Crystal Thickness Research Articles

An improved analytical model of a thick defective phononic crystal for bending wave excitation

This research aims to advance the existing analytical model, which was based on the Euler–Bernoulli beam theory, for predicting the velocity amplitude of bending waves excited while amplified by defective phononic crystals (PnCs) with piezoelectric elements. The previous analytical model falls short for thick defective PnCs and is limited to low frequencies. Hence, the Timoshenko beam theory is newly incorporated to address these issues, considering shear deformations and rotational inertia. Its performance is validated against a finite element model. It confirms that the proposed analytical model outperforms its predecessor, which was suitable for thin defective PnCs with a slenderness ratio above 10 and frequencies around 2 kHz. Remarkably, the improved analytical model demonstrates excellent predictive capabilities up to 40 kHz for thick defective PnCs with a slenderness ratio of around 2.5. With a relative error of less than 1% compared to the finite element model, this advanced approach also demonstrates time-efficient and stable computational performance. This work introduces three contributions. First, it pioneers the proposal of an electromechanically coupled analytical model for predicting the performance of defect-mode-enabled bending wave excitation. Second, this enhanced analytical model surpasses existing counterparts in terms of both accuracy and computational stability, particularly for thick defective PnCs, even at high frequencies. Last, the analytical model dramatically reduces the computation time.

Fabrication and characterization of CaF2 target for multi-nucleon transfer reaction studies

Thin CaF2 targets with carbon backing are fabricated for the first time using physical vapour deposition technique at the target laboratory of IUAC, New Delhi to perform a multi nucleon transfer reaction on 10,11B＋40Ca systems using General Purpose Scattering Chamber (GPSC) facility. Ca being an oxidizing material, we have used molecular CaF2 as the target material instead of Ca. Using turbo-molecular pump based coating unit (TMPU), carbon is deposited on cleaned glass slide and then CaF2 is deposited on diffusion pump based coating unit (DPU). We are successful in fabricating CaF2 targets of thickness ≈264.4μg/cm2 on carbon backing of thickness ≈21.5μg/cm2. The thickness and purity measured by the crystal thickness monitor equipped with the vacuum chamber is verified by different techniques like alpha energy loss, Rutherford Back Scattering (RBS), Electron Diffraction Spectroscopy (EDS) and X-ray Diffraction (XRD). Small impurity level were smeared during the observation and the target is successfully used for nuclear reaction experiment. The fabrication of molecular Ca target is found to be more conducive to experimentation since they exhibit less oxidation and herewith we discuss about fabrication technique, experimentation and characterization.

Enhanced terahertz magneto-optical performance in substrate-free ultra-thick TbErBi:RIG crystal films

A wafer-scale single crystal thick film of rare-earth iron garnet (RIG) has been successfully produced on a 3-in. gadolinium gallium garnet (GGG) substrate using the liquid phase epitaxy method. The RIG crystal's thickness measures ∼550 μm. By removing the GGG substrate through polishing, we improved the terahertz (THz) transmittance of the RIG crystal. In the frequency range of 0.1–1.0 THz, the RIG material exhibits a large refractive index of around 4.50, with a transmittance of ∼60% and an absorption rate of only 10–50 cm−1. Furthermore, we investigated the THz magneto-optical effect in the RIG material through THz time-domain spectroscopy. The observed results demonstrate the presence and significance of the magneto-optical effect in the RIG crystal. To provide further insights, we measured the THz Faraday rotation angle of the 550 μm-thick RIG crystal using the THz-TDS system under an external magnetic field of 0.17 T. The measured Faraday rotation angle reached 22°, and the calculated Verdet constant for the RIG sample was ∼120°/mm/T. Considering these findings, our study highlights the unique properties of this wafer-scale single crystal thick film of RIG, including its low loss, high transmission, and strong magneto-optical effect in the THz range. These characteristics make it a promising candidate for various applications, such as THz magnetic polarization conversion, non-reciprocal phase shifters, and isolators.

Effects of friction stir processing on strength and ductility of laser directed energy deposited AerMet100 steel

Due to thick columnar crystals and obvious pores as typical defects, the mechanical properties of AerMet100 steel fabricated by laser directed energy deposition (LDED) are adversely affected, limiting its applicability in aerospace industry. Aiming at both defects, we developed friction stir processing as a post treatment to strengthen the LDEDed specimens, with two following advantages: (i) it significantly reduced the porosity and homogenized the microstructure, and the average grain size was decreased from 4.75 μm to 1.24 μm; and (ii) it significantly enhanced room temperature tensile properties by eliminating the pores, leading to the fact that the elongation increased from 3.51% to 7.38%, and the ultimate tensile strength increased from 1458 MPa to 1561 MPa. Therefore, FSP generated a simultaneous improvement in both the strength and elongation of the LDEDed AerMet100 steel.

Scanning time-resolved measurement of transient lattice strain on quartz oscillators resonating under alternating electric field

Distributions of transient and local lattice strains on resonating AT-cut quartz oscillators were measured in situ by scanning time-resolved X-ray diffraction under an alternating electric field to reveal the effects of the crystal shape and electrode thickness on their piezoelectric vibration. The concentration of the lattice vibration amplitude and energy at the electrode center in a plano-convex type oscillator and enhancement of the lattice strain in a plano–plano type oscillator within the electrode area with increasing electrode thickness have been unambiguously demonstrated by the method without any surface modifications.

Background energy spectra of lutetium-based inorganic scintillators for radiation detection

In this work the predictions of the background spectra of ten scintillating crystals with lutetium for 1 cm3 cubes are presented. These spectra were obtained by performing Monte Carlo simulations of the 176Lu decay, and the radiation interaction mechanisms in the crystal materials, using PENELOPE and GATE, and by using an analytic model based on the 176Lu decay radiation emission and absorption probabilities. To properly simulate these materials, the mass density and elemental composition were investigated from various references, from which other important properties were derived, such as the expected count rate due to their intrinsic radiation and the intrinsic quantum efficiency for a continuous range of crystal thickness and photon energies, presented as two-dimensional maps. Additionally, other physical properties important for radiation detection of the lutetium-based scintillators are also compared to those of lutetium oxyorthosilicate, the most prevailing material in current detector technology for various imaging applications.

Single-crystallization of electrolytic copper foils

Depending on the production process, copper (Cu) foils can be classified into two types, i.e., rolled copper (r-Cu) foils and electrolytic copper (e-Cu) foils. Owing to their high electrical conductivity and ductility at low cost, e-Cu foils are employed extensively in modern industries and account for more than 98% of the Cu foil market share. However, industrial e-Cu foils have never been single-crystallized due to their high density of grain boundaries, various grain orientations and vast impurities originating from the electrochemical deposition process. Here, we report a methodology of transforming industrial e-Cu foils into single crystals by facet copy from a single-crystal template. Different facets of both low and high indices are successfully produced, and the thickness of the single crystal can reach 500 µm. Crystallographic characterizations directly recognized the single-crystal copy process, confirming the complete assimilation impact from the template. The obtained single-crystal e-Cu foils exhibit remarkably improved ductility (elongation-to-fracture of 105% vs. 25%), fatigue performance (the average numbers of cycles to failure of 1600 vs. 200) and electrical property (electrical conductivity of 102.6% of the international annealed copper standard (IACS) vs. 98.5%) than original ones. This work opens up a new avenue for the preparation of single-crystal e-Cu foils and may expand their applications in high-speed, flexible, and wearable devices.

Long-Range Electron Transport Rates Depend on Wire Dimensions in Cytochrome Nanowires.

The ability to redirect electron transport to new reactions in living systems opens possibilities to store energy, generate new products, or probe physiological processes. Recent work by Huang et al. showed that 3D crystals of small tetraheme cytochromes (STC) can transport electrons over nanoscopic to mesoscopic distances by an electron hopping mechanism, making them promising materials for nanowires. However, fluctuations at room temperature may distort the nanostructure, hindering efficient electron transport. Classical molecular dynamics simulations of these fluctuations at the nano- and mesoscopic scales allowed us to develop a graph network representation to estimate maximum electron flow that can be driven through STC wires. In longer nanowires, transient structural fluctuations at protein-protein interfaces tended to obstruct efficient electron transfer, but these blockages are ameliorated in thicker crystals where alternative electron transfer pathways become more efficient. The model implies that more flexible proteinprotein interfaces limit the required minimum diameter to carry currents commensurate with conventional electronics.

Slow light and photon echoes in the 7F0→5D0 transition of Sm2+ in single-crystal BaFCl.

Slow light effects induced by transient spectral hole-burning in the 7F0→5D0 transition of Sm2+ in BaFCl at 688 nm are reported and a probe pulse delay of 1.25 μs was observed through a 5 mm thick crystal. This delay corresponds to a reduction of the group velocity vG of the transmitted light to ∼4000 m/s. An analysis of the dependence of the slow light effect on the probe pulse timing indicates some broadening of the spectral hole caused by relatively fast excitation energy transfer. We also demonstrate two-pulse (2PE) and (three-pulse) stimulated photon echoes (SPE) for the first time for Sm2+ in the solid state and a homogenous linewidth of 16 kHz (∼2.5·10-8 nm) was obtained at 1.8 K. The echoes in the optically dense medium were very efficient and revealed spectral diffusion on the 100-μs time scale possibly due to flipping of the fluorine and chlorine nuclear spins in the environment of the Sm2+ ions. Furthermore, the SPE also indicates relatively fast energy transfer, commensurate with the hole decay.

Nonlinear THz Generation through Optical Rectification Enhanced by Phonon-Polaritons in Lithium Niobate Thin Films.

We investigate nonlinear THz generation from lithium niobate films and crystals of different thicknesses by optical rectification of near-infrared femtosecond pulses. A comparison between numerical studies and polarization-resolved measurements of the generated THz signal reveals a 2 orders of magnitude enhancement in the nonlinear response compared to optical frequencies. We show that this enhancement is due to optical phonon modes at 4.5 and 7.45 THz and is most pronounced for films thinner than 2 μm where optical-to-THz conversion is not limited by self-absorption. These results shed new light on the employment of thin film lithium niobate platforms for the development of new integrated broadband THz emitters and detectors. This may also open the door for further control (e.g., polarization, directivity, and spectral selectivity) of the process in nanophotonic structures, such as nanowires and metasurfaces, realized in the thin film platform. We illustrate this potential by numerically investigating optical-to-THz conversion driven by localized surface phonon-polariton resonances in sub-wavelength lithium niobate rods.

Effect of Al2Cu constituent layer thickness discrepancy on the tensile mechanical behavior of Cu/Al2Cu/Al layered composites: a molecular dynamics simulation

Nano-polycrystalline Cu/Al2Cu/Al layered composites with different layer thicknesses d of single-crystal Al2Cu constituent are constructed. The effects of d on the strength and fracture modes of nano-polycrystalline Cu/Al2Cu/Al layered composites are systematically investigated by molecular dynamics simulations. The uniaxial tensile results show that the ultimate strength and fracture mode of the nano-polycrystalline Cu/Al2Cu/Al layered composites do not change monotonically with the change of single crystal Al2Cu constituent layer thickness d, the ultimate strength peaking at d = 2.44 nm, and the toughness reaching the optimum at d = 4.88 nm. The improvement of deformation incompatibility between Cu, Al and Al2Cu components increases the ultimate strength of polycrystalline Cu/Al2Cu/Al laminated composites. Due to the high activity of Cu dislocation and the uniformity of strain distribution of single crystal Al2Cu, the fracture of nano-crystalline Cu/Al2Cu/Al layered composites changes from brittleness to toughness. This study is crucial to establish the organic connection between microstructure and macroscopic properties of Cu/Al layered composites. To provide theoretical basis and technical support for the application of Cu/Al layered composites in high-end fields, such as automotive and marine, aerospace and defense industries.

Ultrathin copper films grown on SrTiO3 substrates for epitaxy of single-crystalline γ-FeMn

The topological antiferromagnet γ-FeMn is a fascinating material for studying unconventional magnetic properties and topological transport phenomena though high-quality γ-FeMn has been obtained on the Cu substrate for magnetic research. However, the difficulty of growing single-crystalline γ-FeMn films on insulating substrates has prevented experiments from exactly verifying many theoretical predictions on their transport properties. Ultrathin Cu buffer films have been considered for solving this problem but failed because the crystal quality and thickness of Cu films achieved in previous works do not satisfy the growth buffer criteria for γ-FeMn. In this work, the growth of 5-nm-thick Cu films on SrTiO3(001) substrates was studied. It was found that single-crystalline ultrathin Cu films with roughness as low as 1 nm are obtained on the insulating substrate. We subsequently obtained high crystalline quality γ-FeMn films with smooth surface and verified their antiferromagnetism. Finally, two aspects of the influence of Cu films on adjacent films have been analyzed. Our results facilitate the experimental exploration of the fascinating properties of γ-FeMn.

Response characteristics of MAPbBr3 direct conversion X-ray detectors based on measurements and Monte Carlo simulation

In recent years, higher requirements have been placed on the response characteristics of X-ray detectors in the field of medical diagnostic imaging. Due to this, high sensitivity, high attenuation coefficient and low cost detection materials need to be developed. In this paper, the geometric model of a detector was established by Geant4 code. The absorption efficiency and mass attenuation coefficient of MAPbBr3 crystals were calculated in the energy range of 30 keV to 100 keV. Compared with the mass attenuation coefficient of the NIST database, the deviation was within 1.39%. The signal charge number and detection sensitivity of the X-ray interaction with the MAPbBr3 crystal ware calculated. Compared with the CdTe crystal and α-Se, the MAPbBr3 crystal still had a larger detection sensitivity under a smaller applied electric field, which was approximately 9 times higher than that of α-Se. This result indicated that the detection sensitivity could be greatly improved by using a high atomic number and high charge mobility-lifetime product. Based on the simulation results, the 2 mm thick MAPbBr3 crystal exhibited the highest detection sensitivity at 60 keV X-rays, which was in agreement with the experimental results.

Thermal performance of a laser-diode end-pumped Nd:YVO4 slab crystal cooled by a pair of microchannel heat sinks

Laser crystal is a key component in the all solid-state laser system. The cooling method for the laser crystal is quite critical when the pump power is high. However, due to some measurement difficulties, the surface temperature and heat dissipation pattern of the laser crystal have not been well demonstrated previously. In this work, we use a pair of novel central-jetting microchannel heat sinks (CJ-MCHSs) to cool a Nd:YVO4 slab crystal under high-power pump conditions. The surface temperature of the crystal is measured directly from its side face by using a slot barrier to filter stray light from pump radiation. The temperature distribution and local heat flux of the laser crystal are obtained. It shows that the surface temperature of the slab crystal decreases linearly along the beaming depth. At a typical pump power of 276 W, the temperature reaches 136 °C at the front face of the crystal but it is only 58 °C at the end face. The crystal's thickness affects the rate of heat dissipation. When the crystal's thickness increases from 1.0 mm to 1.5 mm, the temperature of the crystal is enhanced by about 15 °C. In the present experiments, the surface temperature of the CJ-MCHS is also monitored. The front side and back side of the CJ-MCHS show quite different temperature distribution patterns because of the non-uniform heat releasing pattern of the crystal. Comparing with a traditional macrochannel heat sink, the novel CJ-MCHS reduces the temperature gradient along the beaming direction and eliminates the unsymmetrical temperature distributions at the sides of the crystal.

Demonstration of an imaging-free terahertz generation setup using segmented tilted-pulse-front excitation.

A novel, to the best of our knowledge, compact, imaging-free, tilted-pulse-front (TPF) pumped terahertz (THz) source based on a LiNbO3 slab with a small wedge angle (< 8°) and with an echelon microstructure on its input surface has been demonstrated. Single-cycle pulses of more than 40-µJ energy and 0.28-THz central frequency have been generated by 100-mJ, 400-fs pump pulses with 4.1 × 10-4 efficiency and excellent focusability. The peak electric field value focused by a single parabolic mirror was 540 kV/cm. Using 200-fs-long pump pulses, the efficiency increased to 1.0 × 10-3, which is in qualitative agreement with the measured increased diffraction efficiency in the velocity matched diffraction order. A further ∼8x increase in efficiency is expected by pumping a cryogenically cooled wedged echelon with appropriate step sizes, better microstructured surface quality, and antireflection coating on both the input and the output sides. THz generation efficiency maxima were found at ∼2.7-mm crystal thickness for both pump pulse durations. The focused THz beam was diffraction limited within 5% accuracy. Compared to conventional THz sources, this setup is very compact, easy to align, can be pumped by larger beam sizes maintaining the high THz generation efficiency, and produces THz pulses with superior focusability.

All-round inhibition of soot generation during diesel combustion by oxygenated biomass fuels: A numerical simulation and experimental study

Soot generated from diesel combustion is an important pollutant source in the atmosphere. Meanwhile, oxygenated biomass fuels are potential green alternatives to diesel that suppress soot formation. The purpose of this study is to evaluate the effect of oxygenated biomass fuels (biodiesel and ethanol) on all soot precursors (polycyclic aromatic hydrocarbons (PAHs)) and soot particles during diesel combustion. Kinetic modeling results showed that both biodiesel and ethanol monotonically reduced the concentration of PAHs mainly by inhibiting their production and facilitating their oxidation. Specifically, the C2-C4 content reduced and the O, H, and OH concentrations increased. Ethanol has a more profound inhibitory effect on pyrene than biodiesel, mainly due to the reduction of C2-C4 species and increased production of CO2 and CO. Soot particles generated from the combustion of diesel (D100) and its blends (D0.8B0.2/B0.1D0.8E0.1/D0.8E0.2; D and E represent the volume fractions of biodiesel and ethanol in diesel, respectively) were collected on an in-house premix burner. Soot particles were further characterized using thermogravimetric analysis (TGA), elemental analysis (EA), fourier infrared spectroscopy (FTIR), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM). The activation energy of D100 was found to be significantly higher than that of other soot particle samples. After blending oxygenated biomass fuels, there is an increase in O elements, oxygen-containing functional groups, layer spacing and streak distance inside PAH crystals, and streak curvature in soot particles; however, there is a decrease in C elements, aliphatic functional groups, crystal size and crystal stacking thickness of PAHs, and average stripe length. These changes indicate that oxygenated biomass fuels allow soot particles to exhibit a higher oxidation activity, which reduces the amount of soot generated by diesel combustion.

Effect of CH2 run length on the crystallization kinetics of sustainable long-spaced aliphatic polyesters

We have studied the isothermal overall crystallization rates and linear growth rates of four symmetric long-spaced aliphatic polyesters (LSAPEs) with 12, 18, 32, or 48 carbons in both the diol and the diacid monomers. With increasing crystallization temperature (Tc) these LSAPEs display discrete rate minima in a number and depth that depend on the length of the methylene spacer between esters. Two rate minima are observed for PE-48,48 and for PE-32,32; one for PE-18,18 and none for PE-12,12. The rate minima are not associated with a change in the mechanism of nucleation and growth or with a change in crystal packing, but occur at Tc where the crystal thickness transitions from a non-integer to a quantized integer number of monomer units. We explain such rate minima based on the self-poisoning feature used earlier to explain similar findings in n-alkanes and other types of precision polyethylene-like systems. In LSAPEs, metastable non-integer crystals attach to the growth front of stable integer crystals retarding their growth until the non-integer segments thicken to complete the layer. Lack of rate minima in the shortest-spaced symmetric polyester is explained by a small difference in lamellar thickness between non-integer and integer layered crystals which favors the formation of integer crystals at any Tc.

Features of a THz pulse generated in the metallic waveguides partially filled with nonlinear crystals

The experimental and numerical study both generation and transmission terahertz pulse in a nonlinear crystal partially filling the rectangular metallic waveguide cross-section is presented. LiNbO3 and DAST crystals have been studied since they have a high-quality factor (FOM) and optical rectification in them is the most promising method for generating an extremely strong terahertz field. Experimentally and numerically has been shown that, depending on the thickness and permittivity of a nonlinear crystal, a broadband terahertz pulse can propagate with low absorption in the form of the fundamental H10 mode or a linear superposition of Hno modes.

Thickness control of dispersion in opal photonic crystals

Opals are naturally occurring photonic crystals which can be formed easily using low-cost self-assembly methods. While the optical behaviour of opals has received significant attention over the last number of decades, there is limited information on the effect of crystal thickness on the optical properties they display. Here, the relationship between volume fraction and crystal thickness is established with an evaporation-induced self-assembly (EISA) method of formation. The extent to which thickness can be used to manipulate the optical properties of the crystals is explored, focusing on the change in the photonic band gap (PBG). Microscopical structural characterization and angle-resolved transmission spectroscopy are used to examine the quality of the photonic crystals formed using different volume fractions of polystyrene spheres, with thicknesses up to 37 layers grown from volume fractions of 0.125%. This work provides a direct correlation between sphere solution volume fraction and crystal thickness, and the associated optical fingerprint of opal photonic crystals. Maximum thickness is examined, which is shown to converge to a narrow range over several evaporation rates. We identify the criteria required to achieve thickness control in relatively fast evaporation induced self-assembly while maintaining structural quality, and the change to the spectroscopic signature to the (111) stopband and higher order (220) reflections, under conditions where a less ordered photonic crystals are formed.

How entanglements determine the morphology of semicrystalline polymers

Crystallization of polymers from entangled melts generally leads to the formation of semicrystalline materials with a nanoscopic morphology consisting of stacks of alternating crystalline and amorphous layers. The factors controlling the thickness of the crystalline layers are well studied; however, there is no quantitative understanding of the thickness of the amorphous layers. We elucidate the effect of entanglements on the semicrystalline morphology by the use of a series of model blends of high-molecular-weight polymers with unentangled oligomers leading to a reduced entanglement density in the melt as characterized by rheological measurements. Small-angle X-ray scattering experiments after isothermal crystallization reveal a reduced thickness of the amorphous layers, while the crystal thickness remains largely unaffected. We introduce a simple, yet quantitative model without adjustable parameters, according to which the measured thickness of the amorphous layers adjusts itself in such a way that the entanglement concentration reaches a specific maximum value. Furthermore, our model suggests an explanation for the large supercooling that is typically required for crystallization of polymers if entanglements cannot be dissolved during crystallization.