a b s t r a c tIn order to evaluate the effect of water vapor on transient stage oxidation of MCrAlY (M_Ni and Co) bond coats, short-term exposures were carried out on thermally sprayed and cast alloy specimens at 1125°C in a range of N 2 -O 2 -H 2 O environments, and resultant thermally grown oxide (TGO) development was evaluated. On cast metal specimens, for which the three-phase alloy dispersion is coarse, growth of unwanted (Ni,Co)(Al,Cr) 2 O 4 spinel was primarily correlated spatially to the Al-poor phases, namely γ, while the preferred TGO product, α-alumina, correlated to the Al-rich β phase. At higher water vapor contents, spinel nucleated and grew out past the γ boundaries to develop above β, too. By thresholding plan view BSE images of TGO surfaces, the amount of spinel vs. alumina surface area coverage was quantified. Spinel coverage was correlated to increasing P H2O , increasing P O2 and, for the highest surface coverage observed, a combination of high P H2O and low P O2 . When a worst case spinel-creating environment of 30 vol.% H 2 O and 10 vol.% O 2 was presented to the more commercially relevant (i.e. sprayed) version of CoNiCrAlY, complete spinel coverage was achieved, indicating that what is often thought of as a long-term spinel growth problem related to Al-depletion of the bond coat, can be created in several hours. Water vapor may enhance transient spinel growth by extending the lifetime of the metastable γ-and δ-alumina phases, the defect spinel structure of which promotes the diffusion of Cr, Co and Ni cations to the TGO surface, whereupon they participate in the development of non-ideal oxides such as spinel. All exposures were carried out without a yttria-stabilized zirconia (YSZ) top coat present.
Zernike phase contrast optics has been demonstrated to be effective in enhancing image contrast for biological specimens. The Zernike phase plate is a thin carbon film with a central hole positioned at the back focal plane of the objective lens (OL) which shifts the modulation of the contrast transfer function from a sine to a cosine function, thereby enhancing the image contrast at low spatial frequencies while maintaining the high resolution information. Cryo-electron microscopy (cryo-EM) uses a transmission electron microscope (TEM) to study macromolecular complexes in their native state. It can also reveal small molecular components in a large macromolecular assembly or identify various subcellular components in a cryo-electron tomogram of cells [1,2]. In order to study complexes that are either small, conformationally heterogeneous or that reside in a crowded cellular environment, the overall system must be able to provide high resolution images with adequate signal-to-noise ratio. The set up of robust TEM and fabrication of enduring phase plates have been challenging. It is a major challenge to build a fully integrated, high performance TEM that is both user-friendly and robust enough to study various biological systems. Progress has been made by different commercial sources using various methods to make the Zernike phase plate and custom configuration of both hardware and electron optics of JEM2200FS. Over the past several years, we have been optimizing our JEM2200FS for high resolution Zernike phase contrast cryo-EM. An important consideration in configuring such a system depends on a number of parameters including the objective lens configuration, anti-contaminator design and types of cryo-specimen tilt holders. Figure 1 shows two configurations in JEM2200FS electron microscope. Their primary differences lie on the design of the anti-contaminator and the focal length of the objective lens. Due to the limited life span of phase plates, we installed an airlock system to allow for the rapid exchange of phase plates without breaking the microscope vacuum. However, the pole piece had a relatively long focal length as well as a weak prefield, which required a long exposure time to achieve a typical single particle cryo-EM dose rate. We then installed a new pole piece with a shorter focal length that has a smaller illumination area, which increased the dose rate to an optimal value (Fig. 1). However, the degree of contrast improvement from the Zernike phase plate depends on its cut-on frequency, which is inversely proportional to the diameter of the central hole and directly proportional to the effective focal length of the lens. Therefore, to recover the contrast lost with the shorter focal length pole piece, we fabricated smaller diameter holes (Fig. 2). As an alternate strategy, we installed another airlock phase plate system at the selected area (SA) aperture, which is at back focal plane of objective mini lens (conjugate to the back focal plane of the OL). This configuration increased the effective focal ...
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