Competing failure mechanisms in thin films: Application to layer transfer

We investigate the origin of transverse cracks often observed in thin films obtained by the layer transfer technique. During this process, two crystals bonded to each other containing a weak plane produced by ion implantation are heated to let a thin layer of one of the material on the other. The level of stress imposed on the film during the heating phase due to the mismatch of thermal expansion coefficients of the substrate and the film is shown to be the dominent factor in determining the quality of the transferred layer. In particular, it is shown that if the film is submitted to a tensile stress, the microcracks produced by ion implantation are not stable and deviate from the plane of implantation making the layer transfer process impossible. However, if the compressive stress exceeds a threshold value, after layer transfer, the film can buckle and delaminate, leading to transverse cracks induced by bending. As a result, we show that the imposed stress σ_m —- or equivalently the heating temperature -— must be within the range −σ_c<σ_m<0 to produce an intact thin film where σ_c depends on the interfacial fracture energy and the size of defects at the interface between film and substrate

Fresnel transmission coefficients for thermal phonons at solid interfaces

Interfaces play an essential role in phonon-mediated heat conduction in solids, impacting applications ranging from thermoelectric waste heat recovery to heat dissipation in electronics. From a microscopic perspective, interfacial phonon transport is described by transmission and reflection coefficients, analogous to the well-known Fresnel coefficients for light. However, these coefficients have never been directly measured, and thermal transport processes at interfaces remain poorly understood despite considerable effort. Here, we report the first measurements of the Fresnel transmission coefficients for thermal phonons at a metal-semiconductor interface using ab-initio phonon transport modeling and a thermal characterization technique, time-domain thermoreflectance. Our measurements show that interfaces act as thermal phonon filters that transmit primarily low frequency phonons, leading to these phonons being the dominant energy carriers across the interface despite the larger density of states of high frequency phonons. Our work realizes the long-standing goal of directly measuring thermal phonon transmission coefficients and demonstrates a general route to study microscopic processes governing interfacial heat conduction

An energy-based model of longitudinal splitting in unidirectional fiber-reinforced composites

Unidirectional fiber-reinforced composites are often observed to fail in a longitudinal splitting mode in the fiber direction under far-field compressive loading with weak lateral confinement. An energy-based model is developed based on the principle of minimum potential energy and the evaluation of effective properties to obtain an analytical approximation to the critical stress for longitudinal splitting. The analytic estimate for the compressive strength is used to illustrate its dependence on material properties, surface energy, fiber volume fraction, fiber diameter, and lateral confining pressure. The predictions of the model show good agreement with available experimental data

A Comparative Performance Evaluation Between Large-Cap and Mid-Cap Mutual Fund Returns

In Indian capital market provide various investment avenues to the investors, to help them to invest in various industries and ensure the profitable return. Among various financial products, a mutual fund ensures minimum risks and maximum return to the investor. Mutual fund industry has grown extensively in the last five years and has become a popular mode of investment.  There are umpteen numbers of schemes available to the investor based on capitalisation of the company such as large cap, mid cap and micro cap funds. This study attempts to measures the risk adjusted performance of large cap and mid cap funds based on several parameters and know which fund has performed better and led to better wealth creation for an investor in the last five years. For this purpose, five different Asset Management Companies have been chosen for the study and under each Asset Management Company, two funds- one large cap fund and one midcap fund have been studied. Based on the study conducted , it is clear that Mid cap funds have shown better performance than Large cap funds in the last five years and it is likely that the same trend will continue in the next five years. Keywords: Large-cap and Mid-cap mutual funds, Absolute returns, Performance, Risk

Phonon-Phonon Interactions in Strongly Bonded Solids: Selection Rules and Higher-Order Processes

We show that the commonly used lowest-order theory of phonon-phonon interactions frequently fails to accurately describe the anharmonic phonon decay rates and thermal conductivity ($\kappa$), even among strongly bonded crystals. Applying a first principles theory that includes both the lowest-order three-phonon and the higher-order four-phonon processes to seventeen zinc blende semiconductors, we find that the lowest-order theory drastically overestimates the measured $\kappa$ for many of these materials, while inclusion of four-phonon scattering gives significantly improved agreement with measurements. We have identified new selection rules on three-phonon processes that help explain many of these failures in terms of anomalously weak anharmonic phonon decay rates predicted by the lowest-order theory competing with four-phonon processes. We also show that zinc blende compounds containing boron (B), carbon (C) or nitrogen (N) atoms have exceptionally weak four-phonon scattering, much weaker than in compounds that do not contain B, C or N atoms. This new understanding helps explain the ultrahigh $\kappa$ in several technologically important materials like cubic boron arsenide, boron phosphide and silicon carbide. At the same time, it not only makes the possibility of achieving high $\kappa$ in materials without B, C or N atoms unlikely, but it also suggests that it may be necessary to include four-phonon processes in many future studies. Our work gives new insights into the nature of anharmonic processes in solids and demonstrates the broad importance of higher-order phonon-phonon interactions in assessing the thermal properties of materials.Comment: 18 pages, 7 figure

Role of thermalizing and nonthermalizing walls in phonon heat conduction along thin films

Phonon boundary scattering is typically treated using the Fuchs-Sondheimer theory, which assumes that phonons are thermalized to the local temperature at the boundary. However, whether such a thermalization process actually occurs and its effect on thermal transport remains unclear. Here we examine thermal transport along thin films with both thermalizing and nonthermalizing walls by solving the spectral Boltzmann transport equation for steady state and transient transport. We find that in steady state, the thermal transport is governed by the Fuchs-Sondheimer theory and is insensitive to whether the boundaries are thermalizing or not. In contrast, under transient conditions, the thermal decay rates are significantly different for thermalizing and nonthermalizing walls. We also show that, for transient transport, the thermalizing boundary condition is unphysical due to violation of heat flux conservation at the boundaries. Our results provide insights into the boundary scattering process of thermal phonons over a range of heating length scales that are useful for interpreting thermal measurements on nanostructures

Dramatic Failure of the Callaway Description of Heat Flow in Boron Arsenide and Boron Antimonide Driven by Phonon Scattering Selection Rules

Callaway's simplified heat flow model is often used to confirm experimental realizations of unconventional, hydrodynamic and Poiseuille phonon transport in ultrahigh thermal conductivity ($\kappa$) materials, due to its simplicity and low computational cost. Here, we show that the Callaway model works exceptionally well for most ultrahigh-$\kappa$ materials like diamond and boron nitride, but fails dramatically for boron arsenide (BAs) and boron antimonide (BSb). This failure is driven by the inability of the Callaway model to effectively describe the severely restricted phonon scattering in BAs and BSb, where many scattering selection rules are activated simultaneously. Our work highlights the powerful predictive capability of the Callaway model, and gives insights into the nature of phonon scattering in ultrahigh-$\kappa$ materials and the suitability of the Callaway's description of heat flow through them

जलकृषि उत्पादों में दु:स्वाद(ओफ-फ्लेवर) की समस्याएं

कृपया पूरा लेखा पढ

Mechanics of large electrostriction in ferroelectrics

The complex arrangement of domains observed in ferroelectric crystals is a consequence of multiple energy minima of the crystal free energy density. Since the total energy is a sum of the free energy, and electrical and mechanical work, switching between the different energetically equivalent domain states can be achieved by both electrical and mechanical means. For many ferroelectric materials, this results in an electrostrictive phenomenon resulting from domain switching. In the current study, the electrostrictive behavior of single crystal ferroelectric perovskites has been investigated experimentally. Experiments have been performed in which a crystal of barium titanate is exposed to a constant compressive stress and an oscillating electric field and global deformation is measured. The combined electromechanical loading results in a cycle of stress and electric field induced 90-degree domain switching. The domain switching cycle results in a measurable strain response theoretically limited by the crystallographic unit cell dimensions. Induced strains of more than 0.8% have been measured in barium titanate. Larger strains of up to 5% are predicted for other materials of the same class