Effect In Crystalline Materials Research Articles

Recent Development of Quantitative Microanalysis Method Based on Electron Channeling Effects in Crystalline Materials

Recent Development of Quantitative Microanalysis Method Based on Electron Channeling Effects in Crystalline Materials

Modeling of surface effects in crystalline materials within the framework of gradient crystal plasticity

A finite-deformation gradient crystal plasticity theory is developed, which takes into account the interaction between dislocations and surfaces. The model captures both energetic and dissipative effects for surfaces penetrable by dislocations. By taking advantage of the principle of virtual power, the surface microscopic boundary equations are obtained naturally. Surface equations govern surface yielding and hardening. A thin film under shear deformation serves as a benchmark problem for validation of the proposed model. It is found that both energetic and dissipative surface effects significantly affect the plastic behavior.

General method of extreme surfaces for geometry optimization of the linear electro-optic effect on an example of LiNbO3:MgO crystals.

A general method for determining the global maximum of the linear electro-optic effect in crystalline materials based on the construction and analysis of extreme surfaces obtained as a result of the optimization procedure is proposed. The electrically induced optical path length changes for ordinary and extraordinary waves as well as the optical path difference for orthogonally polarized waves were used as the objective functions in the optimization. The objective functions were determined for units of the electric field and crystal thickness in the light pass direction. In the example of LiNbO3:MgO, it is shown that the maximal achievable given values of the optical path length change (global maxima) for ordinary and extraordinary waves are 119pm/V and 277pm/V, respectively. The global maximum of the optical path difference for orthogonally polarized waves is 269pm/V (for 632.8nm wavelength and at room temperature). These global maxima are exceeded by ∼1.5, 1.7, and 2.3 times the respective maximum values on direct cut crystals of LiNbO3:MgO and are ∼5%, 9%, or 11% larger than the global maxima for undoped LiNbO3 crystal. This ensures a possibility to increase the energy efficiency by ∼2.9 or 5.3 times in the case of using of LiNbO3:MgO crystals with optimal cuts as sensitive elements of electro-optic devices.

Plastic Transition to Switch Nonlinear Optical Properties Showing the Record High Contrast in a Single-Component Molecular Crystal.

To switch bulk nonlinear optical (NLO) effects represents an exciting new branch of NLO material science, whereas it remains a great challenge to achieve high contrast for "on/off" of quadratic NLO effects in crystalline materials. Here, we report the supereminent NLO-switching behaviors of a single-component plastic crystal, 2-(hydroxymethyl)-2-nitro-1,3-propanediol (1), which shows a record high contrast of at least ∼150, exceeding all the known crystalline switches. Such a breakthrough is clearly elucidated from the slowing down of highly isotropic molecular motions during plastic-to-rigid transition. The deep understanding of its intrinsic plasticity and superior NLO property allows the construction of a feasible switching mechanism. As a unique class of substances with short-range disorder embedded in long-range ordered crystalline lattice, plastic crystals enable response to external stimuli and fulfill specific photoelectric functions, which open a newly conceptual avenue for the designing of new functional materials.

Criterion for the softening effect in crystalline materials under their doping

A dislocation model of the effect of softening (i.e., decrease in the microhardness or yield stress of the doped material as compared to the initial material) is developed. The motion of dislocations in metals with the body-centered cubic structure, semiconductors, and other materials in accordance with the kink mechanism is described taking into account the effect of impurity atoms creating the centers for benign generation of kink pairs and obstacles for propagation of kinks. The conditions under which the velocity of dislocations increases after the introduction of impurities are determined using the equation describing the kinetics of passage of dislocations through the Peierls barrier in the doped material.

Angular stability of electric field-induced effects in crystalline materials

We analyze angular bandwidths of extrema of the indicative surfaces describing spatial anisotropy of piezoelectric (PE) and electrooptic (EO) properties of doped lithium niobate (LiNbO3:MgO) and langasite (La3Ga5SiO14) crystals. A number of highly efficient experimental geometries are suggested, which are promising for PE and EO devices. Our data obtained with both analytical and numerical techniques characterize angular stability of those devices and, in particular, their angular aperture. We show that, besides of a maximal size of the electric field-induced effects, ‘nondirect crystal cuts’ offer considerably higher angular stability of their characteristics, when compared with that typical for ‘direct crystal cuts’ usually employed in PE and EO devices.

On the νSiO infrared absorption of polysiloxane films

The infrared absorption of polysiloxanes involves a strong band at around 1050cm−1, attributed to the antisymmetric vibration of siloxane bridges. The splitting of this band into two components is generally attributed to coupling between next-neighbor siloxane groups along the polysiloxane chain. From a quantitative analysis of the spectra of these materials, we find that this splitting is larger when the material is in thin-film form, and that the relative intensity of the two components is polarization dependent. We show that these effects are fully understandable in the theoretical framework of infrared absorption by thin films, and are related to long-range dipolar interactions responsible for the longitudinal–transverse splitting effect in crystalline materials. As a consequence, the polarization dependence of the infrared absorption observed for thin films does not appear to be associated with an orientational ordering in the film.

Effect of grain size distribution on plastic strain recovery

Plastic deformation is size dependent, this is observed in experiments and simulations and has a fundamental effect in crystalline materials with average grain size in the nanometer range. Recent experiments show that plastic deformation is not only affected by the average grain size but also by the grain size distribution. One of the most notorious aspects is that nanocrystalline metals recover plastic strain after unloading. Here, we perform numerical simulations that show that plastic strain recovery is driven by the inhomogeneous stress distribution observed in samples with large variations in grain size. Our model predicts that the fraction of plastic strain recovery increases with the macroscopic strain applied to the sample and that the recovery rate increases as the volume fraction of larger grains decreases.

The equivalent axisymmetric model for Berkovich indenters in power-law hardening materials

Nix and Gio [Nix, W.D., Gao, H.J., 1998. Indentation size effects in crystalline materials: a law for strain gradient plasticity. Journal of the Mechanics and Physics of Solids 46, 411–425] established an important relation between the micro-indentation hardness and indentation depth for axisymmetric indenters. For the Berkovich indenter, however, this relation requires an equivalent cone angle. Qin et al. [Qin, J., Huang, Y., Xiao, J., Hwang, K.C., 2009. The equivalence of axisymmetric indentation model for three-dimensional indentation hardness. Journal of Materials Research 24, 776–783] showed that the widely used equivalent cone angle from the criterion of equal base area leads to significant errors in micro-indentation, and proposed a new equivalence of equal cone angle for iridium. It is shown in this paper that this new equivalence holds for a wide range of plastic work hardening materials. In addition, the prior equal-base-area criterion does not hold because the Berkovich indenter gives much higher density of geometrically necessary dislocations than axisymmetric indenter. The equivalence of equal cone angle, however, does not hold for Vickers indenter.

Discrete dislocation plasticity analysis of the wedge indentation of films

The plane strain indentation of single crystal films on a rigid substrate by a rigid wedge indenter is analyzed using discrete dislocation plasticity. The crystals have three slip systems at ± 35 . 3 ∘ and 90 ∘ with respect to the indentation direction. The analyses are carried out for three values of the film thickness, 2 , 10 and 50 μ m , and with the dislocations all of edge character modeled as line singularities in a linear elastic material. The lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation are incorporated through a set of constitutive rules. Over the range of indentation depths considered, the indentation pressure for the 10 and 50 μ m thick films decreases with increasing contact size and attains a contact size-independent value for contact lengths A > 4 μ m . On the other hand, for the 2 μ m films, the indentation pressure first decreases with increasing contact size and subsequently increases as the plastic zone reaches the rigid substrate. For the 10 and 50 μ m thick films sink-in occurs around the indenter, while pile-up occurs in the 2 μ m film when the plastic zone reaches the substrate. Comparisons are made with predictions obtained from other formulations: (i) the contact size-independent indentation pressure is compared with that given by continuum crystal plasticity; (ii) the scaling of the indentation pressure with indentation depth is compared with the relation proposed by Nix and Gao [1998. Indentation size effects in crystalline materials: a law for strain gradient plasticity. J. Mech. Phys. Solids 43, 411–423]; and (iii) the computed contact area is compared with that obtained from the estimation procedure of Oliver and Pharr [1992. An improved technique for determining hardness and elastic-modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7, 1564–1583].

A model of size effects in nano-indentation

The indentation hardness–depth relation established by Nix and Gao [1998. Indentation size effects in crystalline materials: a law for strain gradient plasticity. J. Mech. Phys. Solids 46, 411–425] agrees well with the micro-indentation but not nano-indentation hardness data. We establish an analytic model for nano-indentation hardness based on the maximum allowable density of geometrically necessary dislocations. The model gives a simple relation between indentation hardness and depth, which degenerates to Nix and Gao [1998. Indentation size effects in crystalline materials: a law for strain gradient plasticity. J. Mech. Phys. Solids 46, 411–425] for micro-indentation. The model agrees well with both micro- and nano-indentation hardness data of MgO and iridium.

Harecxs Measurements of Elemental Disordering

It has been long known that orientation effects in crystalline materials can influence characteristic x-ray emission and microanalysis1-7. High Angular Resolution Electron Channeling X-ray Spectroscopy (HARECXS)6-7. a variation of ALCHEMI4-5, has been used at ANL for the last few years to investigate the effects of channeling on quantitative XEDS analysis of materials. More recently we have also been using HARECXS to carefully measure elemental disordering in a number of systems and have found that it can be used very successfully to elucidate the various stages of disorder.Perovskite (nominally CaTiO3) is a host phase for actinides in various wasteforms for the immobilization of high level radioactive nuclear waste. Over geologic time, alpha decay damage of the actinides in perovskite will cause displacive effects that influence the dimensional and chemical stability of the wasteform. in the past, the progression of damage has been studied by monitoring changes in selected area electron diffraction (SAED) patterns with increasing dose (e.g. 11).

HARECX Measurements of Electron Channeling Effects on Quantitative AEM-Based XEDS

It has been long known that orientation effects in crystalline materials can influence characteristic x-ray emission and microanalysis. Most of the recent studies have concentrated upon using the phenomenon to perform site specific distributions of impurity elements in ordered compounds using the ALCHEMI methodology. For the most part, it has been asserted that increasing the parameters of thickness, orientation and beam convergence effectively averages out these effects. Using High Angular Resolution Electron Channeling X-ray Spectroscopy (HARECXS) we have carefully measured the phenomenon in a number of ordered systems and find that it must be considered in many cases.All experimental measurements presented here were conducted on a Philips EM 420T analytical electron microscope. The instrument was operated in the TEM mode, at 120 kV using a LaB6 electron source. The characteristic x-ray emission was measured using an ED AX ultra thin window Si(Li) detector having a FWHM of ∼ 145 eV at Mn Ka.

Indentation size effects in crystalline materials: A law for strain gradient plasticity

We show that the indentation size effect for crystalline materials can be accurately modeled using the concept of geometrically necessary dislocations. The model leads to the following characteristic form for the depth dependence of the hardness: H H 0 1+ h ∗ h where H is the hardness for a given depth of indentation, h, H 0 is the hardness in the limit of infinite depth and h ∗ is a characteristic length that depends on the shape of the indenter, the shear modulus and H 0. Indentation experiments on annealed (111) copper single crystals and cold worked polycrystalline copper show that this relation is well-obeyed. We also show that this relation describes the indentation size effect observed for single crystals of silver. We use this model to derive the following law for strain gradient plasticity: ( σ σ 0 ) 2 = 1 + l ̂ χ , where σ is the effective flow stress in the presence of a gradient, σ 0 is the flow stress in the absence of a gradient, χ is the effective strain gradient and l ̂ a characteristic material length scale, which is, in turn, related to the flow stress of the material in the absence of a strain gradient, l ̂ ≈ b( μ σ 0 ) 2 . For materials characterized by the power law σ 0 = σ ref ε 1 n , the above law can be recast in a form with a strain-independent material length scale l. ( builtσ σ ref ) 2 = ε 2 n + l χ l = b( μ σ ref ) 2 = l ̂ ( σ 0 σ ref ) 2 . This law resembles the phenomenological law developed by Fleck and Hutchinson, with their phenomenological length scale interpreted in terms of measurable material parametersbl].

Debye-Waller Factor Measurements by Quantitative Convergent Beam Electron Diffraction (CBED)

Quantitative CBED techniques are now capable of making low-order structure factor measure-ments with sufficient accuracy to study bonding effects in crystalline materials. The main limitation of these techniques has been identified as the accuracy with which one knows the Debye-Waller factor(s) (DWF(s)). Even where X-ray measurements exist, values have usually been determined at room temperature whereas we often want to perform our electron diffraction experiments at liquid nitrogen temperatures to reduce the effects of thermal diffuse scattering (TDS). Attempts to calculate theoretical DWF values have been shown to have limited accuracy when compared to experimental measurements.This has led to a search for new electron diffraction methods for DWF determination such as the use of HOLZ line segments, high-index systematic rows and electron precession patterns. The aim should be to measure the DWF(s) under identical conditions to those used for the charge density studies, e.g. the same sample thickness, temperature and microscope settings.

Ionic conductivities of spinel-type quaternary lithium chlorides-phase diagrams of LiClM IClM IICl 2 (M ICu, Na; M IIMn, Cd, Mg)

The phase relationships of the systems Li 2M IICl 4M 2 IM IICl 4 (M ICu, Na; M IIMg, Mn, Cd) were studied by thermal analyses (DTA), X-ray and Raman methods. The electric conductivities of the Li 2−2xM 2x IM IICl 4 solid solutions obtained were determined by impedance spectroscopy. In the ternary Li 2M IICl 4 as much as 100 mol% of lithium can be replaced by copper (I) (at 650 K, M IICd) and 80 mol% by sodium (at 720 K). At ambient temperature, the mutual solubilities are considerably smaller, i.e., negligible for M INa and only up to 40 mol% for M ICu. In case of the copper containing compounds, the transitions of the spinel-type solid solutions present at ambient temperature to deficient NaCl superstructures at elevated temperatures can be studied by both X-ray and Raman methods. The ionic conductivities of the solid solutions decrease (i) slightly with increase in sodium content, but (ii) strongly with increasing copper content at least for x<0.5. Since copper (I) primarily replaces lithium ions in tetrahedral voids whereas sodium substitutes those in octahedral ones, these conductivity data confirm the conduction mechanism proposed for spinel-type chlorides formerly, namely an interstitialcy diffusion of the lithium ions located in tetrahedral sites including octahedral interstitial sites. The spinel-type Li 2−2xCu 2x IM IICl 4 are a further example of the mixed alkali effect in crystalline materials.