Electron Microscopy

NPS hosts three electron microscopes across MAE and Physics. This includes a single beam scanning electron microscope (SEM), one dual-beam SEM, and a transmission electron microscope. The microscopes afford NPS faculty, staff, and students the ability to image and analyze samples at the microstructural down to atomic scales.
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NPS hosts a Thermo-Fisher Scientific (FEI) Helios 5 UX dual-beam SEM with focused ion beam (FIB) capabilities. The FIB, coupled with FEI’s Easylift system allows users to prepare lamellas (50 – 150 nm thick slices) out of bulk samples for nanometer scale analysis in a transmission electron microscope (TEM). The FIB also allows for serial sectioning of a material, which in turn allows for tomographic imaging and analysis.

The Helios SEM has a Schottky FEG with resolutions down to 0.7 nm at 30 kV or 0.7 nm at 1.0 kV (good for imaging ceramic or non-conductive samples). For materials that are especially sensitive to an electron beam, e.g., most polymers, a bias can be applied to the stage to reduce electron landing energies, improving imaging on these surfaces. An immersion mode is included for higher resolution of nano particles and sub-micron structures on surfaces. The Helios contains five separate detectors for imaging with electrons and ions for enhanced contrast of materials during imaging.
 
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The FEI Inspect f50 SEM is a general purpose SEM used for imaging material's microstructure. With its cold field emission gun, the microscope allows for high resolution imaging. 
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The FEI Tecnai Osiris transmission electron microscope (TEM) is a fully digital 200 kV S/TEM system with a field emission gun (FEG). Bright-field, dark-field, and electron diffraction imaging can be performed. Phase contrast and atomic resolution is also possible in this TEM. The TEM includes ChemiSTEM technology for nanoscale energy dispersive spectroscopy (EDS). The high-sensitivity EDS system allows for the detection of all elements down to and including boron. The instrument is equipped with a high-speed digital camera (Gatan). The TEM also comes with an electron energy loss spectrometer (EELS), which is an analytical technique that measures the change in kinetic energy of electrons after they have interacted with a sample.

The X-FEG system combines the benefits of high total current, long-term stability and long lifetime with the high brightness values of a cold FEG. All without the increased vacuum requirements or tip-flashing. The X-FEG delivers about five times the beam current of a standard Schottky FEG while keeping the convergence angle small. This provides improved signal-to-noise ratio in STEM and EDS/EELS and improves spatial coherence for holography and HRTEM applications.

Optical Microscopy

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A Nikon Epiphot 200 optical microscope used for optical imaging of material microstructures. Faculty, staff, and students have access to a fully equipped metallography lab that includes equipment for sectioning, encapsulating, and polishing of metal, ceramic, polymer, and composite samples. This lab also contains stereomicroscopes for low magnification, wide-angle imaging of samples.  

Surface Analysis

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NPS’s inVia Confocal Raman Microspectrometer (Renishaw Inc.) offers one of the largest arrays of excitation wavelengths in the US with UV, visible and near-IR laser excitation. The spectrometer is equipped with a heating/cooling sampling stage (-170 to 600ºC), tensile testing stage (0.01-200N), and high pressure cell (up to 14bar). Operation modes include confocal Raman spectroscopy, polarized Raman spectroscopy, ultrafast Raman mapping and imaging, in situRaman spectroscopy, and surface-enhanced Raman spectroscopy.
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The surface area and pore size of a material can be estimated using a Quantachrome Nova 4200e analyzer. In practice, the sample is degassed in a glass cell and then small amounts of gas (typically N2) are admitted, in steps, into the evacuated sample chamber. During this step gas molecules stick to the surface of the solid and form a thin layer. The number of molecules required to cover the adsorbent surface with a monolayer of adsorbed molecules can be estimated, and the surface area calculated, using the BET (Brunauer, Emmett and Teller) method.

The addition of gas beyond the monolayer formation will cause the equilibrium adsorbate pressures to approach saturation and pores will completely fill with adsorbate, what will produce an adsorption isotherm. By measuring the volume of gas absorbed by the material across a range of preset pressures, the pore sizes could be computed from equilibrium gas pressures using diverse methods. Knowing the density of the adsorbate, the volume it occupies and hence the total pore volume of the sample can be estimated.


Thermal Analysis

These tools are used to determine the thermal stability of samples and the precise temperature at which reactions (oxidation, reduction, phase transformations, etc.) occur.
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The Netzsch STA 449 F3 Jupiter located in Watkins Hall Labs at NPS is a simultaneous thermogravimetric analyzer – differential scanning calorimeter. Weight changes in the sample under study can be recorded as a function of temperature at the same time that heat flows are measured. The maximum temperature is 1550 °C and the heating rates could be varied from 0.001 to 50 K/min. The atmosphere of the process could be controlled and the volatile components in the exhaust identified by the use of a MS403C Aëolos mass spectrometer.

X-ray Diffraction

X-ray diffraction (XRD) allows for the characterization of crystal structure. Our labs host a powder XRD and a portable XRD for residual strain measurement.
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NPS has a Rigaku MiniFlex 600, a general purpose X-ray powder diffractometer (XRD) that can perform qualitative and quantitative analysis of polycrystalline materials. The instrument comes with Cu tube and PDXL, Rigaku's full-function powder diffraction analysis package. The later includes a fundamental parameter method (FP) for more accurate peak calculation, phase identification using the Crystallography Open Database (COD), and a wizard for ab initio crystal structure analysis.
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A Proto iXRD is a portable x-ray diffractometer used for residual strain measurements. Notably, the Proto was used to measure residual strains in AA5456 shipboard deck plates near weldments.