circatidal (12.4h) and circadian (24h) behavioral phenotypes. We show that both species express a single CHH transcript in the cerebral ganglia, encoding peptides featuring all expected, conserved characteristics of other CHHs. E. pulchra preproCHH is an amidated 73 amino acid peptide N-terminally flanked by a short, 18 amino acid precursor related peptide (CPRP) whilst the T. saltator prohormone is also amidated but 72 amino acids in length and has a 56 residue CPRP. The localization of both was mapped by immunohistochemistry to the protocerebrum with axon tracts leading to the sinus gland and into the tritocerebrum, with striking similarities to terrestrial isopod species. We substantiated the cellular position of CHH immunoreactive cells by in situ hybridization. Although both species showed robust activity rhythms, neither exhibited rhythmic transcriptional activity indicating that CHH transcription is not likely to be under clock control. These data make a contribution to the inventory of CHHs that is currently lacking for non-decapod species.