Hendrik O. Colijn

Preparation of Amorphous Boron Nitride and Its Conversion to a Turbostratic, Tubular Form

Amorphous boron nitride, BN, is obtained from the reaction of B-trichloroborazine, (BCINH)3, with cesium metal. The amorphous product is converted to a turbostratic form upon heating to 1100�C. Scanning electron microscopy reveals a previously unreported morphology composed of hollow tubular structures. The largest of these appear to be approximately 3 micrometers in external diameter and 50 to 100 micrometers in length. Transmission electron microscopy and selected-area electron diffraction also indicate the tube walls to be turbostratic in nature. The mechanism by which the tubes form is not known, although apparent sites of incipient tube growth have been observed.

Rationalization of Microstructure Heterogeneity in INCONEL 718 Builds Made by the Direct Laser Additive Manufacturing Process

Simulative builds, typical of the tip-repair procedure, with matching compositions were deposited on an INCONEL 718 substrate using the laser additive manufacturing process. In the as-processed condition, these builds exhibit spatial heterogeneity in microstructure. Electron backscattering diffraction analyses showed highly misoriented grains in the top region of the builds compared to those of the lower region. Hardness maps indicated a 30 pct hardness increase in build regions close to the substrate over those of the top regions. Detailed multiscale characterizations, through scanning electron microscopy, electron backscattered diffraction imaging, high-resolution transmission electron microscopy, and ChemiSTEM, also showed microstructure heterogeneities within the builds in different length scales including interdendritic and interprecipitate regions. These multiscale heterogeneities were correlated to primary solidification, remelting, and solid-state precipitation kinetics of γ″ induced by solute segregation, as well as multiple heating and cooling cycles induced by the laser additive manufacturing process.

Transmission electron microscopic investigation of high-palladium dental casting alloys

OBJECTIVE The purpose of this study was to use transmission electron microscopy to examine four representative high-palladium alloys and gain insight into possible strengthening mechanisms. METHODS Castings of two Pd-Cu-Ga alloys and two Pd-Ga alloys were thinned by jet polishing and ion milling, followed by plasma cleaning, to yield foil specimens. Multiple specimens were prepared for each alloy. Bright-field images, dark-field images and selected-area electron diffraction patterns for the alloys in the as-cast condition, after simulated porcelain-firing heat treatment, and after annealing at 980 degrees C were analyzed by standard transmission electron microscope (TEM) techniques. The overall compositions of the ultrastructures for the specimen foils were determined by conventional standardless energy-dispersive spectroscopic analyses with the TEM, and mean values of the elemental compositions were compared to the nominal alloy compositions provided by the manufacturers. RESULTS There was generally good agreement (differences less than 2 wt%) between the overall ultrastructure composition and each nominal alloy composition, except for Protocol from which in may have been lost during casting or formed intermetallic compounds that were not detected by TEM. The same fine-scale tweed structure within parallel bands of approximately 100-200 nm width was observed for all four alloys in the as-cast condition and after simulated porcelain-firing heat treatment. The persistence of the ultrastructure in the specimens of the two Pd-Cu-Ga alloys annealed at 980 degrees C and quenched in ice water indicated very rapid formation from the palladium solid solution. The presence of ¿100¿ and ¿110¿ forbidden reflections for the <001> zone suggested that the tweed structure is ordered, although further research is necessary to establish this conclusion. SIGNIFICANCE The presence of a similar tweed structure in both the Pd-Cu-Ga alloys and the Pd-Ga alloys of substantially lower hardness shows that some other strengthening mechanism accounts for the high hardness and strength generally observed for Pd-Cu-Ga alloys.

£ Factor and k-Factor Determination Using Needle Samples

The Cliff-Lorimer method for TEM/STEM EDX analysis is the most common method of EDX quantification. [1] While most researchers use k-factors supplied by the detector manufacturer, it is generally preferable to use experimentally determined standards. Acquiring appropriate multi-element standards can be challenging since these standards need to be homogeneous on a small scale with no surface films. Once adequate standards have been obtained, it is still necessary to account for absorption in the sample.

Initial Results From a CdTe High-Energy X-ray Detector on a TEM

Energy Dispersive X-ray analysis (EDX) in the transmission electron microscope (TEM) is a well-established technique for thin films. The higher accelerating voltage and small analysis volume allow experiments not be possible in the SEM. However, there are instances where characteristic X-rays from different elements are not resolvable using conventional Si based detectors. Detection of higher energy X-rays from these elements may circumvent the overlap issues. CdTe and CdZnTe detectors have been used for a number of years in nuclear and gamma-ray spectroscopy [1]. The energy response of a typical 1mm thick detector is shown in Figure 1 [2]. These experiments were intended to see if the ability of a CdTe X-ray detector to see higher energy photons would provide the ability to see and resolve high energy X-ray peaks in the TEM.

Multilayer Thin Films as Pseudo-Homogeneous EDX Standards

Quantifying EDX data in the TEM/STEM is generally accomplished using either the Zeta-factor or the Cliff-Lorimer method (k-factors) [1]. The Zeta-factor method requires accurate measurement of the beam current which can be difficult to measure accurately on most microscopes. Consequently the Zeta-factor approach is not frequently used. The Cliff-Lorimer method is much more commonly used but requires accurate k-factors in the analysis. While theoretical k-factors are available there can easily be large variations in the values of the k-factor depending on the theoretical model chosen. These variations can lead to errors in the final composition values. Experimentally determined k-factors are generally more reliable for doing the analysis but require the availability of suitable experimental standards.

Performance of an Improved TEM SDD Detector

External SDD systems can also replace Si(Li) detectors on existing microscopes and offer many of the advantages of the in-situ SDD systems. Since the SDDs do not operate at liquid nitrogen temperatures, the insulation gaps in the nosepiece/collimator assemblies can be reduced or eliminated. Also, the detector window and the associated support frame are no longer used. This means that the detecting area can be moved closer to the sample improving the solid angle for signal collection. In addition, the fact that the detector operates at -20C to -30C instead of cryogenic temperatures means that ice buildup on the detector is no longer an issue. One limitation to increasing the solid angle is that the detector must fit through the port in the sample stage area. While detectors are being made with larger and larger active areas, if they can’t fit through the microscope port they are useless.