Materials Characterization - Center for Materials Research
Some of the techniques to characterize solid materials available at the Naval Postgraduate School include: X-ray diffraction (XRD), Physisorption surface area analysis (BET), Thermogravimetry (TGA)-Differential Scanning Calorimetry (DSC), Mass Spectroscopy (MS), Energy Dispersive Spectroscopy (EDS), Electron Microscopy (SEM and TEM) and Energy Electron Loss Spectroscopy (EELS). A solid sample could be analyzed to determine features such as crystal structure, surface area, size and shape of pores, thermal stability, phase distribution and bonding environment among others.
NPS has Rigaku MiniFlex, a general purpose X-ray powder diffractometer 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 initiocrystal structure analysis.
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.
The Netzsch STA 449 F3 Jupiter located at 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 furnace maximum temperature is 1550 degrees C and the heating rates could be varied from 0.001 to 50 K/min. This tool is indispensable to determine thermal stability of samples and precise temperature at which reactions (oxidation, reduction, phase transformations, etc.) occur. 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.
Scanning Electron Microscope / Energy Dispersive Spectroscopy
The morphological features on the samples could be studied at the Zeiss Neon 40. This instrument can perform the typical high resolution scanning electron microscopy tasks to reveal the topographical details, shape and size of sample structures to the nanometric level. This SEM is also useful for many specialized purposes, including being able to section materials in real time, deposit materials, erode materials or conduct electron tomographic reconstructions. It consists of a dual beam SEM/FIB working at accelerating voltages: 0.1-30 kV. It has a Schottky emitter, 2-30 kV Ga metal ion source and a 1.1 nm resolution. The instrument is coupled with an INCA Energy 250 Energy Dispersive X-ray Microanalysis system with Analytical Drift Detector.
Transmission Electron Microscope / EDS / EELS
The Tecnai Osiris located at NPS is a fully digital 200 kV S/TEM system, designed to deliver outstanding quality in TEM and STEM imaging. Apart of the basic internal structure images of the sample in dark and bright fields along electron diffraction, this model includes ChemiSTEM technology for EDX signal detection. It has a high brightness electron source, Schottky FEG, and Silicon Drift Detector (SDD), which could achieve a factor of more than 50 enhancement in acquisition speed of EDX chemical mappings. X-FEG is a high brightness electron source that combines benefits such as a high total current, long-term stability and long lifetime with brightness values of a Cold FEG, without the increased vacuum requirements or tip-flashing. The X-FEG delivers about 5 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 EDX/EELS, and improved spatial coherence for holography and HRTEM applications.
The high sensitivity EDX system allows the detection of all elements down to and including boron. The instrument is equipped with a high speed digital camera. The TEM is coupled with an Energy Electron Loss Spectrometer. EELS is an analytical technique that measures the change in kinetic energy of electrons after they have interacted with a sample, in conjunction with the TEM this tool is capable of giving structural and chemical information about the solid samples as needed. NPS also has tensile testers, Raman Spectroscopy, Nanoindentor, Microhardness testers, Sputtering chamber, Optical microscopes and multiple approaches to fabricate materials.
The capabilities include (a) Fourier transform infrared spectrometer (FTIR) for spectral characteristic of materials from 1.5 to 20 THz and (b) microbolometer focal plane array camera and THz quantum cascade lasers (QCLs) for real time imaging in 1-5 THz spectral band. These facilities are being used for developing high THz absorbing materials and video rate imaging of concealed objects.