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Surfaces
Published in Gerald L. Schneberger, Adhesives in Manufacturing, 2018
The crystal structure of ideal surfaces for most practically used materials is generally one of three major types: body-centered cubic, face-centered cubic, or close-packed hexagonal. All engineering surfaces vary from these ideal structures. Most real surfaces have grain boundaries which develop during the solidification of crystalline solids. These grain boundaries are, in a strict sense, defects which exist in the bulk solid and extend to the surface. They are atomic bridges linking the crystal structure of the two adjacent grains. Because of their role they do not possess a regular structure; they are highly active and are very energetic. Grain boundaries are large defects, readily observable on real surfaces. In addition to these there are, however, many lesser defects that may exist. These include subboundaries, twins, dislocations, interstitials, and vacancies.
Inorganic Photovoltaic Materials and Devices: Past, Present, and Future
Published in Sun Sam-Shajing, Sariciftci Niyazi Serdar, Organic Photovoltaics, 2017
Aloysius F. Hepp, Sheila G. Bailey, Ryne P. Raffaelle
A lower cost alternative, although less efficient, to standard crystalline silicon technology is the use of polycrystalline silicon [18]. This material is manufactured by pouring liquid silicon into a mold. Upon solidification, multicrystallites form with associated grain boundaries. The resulting blocks of material are sliced into suitable wafers. Polycrystalline silicon (p-Si) has been extensively studied over the past decade due to its applications in optoelectronics. The light absorption properties along with its simple manufacture and broadly tunable morphology have made it an attractive photovoltaic material for some time now. Due to the defects associated with the grain boundaries, the best p-Si solar cell efficiencies stand at 19.8%, less than its mono-crystalline silicon counterpart [19].
Contact Materials
Published in Milenko Braunovic, Valery V. Konchits, Nikolai K. Myshkin, Electrical Contacts, 2017
Milenko Braunovic, Valery V. Konchits, Nikolai K. Myshkin
It was first proposed by Watanabe269 that by controlling thermo-mechanical processing, the type of boundaries in a polycrystalline material could also be controlled by deliberately incorporating materials into the material boundaries which have particularly low values for properties such as energy, diffusivity, and resistivity. This is how the concept of grain-boundary engineering (GBE) was born, which is essentially the manipulation of grain-boundary structure to improve material properties. In other words, grain-boundary engineering allows the production of polycrystalline material whereby the character and distribution of grain boundaries suppress their detrimental effects and enhance their beneficial effects to a maximum extent. These remarkable property enhancements are possible through tailoring the grain-boundary network and promoting the development of grain boundaries with special crystallography and properties. As a result, material properties such as resistance to intergranular corrosion and fracture, creep, and electromigration were greatly improved.270–272
Wear properties of Fe-16Mn-10Al-5Ni-0.86C lightweight steel manufactured by laser powder bed fusion
Published in Powder Metallurgy, 2023
Tae-Hoon Kang, Amol B. Kale, Han-Soo Kim, Kee-Ahn Lee
Through the XRD analysis results depicted in Figure 2, the constituent phases of the LPBF-built LWS and conventional LWS were identified. As a result, the γ-austenite and B2-IMC phases observed in the EBSD phase map were identified as identical. The LPBF LWS presents a broaden peak compared to the conventional LWS, which can contribute to the dislocation pile up mentioned in the paragraph above and sub-grain boundary formation. The broadened peak observed in the XRD analysis of the LPBF LWS is an indication of a smaller crystallite size and a higher density of defects, such as dislocations and vacancies, compared to the conventional LWS. This is due to the rapid solidification rate and high cooling rate during the LPBF process, which results in a finer microstructure and more defects. The presence of these defects and the formation of sub-grain boundaries can have a significant impact on the material’s mechanical properties. The increased density of defects can hinder dislocation motion and contribute to strain hardening, resulting in improved strength and ductility. The formation of sub-grain boundaries can also act as barriers to dislocation motion, effectively strengthening the material. B2-IMCs were represented by FeAl and NiAl, and they have compositions that are different from δ-ferrite.
Investigation of the vacuum hot pressing process on the mechanical properties, microstructures and material characteristics of Ni–35Mo–15Cr alloys
Published in Powder Metallurgy, 2022
Shih-Hsien Chang, Yi-Chang Hsiao, Kuo-Tsung Huang
In addition, grain size is another influential factor. Generally, grain boundaries are lattice defects that lower the conductivity. In this work, the mean grain size of the hot pressing specimens was measured using the linear intercept method [11,12]. As seen in Figures 4(d) and 5(a), the mean grain size of the 1200°C-sintered specimen increased (15.7 ± 0.4→18.9 ± 0.5 μm), which resulted in declined conductivity (8.59 ± 0.14→7.23 ± 0.19 × 103 S·cm−1), thus, it was reasonable to suggest that increasing the mean grain size led to a grain coarsening phenomenon of the Ni–35Mo–15Cr alloys, which affected the conductivity. According to above the discussion and test results, although the mean grain size of Ni–35Mo–15Cr alloys has an important effect on conductivity; but the electrical properties seem to be related to the overall crystal structure in this study.
Improving the thermoelectric figure of merit
Published in Science and Technology of Advanced Materials, 2021
Another possible way of reducing the lattice thermal conductivity is by making the crystal size small. In most materials the phonons are predominantly scattered by other phonons. However, it has long been known that phonons can also be scattered at the crystal boundaries. This effect is usually most obvious at low temperatures when the mean free path of the phonons is greatest. At first sight it is not expected that boundary scattering of phonons would have a significant effect at ordinary temperatures. However, a substantial amount of heat is carried by low frequency phonons and these have a long free path and are more likely to be scattered on grain boundaries. The effect should be greatest for solid solutions because alloy scattering preferentially affects the high frequency phonons [8]. This means that much of the heat transport in solid solutions is carried by the low frequency phonons, the very group that is sensitive to boundary scattering.