This manuscript describes, defines, and discusses the process of cold sintering, which can consolidate a broad set of inorganic powders between room temperature and 300 °C using a standard uniaxial press and die. This temperature range is well below that needed for appreciable bulk diffusion, indicating immediately the distinction from the well-known and thermally driven analogue, allowing for an unconventional method for densifying these inorganic powders. Sections of this report highlight the general background and history of cold sintering, the current set of known compositions that exhibit compatibility with this process, the basic experimental techniques, the current understanding of physical mechanisms necessary for densification, and finally opportunities and challenges to expand the method more generically to other systems. The newness of this approach and the potential for revolutionary impact on traditional methods of powder-based processing warrants this discussion despite a nascent understanding of the operative mechanisms.

The refractory nature of BaTiO3 leads to limited densification and grain growth for films processed at low temperatures and a modest nonlinear dielectric response due to a marked sensitivity to physical scale and material quality. Adding liquid-forming sintering aids, common in bulk ceramics, to thin films enhances mass transport, leading to enhanced grain growth at lower temperatures. This work explores the effectiveness of a sputtered CuO buffer layer with BaO–B2O3 (BBO) fluxes to engineer the microstructure of BaTiO3 films. Grain size and homogeneity increase in the presence of even a ∼1 nm CuO layer. In general, grain size increases from 75 to 370 nm with an addition of 2.2% BBO and 8 nm CuO. Room temperature capacitance in fluxed films increases by a factor of 5 over pure films, and ferroelectric phase transitions are clearly observable in dielectric measurements. CuO–BBO proves effective on (0001) Al2O3 and (100) MgO substrates, although all microstructures are notably finer for the latter.

A low-temperature thin-film processing method for BaTiO3 is studied to understand microstructure development in the presence of a liquid-forming phase. The addition of a eutectic barium borate flux is found to prevent nucleation of BaTiO3 during pulsed-laser deposition on sapphire substrates at 400 °C. Subsequent thermal annealing above the flux's eutectic temperature dramatically enhances the film's microstructural development and crystallinity. A secondary reaction phase of barium aluminate is identified at the substrate interface in both unfluxed and fluxed films, although it is more pronounced in the fluxed films. This barium aluminate phase in conjunction with the liquid flux serves to nucleate {111} twins in the barium titanate, which subsequently lead to enhanced grain growth. The resulting large-grained and dense thin films result in markedly improved dielectric properties.

Epitaxially oriented silicon nanowires (SiNWs) were grown on (111) Si substrates by the vapor–liquid–solid technique in an atmospheric-pressure chemical vapor deposition (APCVD) system using Au as the catalyst and SiCl4 as the source gas. The dependencies of SiNW growth rate on the growth temperature and SiCl4 partial pressure (PSiCl4) were investigated, and the experimental results were compared with calculated supersaturation curves for Si obtained from a gas phase equilibrium model of the SiCl4–H2 system. The SiNW growth rate was found to be weakly dependent on temperature but strongly dependent on the PSiCl4, exhibiting a maximum value qualitatively similar to that predicted from the equilibrium model. The results indicate that SiNW growth from SiCl4 is limited by gas phase chemistry and transport of reactant species to the growth surface under APCVD conditions. The experimental results are discussed within the context of a gas phase mass transport model that takes into account changes in equilibrium partial pressure due to curvature-related Gibbs–Thomson effects.

Vanadium oxide thin films were deposited using pulsed direct current (dc) magnetron sputtering in an atmosphere containing argon and oxygen. The total pressure was varied from 2.5 to 15 mTorr, and the oxygen-to-argon ratio was varied from 2.5 to 30%. The resulting films were characterized using Rutherford backscattering spectroscopy (RBS), transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and glancing incidence x-ray diffraction (GIXRD). Electrical resistivity was calculated from I–V curves acquired from two-point-probe measurements and thicknesses measured from bright-field TEM images of cross-sectioned samples. TEM and GIXRD were used to characterize the crystallinity of each film. A transition from nanocrystalline to amorphous growth was observed with increasing partial pressure of oxygen. In all samples, the only crystalline phase observed was cubic vanadium oxide with the sodium chloride structure. Though the cubic VOx equilibrium phase field is limited to a maximum of x = 1.3, the cubic phase was observed with a value of x up to 2 in the present work. It was apparent from electron diffraction data that increased oxygen content correlated with an increase in the film disorder. The increase in oxygen content also corresponded with an increase in the film resistivity, which varied over 7 orders of magnitude from 1.18 × 10−3 to 2.98 × 104 Ω·cm. The temperature coefficient of resistance was found to increase with increasing oxygen content from −0.1 to −3.5%/°C. A direct correlation between film disorder and temperature coefficient of resistivity (TCR) was observed and could be exploited to engineer materials with the desired TCR.

High-resolution transmission electron microscopy and electron energy loss spectroscopy (EELS) were performed on electrochemically anodized niobium and niobium oxide. Sintered anodes of Nb and NbO powders were anodized in 0.1 wt% H3PO4 at 10, 20, and 65 V to form surface Nb2O5 layers with an average anodization constant of 3.6 ± 0.2 nm/V. The anode/dielectric interfaces were continuous and the dielectric layers were amorphous except for occurrences of plate-like, orthorhombic pentoxide crystallites in both anodes formed at 65 V. Using EELS stoichiometry quantification and relative chemical shifts of the Nb M4,5 ionization edge, a suboxide transition layer at the amorphous pentoxide interface on the order of 5 nm was detected in the Nb anodes, whereas no interfacial suboxide layers were detected in the NbO anodes.

Silicon-germanium (Si1−x Ge x :H) thin films have been prepared by plasma enhanced chemical vapor deposition of SiH4 and GeH4 and measured during growth using real time spectroscopic ellipsometry. A two-layer virtual interface analysis has been applied to study the structural evolution of Si:H films prepared in multistep processes utilizing alternating intermediate and low H2-dilution material layers, which have been designed to produce predominately amorphous films with a controlled distribution of microcrystalline particles. The compositional evolution of alloy-graded a-Si1−x Ge x :H has been studied as well using similar methods. In each case, depth profiles of microcrystalline content, f μc , or Ge content, x, have been extracted. Additionally, real time spectroscopic ellipsometry has been used to monitor post-deposition exposure of a-Si:H, a-Si1−xGex:H, and a-Ge:H films to a hydrogen plasma in situ in order to determine sub-surface amorphous film modification similar to that which would occur when a highly H2-diluted layer is deposited on a layer prepared with lower dilution. These analyses provide guidance for enhanced performance of Si:H based solar cells, through controlled bandgap grading using compositionally graded amorphous binary alloys (a-Si1−xGe x :H) or the incorporation of controlled fractions of microcrystallites into bulk amorphous i-layer materials, and by providing a fundamental understanding of the modification of component layers during the deposition of subsequent layers in multilayer stacks.

The effect of growth conditions on the composition and structure of Si1−xGex nanowires grown by the vapor–liquid–solid method using gaseous precursors (SiH4 and GeH4) was investigated. Transmission electron microscopy was used to characterize the structural properties and elemental composition of the nanowires. At higher growth temperatures (>425 °C), Ge thin film deposition on the nanowire surface resulted in Au loss during growth and the formation of tapered structures. By simultaneously reducing the growth temperature from 425 to 325 °C to suppress the rate of Ge film deposition and increasing the GeH4/(GeH4 + SiH4) gas ratio, Si1−xGex nanowires were produced with Ge fractions spanning the entire composition range. The Ge fraction follows that predicted from the elemental nanowire growth rates in the Ge-rich (x > 0.5) regime, but deviates to higher Ge fractions in Si-rich (x < 0.5) nanowires. A mechanism was proposed whereby surface diffusion provides an additional pathway to Ge incorporation in Si-rich Si1−xGex nanowires.

We have studied the evolution of stress and microstructure of compositionally graded Al1-xGaxN (0 ≤ x ≤1) buffer layers on (111) Si substrates with varying thicknesses. In-situ stress measurements reveal a tensile-to-compressive stress transition that occurs near the half-thickness in each buffer layer. Cross-sectional transmission electron microscopy (TEM) shows a significant reduction in threading dislocation (TD) density in the top half of the buffer layer, suggesting that the compressive stress enhances the threading dislocation annihilation. The composition of the buffer layers varies linearly with thickness, as determined by X-ray energy dispersive spectrometry (XEDS). The composition grading-induced compressive stress offsets the tensile stress introduced by microstructure evolution, thus yielding a tensile-to-compressive stress transition at x ≈ 0.5.