Monolithic Single Crystals Research Articles

Dopant segregation in YAG single crystal fibers grown by the laser heated pedestal growth technique

In this study, we report self-segregation of dopants in a crystal matrix within a single crystal (SC) fiber. Neodymium and holmium-doped yttrium aluminum garnet (YAG) fibers were grown using the Laser Heated Pedestal Growth (LHPG) technique and cross-sectional dopant concentration was measured using electron-probe micro-analysis. It was observed that the degree of auto-segregation of the rare-earth dopant depended on the difference in ionic size of the dopant ion and the Y3+ ion in the YAG matrix. While holmium showed little tendency to self-segregate, the concentration of neodymium ions varied as much as 25% across the cross-section of the fiber. Strong correlation between the dopant concentration profile and fiber draw speed was also demonstrated. Since the local refractive index depends on the concentration of dopants, a refractive index profile can be achieved by a dopant profile across the fiber cross-section. Engineered index profiles can be realized by varying growth conditions, dopants, crystal matrix, etc. Such an approach is promising in applications such as the development of monolithic SC fibers with graded-index profiles.

Biologically inspired far-from-equilibrium materials

Abstract

Divalent europium doped and un-doped calcium iodide scintillators: Scintillator characterization and single crystal growth

The alkaline-earth scintillator, CaI2:Eu2+, was initially discovered around 1964 by Hofstadter, Odell, and Schmidt. Serious practical problems quickly arose, however, that were associated with the growth of large monolithic single crystals of this material due to its lamellar, mica-like structure. As a result of its theoretically higher light yield, CaI2:Eu2+ has the potential to exceed the excellent scintillation performance of SrI2:Eu2+. In fact, theoretical predictions for the light yield of CaI2:Eu2+ scintillators suggested that an energy resolution approaching 2% at 662keV could be achievable. As in the case of the early SrI2:Eu2+ scintillator, the performance of CaI2:Eu2+ scintillators has traditionally suffered due, at least in part, to outdated materials synthesis, component stoichiometry/purity, and single-crystal-growth techniques. Based on our recent work on SrI2:Eu2+ scintillators in single-crystal form, we have developed new techniques that are applied here to CaI2:Eu2+ and pure CaI2 with the goal of growing large un-cracked crystals and, potentially, realizing the theoretically predicted performance of the CaI2:Eu2+ form of this material. Calcium iodide does not adhere to modern glassy carbon Bridgman crucibles—so there should be no differential thermal-contraction-induced crystal/crucible stresses on cooling that would result in crystal cracking of the lamellar structure of CaI2. Here we apply glassy carbon crucible Bridgman growth, high-purity growth-charge compounds, our molten salt processing/filtration technique, and extended vacuum-melt-pumping methods to the growth of both CaI2:Eu2+ and un-doped CaI2. Large scintillating single crystals were obtained, and detailed characterization studies of the scintillation properties of CaI2:Eu2+ and pure CaI2 single crystals are presented that include studies of the effects of plastic deformation of the crystals on the scintillator performance.

From colloidal nanoparticles to a single crystal: New insights into the formation of nacre’s aragonite tablets

Nacre has long served as a model for understanding the biomineralization mechanism and designing bio-inspired materials. However, its basic building blocks, the aragonite tablets, are still under debate in terms of their fine structure at the nanoscale and corresponding formation mechanism. Here, using a field emission scanning electron microscope (SEM), high resolution transmission electron microscope (HRTEM), and X-ray diffractometer, we comparatively investigate the immature and mature tablet from the green mussel's nacre. We find that: (1) the early immature tablet consists of closely-packed colloidal nanoparticles, which contain nanocrystals surrounded by the amorphous calcium carbonate (ACC) phase. Moreover, these nanocrystals are generally different in shape, size, and orientation; (2) the immature tablet can grow via oriented attachment besides via transformation of the ACC phase; and (3) with growth, the colloidal nanoparticles gradually increase in crystallinity and size until fully crystallized and fused together, leading to a mature tablet that is a monolithic single crystal of aragonite. Based on these findings, we propose a new model showing how the mature tablet evolves from the primary colloidal ACC nanoparticles. We expect this work will provide new insights into the formation of single crystal biominerals via the amorphous precursor route.

Phenomenological crystal plasticity modeling and detailed micromechanical investigations of pure magnesium

We present a single crystal plasticity model for pure Mg incorporating slip and deformation twinning. The model uses the basic framework of Kalidindi (1998), but proposes constitutive descriptions for the slip and twin evolution and their interactions that are motivated by experimental observations. Based on compelling experimental evidences, we distinguish between the constitutive descriptions of the tension and compression twinning to better represent their roles in the overall hardening of Mg single crystals. With these improved phenomenological descriptions, we first calibrate material parameters for the different slip and twin modes by performing three-dimensional simulations mimicking the plane-strain compression experiments by Kelley and Hosford (1967, 1968) on single crystal pure Mg. In doing so, these computational responses are critically compared with their corresponding orientation-dependent microscopic (slip and twin activities) and macroscopic (stress–strain responses) experimental observations. Then, the calibrated parameters are used to predict several other experimental results on pure single- and poly-crystal Mg under different loading conditions. We also investigate the role of pre-existing heterogeneities such as initial twin population and stiff, elastic inclusions on the single crystal macroscopic and microscopic responses. Microstructural characteristics show that such heterogeneities strongly influence the local and global evolution of the slip and twin activities, and in some cases modulate the strength anisotropy that is commonly observed in monolithic single crystals. These results may provide useful indicators toward designing novel composite Mg microstructures.

Branched Copper Nanocrystal Corals by Room-Temperature Galvanic Displacement

Coral-like branched architectures comprised of single-crystal copper nanocrystals were synthesized at room temperature through a galvanic displacement reaction between aqueous CuCl2 and Al foil in the presence of a cationic double-chain surfactant diocta-decyl-dimethyl-ammonium bromide. The corals are monolithic single crystals consisting of nanorod stems with an axis along ⟨001⟩ and orthogonal branches along ⟨110⟩. The branch diameters fall in a narrow range between 80 and 100 nm, and the branch lengths vary between 200 and 800 nm. The branch density is controllable by adjusting the surfactant/metal-ion ratio in solution and reaction time. These branched structures could serve as attractive building blocks for creating interconnected nanorod networks or porous materials for diverse applications, including catalysts, sensors, solid-state refrigerators, and nanodevices.

Aragonite twinning in gastropod nacre

Aragonite twinning in gastropod nacre has been investigated using scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), and transmission electron microscopy (TEM) with the focused ion beam (FIB) sample preparation technique. At the growth front of nacre, Haliotis discus hannai and Omphalius rusticus form a “stack-of-coins” structures, which consist of pseudo-hexagonal and elliptical aragonite tablets, respectively. SEM, EBSD, and TEM analyses revealed that these tablets are monolithic single crystal and almost free of the {1 1 0} twins that are common in aragonite of biotic or abiotic origin. The longest diagonal of the hexagon and the longer axis of the ellipse are parallel to the a -axis of aragonite. The crystal orientation of each tablet in a stack, measured by TEM-Kikuchi pattern analysis, is almost the same but there is occasionally {1 1 0} twin-like relationship between adjacent tablets along the stacking. On the other hand, the fibrous aragonite layer formed prior to the nacreous structure is composed of polycrystalline aragonite with high density of {1 1 0} twins. TEM observation suggests that the interlamellar organic sheet prevents the inheritance of the twins, by selecting only single domain of the twins, through the mineral bridge.

On the feasibility of investigation of some defect and porous systems by means of positron annihilation spectroscopy

It was shown that positron annihilation spectroscopy is an effective technique for the determination of the size of nanodefects (vacancies, vacancy clusters); the free volumes of pores, cavities, and voids; and their concentration and chemical composition at the site of annihilation in some defect and porous systems. Experimental studies of nanodefects in porous silicon, proton-irradiated monolithic silicon and quartz single crystals, and quartz powders are briefly surveyed.

New insights into the widmanstätten proeutectoid ferrite transformation: Integration of crystallographic and three-dimensional morphological observations

The crystallography and three-dimensional (3-D) morphology of Widmanstatten proeutectoid ferrite precipitates are examined in an Fe-0.12 wt pct C-3.28 wt pct Ni steel isothermally reacted at 650 °C, 600 °C, and 550 °C. This article integrates new orientation mapping (OM) results with the findings of a companion article to this one on the 3-D morphology of proeutectoid ferrit[1] and an earlier transmission electron microscopy (TEM) study which is reanalyzed here in light of the new OM and 3-D results. All of these studies were performed for the same alloy and heat treatments. The 3-D morphologies and distributions of proeutectoid ferrite precipitates are now known to often be quite different from those deduced by conventional two-dimensional (2-D) microscopy techniques. The present crystallographic studies indicate that “primary” ferrite (nucleated directly on prior austenite grain boundaries) forms monolithic single crystals and can be approximated as elongated triangular pyramids. “Secondary” ferrite morphologies can be described as laths and plates branching into the austenite from a thick and/or broad allotriomorphic ferrite base. These secondary Widmanstatten branches are composed of many misoriented crystals with ferrite: ferrite boundaries between them and appear to approach a common orientation as they extend into the austenite grain. Implications of the current findings on existing growth and crystallography models are discussed, and a preliminary hypothesis or mechanism of ferrite formation has been proposed to account for the present observations.

Cyclic saturation and fatigue fracture of tungsten monofilament-reinforced multicrystalline copper composites

Studies on cyclic saturation and fatigue fracture behavior have been conducted on tungsten monofilament-reinforced multicrystalline copper composites. A model was proposed to link the cyclic stress–strain response of the multicrystalline composites to those of monolithic single crystals and fibers. The results calculated by the model show very good agreement with the experimental data at all applied strain amplitudes by which the composites were fatigued. The fatigue fracture mechanism of the composites highly depends on applied plastic strain amplitudes. Most cracks initiate at grain boundaries in the matrix, and propagate toward and break the fiber at low and intermediate plastic strain amplitudes. At a high plastic strain amplitude, fatigue cracks initiate at the fiber even before surface cracks become pronounced, and propagates from inside outwards at a high growth rate.

Application of orientation imaging microscopy to study phase transformations in steels

In this investigation, the methodology, advantages, and limitations of using Orientation Imaging Microscopy (OIM) to study microstructures in multiphase steels are examined. In particular, proeutectoid ferrite precipitation has been investigated in a hypoeutectoid steel (Fe–0.12%C–3%Ni), while the proeutectoid cementite transformation has been studied in a hypereutectoid steel (Fe–1.3%C–13%Mn). By utilizing misorientation maps and inverse pole figure maps to determine the orientation differences between individual crystals, the internal morphology of the solid-state precipitates has been determined. What appear by other types of imaging to be monolithic single crystals are shown to be aggregates of finer crystals with misorientations between them. The relative quality of the Electron Backscatter Diffraction (EBSD) patterns has been correlated to specific phases and to interphase and grain boundaries using pattern quality maps (also referred to as image quality maps). These techniques have been adapted for quantitative identification of different microconstituents, including volume fraction measurements. By making comparisons among optical micrographs, conventional SEM micrographs, pattern quality maps, inverse pole figure maps, and misorientation maps, the advantages and limitations of different types of OIM mapping for studying different aspects of phase transformations in steels are elucidated.

Cyclic deformation of tungsten monofilament-reinforced multicrystalline copper composites

In an attempt to understand the cyclic deformation behavior of continuous fiber reinforced metal matrix composites, plastic strain controlled tests have been performed on tungsten monofilament-reinforced, multicrystalline, copper composites. The cyclic hardening response of the composites greatly depends on the fatigue dislocation structures corresponding to the strain amplitude. For example, at high strain amplitude, i.e. 1×10 −3, secondary slip stimulated by the self-stresses of the primary dislocations becomes more active, and secondary hardening even occurs during saturation. At low strains, loop patches form and are associated with fine slip. At intermediate strains, persistent slip bands occur, but their distribution is altered by the presence of the fiber. The paper introduces a simple model to link the cyclic stress–strain response of the multicrystalline composites to those of monolithic single crystals and fibers. This model not only represents the fiber reinforcement by the rule of mixtures, but also adopts the Sachs model for the single crystal–polycrystal conversion factor. The results calculated by the model show very good agreement with the experimental data in all strain amplitudes at which the composites were fatigued. This encouraging outcome suggests that the new model could be applied to high-cycle fatigue of commercial continuous-fiber-reinforced polycrystalline metal matrix composites.

Lamb-wave microdevices fabricated on monolithic single crystal silicon wafers

Reliable and inexpensive microdevices that can be used in physical, chemical, biological, and environmental applications are in great demand. Lamb-wave (LW) ultrasonic devices have been shown to be extremely useful for applications in liquid environments. Working LW devices have been fabricated in this investigation on monolithic single crystal membranes using well-established integrated-circuit technology. The detailed fabrication procedure, photoacoustic testing of the piezoelectric material film, and a demonstration of device performance are presented in this paper. In practice, device performance was very sensitive to the surface quality of the membrane structure. This process presents the possibility for further size reduction, increased integration, and large-scale production.

Cyclic saturation behavior of tungsten monofilament-reinforced monocrystalline copper matrix composites

Studies on saturation behavior produced by cyclic deformation have been conducted on tungsten monofilament-reinforced monocrystalline copper composites. The effect of the fiber on strain localization has been investigated using interferometry. For a given applied strain amplitude, local strain and volume fraction of the persistent slip bands (PSBs) in the composite appeared no different from those observed in monolithic copper single crystals. However, the distribution of the PSBs was observed to be more uniform, and the total number of PSBs is substantially higher than that in monolithic crystals. The PSBs appeared mostly in the form of micro-PSBs or macro-PSBs with very limited width. Instead of expanding existing PSBs, new PSBs were more likely to nucleate at new locations during cyclic deformation. The volume fraction and width of the PSBs were observed to increase during saturation, which indicates that some of the PSBs become aged and new PSBs form in order to continue to carry the plastic strain. A rule of mixtures model was established to link the cyclic stress–strain response of the monocrystalline composites to the behavior of monolithic single crystals and fibers. The results calculated from the model show very good agreement with the experimental data.

The formation mechanism(s), morphology, and crystallography of ferrite sideplates

The formation mechanism(s), morphology, and crystallography of secondary ferrite sideplates were investigated with transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron backscatter pattern (EBSP) analysis, and optical microscopy in a high-purity Fe—0.12 wt pct C—3.3 wt pct Ni alloy isothermally transformed at temperatures of 550 °C, 600 °C, 650 °C, and 675 °C. The results indicate that two different mechanisms contribute to the formation of these sideplates at austenite grain boundaries. On the first mechanism, primary sideplates form initially, followed by rapid lateral impingement along their bases, resulting in a region along the grain boundary which very early in the growth process resembles an allotriomorphic film. On the second mechanism, sympathetic nucleation of ferrite sideplates occurs atop pre-existing ferrite allotriomorphs, resulting in ferrite:ferrite grain boundaries and significant crystallographic misorientations between the sideplates and the allotriomorphs with which they are associated. These results indicate that “secondary sideplates” and the allotriomorphs from which they evolve arenot composed of monolithic single crystals formed by a morphological instability mechanism but are instead composed of multiple crystals formed by individual nucleation events. Previous investigations in Ti-Cr alloys and a high chromium stainless steel suggest that the findings presented here may be applicable to a number of other alloy systems as well.

Neutron storage between perfect silicon crystal plates

Backreflection at magnetically gated perfect crystal plates has been used to capture highly monochromatic neutrons (6.27 A) in between two such plates on a 1 m long monolithic single crystal of silicon. Up to 78 back and forth reflections corresponding to storage periods up to 263 ms have been achieved. Several improvements are discussed which can further increase the efficiency of such a system by more than a factor of 10. From the measured loss rates we are able to deduce a lower limit for the mean reflectivity of the silicon crystal in the Darwin plateau region of 0.9978.

Characterization of porous silicon layers by means of X-ray double-crystal diffractometry.

(111) and (100) p-type silicon wafers of 0.55 to 33 Ωcm resistivities were anodized in 50 wt% HF solution and formed porous silicon layers were investigated by using X-ray double-crystal diffraction techniques. The porous layer is shown to be a monolithic single crystal, and its lattice spacing is slightly larger than that of the unanodized substrate, resulting in elastic bending of the wafer. The difference in lattice spacing between the porous layer and the substrate was nearly identical independent of wafer orientation and wafer resistivity. The crystal lattice of porous silicon layer is found to be elastically distorted predominantly in the direction normal to the wafer surface. The lattice spacing of porous layer increased slightly with time and the crystalline quality degraded. The results suggest that stresses, generated by the growth of native oxide layers on pore surfaces, are responsible for the elastic lattice distortion of porous layer. It is also shown tht the crystalline quality of the porous layer produced on the (111) wafers appears superior to that of the (100) wafers for all the wafer resistivity studied.

Analog Integral Circuit Packages at Voice Frequency, 12 MC and 88 MC

" Integral Circuit Packages," or "Integrated Electronic Stages," offer the attractive features of small size, light weight, and the promise of ultra-reliability (particularly when combined redundantly) to the aerospace technology. While digital devices are becoming common, analog devices are not. An analog voice uhf receiver is discussed with emphasis on performance data at voice frequency, 12 Mc and 88 Mc. Four device types are included -- the "hybrid" microassembly, the thin film, the monolithic single crystal, and the combined monolithic-thin film. It is shown that stable gain per stage of 23 db can be achieved at 88 Mc, and of 26 db at 12 Mc. Device limitations and peculiarities are discussed.