Difference In Lattice Constants Research Articles

Evaluation of the fracture toughness of Ti1-xZrxN hard coatings: Effect of compositions

The objectives of this study were to evaluate the fracture toughness of Ti1-xZrxN hard coatings using the internal energy induced cracking (IEIC) method and to investigate the compositional effect on the fracture toughness, from which the optimum composition for fracture toughness could be attained. Ti1-xZrxN was selected to be the model system, because Ti1-xZrxN remained single phase structure in the entire compositional range at deposition temperature below 500 °C. Three compositions of Ti1-xZrxN coatings, x = 0.25, 0.55 and 0.85, were deposited by unbalanced magnetron sputtering. The parameters of IEIC method included the residual stress determined by the laser curvature method, Young's modulus obtained from nanoindentation and the film thickness measured from SEM cross-sectional image. The residual stress and film thickness before specimen fracture were used to calculate the elastic stored energy (Gs), from which the fracture toughness (Gc) could be derived. The resultant Gc of the Ti1-xZrxN coatings varied with Zr fraction, ranging from 26.0 to 48.7 J/m2, and reaching a maximum at a composition of Ti0.15Zr0.85N. The results showed that adding Zr atoms into TiN could effectively increase the fracture toughness. The maximum increase of fracture toughness lay in the intermediate range of composition (Zr = 0.55–0.85), suggesting that different properties of Ti and Zr atoms may play an important role on the fracture toughness of single phase Ti1-xZrxN thin films. The increase of Gc with Zr composition may be correlated with the change of configurational entropy of the Ti-Zr-N system. The atomic size difference of Zr and Ti may be crucial on increasing fracture toughness. The increase of fracture toughness for Zr-dominant Ti0.15Zr0.85N coating was higher than that for Ti-dominant Ti0.75Zr0.25N coating. This asymmetrical behavior could be attributed to the difference in lattice constants between Ti-rich and Zr-rich compounds, where the capability of increasing Gs may be higher for a smaller Ti atom incorporated into a larger Zr site in ZrN lattice.

Lattice Distortion and Phase Stability of Pd-Doped NiCoFeCr Solid-Solution Alloys.

In the present study, we have revealed that (NiCoFeCr)100−xPdx (x= 1, 3, 5, 20 atom%) high-entropy alloys (HEAs) have both local- and long-range lattice distortions by utilizing X-ray total scattering, X-ray diffraction, and extended X-ray absorption fine structure methods. The local lattice distortion determined by the lattice constant difference between the local and average structures was found to be proportional to the Pd content. A small amount of Pd-doping (1 atom%) yields long-range lattice distortion, which is demonstrated by a larger (200) lattice plane spacing than the expected value from an average structure, however, the degree of long-range lattice distortion is not sensitive to the Pd concentration. The structural stability of these distorted HEAs under high-pressure was also examined. The experimental results indicate that doping with a small amount of Pd significantly enhances the stability of the fcc phase by increasing the fcc-to-hcp transformation pressure from ~13.0 GPa in NiCoFeCr to 20–26 GPa in the Pd-doped HEAs and NiCoFeCrPd maintains its fcc lattice up to 74 GPa, the maximum pressure that the current experiments have reached.

Ab initio calculations of electronic band structure and effective-mass parameters of thermoelectric Mg2X1−xYx(X,Y=Si , Ge, or Sn) pseudobinary alloys

${\mathrm{Mg}}_{2}{X}_{1\ensuremath{-}x}{Y}_{x}$ thermoelectric materials and their pseudobinary related alloys, for $X,Y$ = Si,Ge,Sn, have been the subject of intense research activity. Studies have revealed their electronic nature, but important properties are still unclear or missing in the case of intermediate solid solutions. Within the CPA-KKR formalism, in the full-relativistic description, we observe stronger deviations from Vegard's law as the lattice constant difference increases. We compute the Bloch spectral density function to map a local-parabolic band structure in the alloys, whenever the disorder-induced broadening is small. We trace nonlinear trends for the indirect and direct band gaps, spin-orbit coupling and the crossing between the low-lying conduction bands observed in the ${\mathrm{Mg}}_{2}{X}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}$ systems. Our computations show that the broadening of the heavy- and light-hole bands is the smallest, but, in either case, we discuss the trends and anisotropy effects of band- and direction-dependent effective masses.

Morphological, structural and hardness changes of human dental enamel irradiated with a Nd:YAG laser

Laser technology has become an indispensable part of modern dental practice, used in conjunction with or as an addition to other techniques. This paper focuses on applications of lasers in caries progression and surface hardness/acid resistance and the laser settings needed for clinical application. Enamel of human molar teeth was treated with a Nd:YAG laser (1064 nm) with varying numbers of laser pulses. The corresponding changes induced in the morphology, chemical composition, hardness and structure of human enamel were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, the Vickers hardness test and x-ray diffractometry, respectively. Etching, eruption, melting, recrystallization and cracking were observed in the morphology of enamel, while the hardness of the material increased as the number of pulses was varied. A decrease in intensity position of the hydroxyl group was observed, which is associated with the chemical change. The carbonate content also decreased, which accounts for the increase in acid resistance, but no such difference was observed in the phosphate content. A slight difference in lattice constant was also observed due to loss of structural water from hydroxyapatite. The results show that lasers can be additional resource in controlling the progression of caries in enamel.

Structural, mechanical and electronic properties of (TaNbHfTiZr)C high entropy carbide under pressure: Ab initio investigation

The structural, mechanical and electronic properties of (TaNbHfTiZr)C high entropy carbide are studied by using density functional theory in conjunction with special quasi-random structures. The proposed lattice constant difference as an empirical criterion for high entropy compounds and mixing enthalpy show the formation of (TaNbHfTiZr)C solid solution. The derived elastic stiffness constants also indicate the mechanical stability of (TaNbHfTiZr)C high entropy carbide. At zero pressure, the calculated elastic mechanics obeys the rule of mixture, whereas Vickers hardness is slightly larger than the average value of constituent binary carbides. The computed elastic parameters show that (TaNbHfTiZr)C is brittle, similar to constituent binary carbides. Under high pressure, the lattice constants decrease slightly, and mechanical properties are improved, even the brittleness-ductility transition takes place. The calculated electronic structures show that covalence in (TaNbHfTiZr)C is relatively weaker than ionic bonding. With increasing pressure, covalence in (TaNbHfTiZr)C decreases while ionicity increases. The present research will be valuable for understanding and designing of high-entropy carbides.

First principles calculation of the nonhydrostatic effects on structure and Raman frequency of 3C-SiC

For understanding the quantitative effect of nonhydrostatic stress on properties of material, the crystal structure and Raman spectra of 3C-SiC under hydrostatic and nonhydrostatic stress were calculated using a first-principles method. The results show that the lattice constants (a, b, and c) under nonhydrostatic stresses deviate those under hydrostatic stress. The differences of the lattice constants under hydrostatic stress from nonhydrostatic stresses with differential stress were fitted by linear equation. Nonhydrostatic stress has no effect on density of 3C-SiC at high pressure, namely the equations of state of 3C-SiC under hydrostatic stress are same as those under nonhydrostatic stress. The frequencies and pressure dependences of LO and TO modes of 3C-SiC Raman spectra under nonhydrostatic stress are just same as those under hydrostatic stress. Under nonhydrostatic stress, there are four new lines with 361, 620, 740, and 803 cm−1 appeared in the Raman spectra except for the LO and TO lines because of the reduction of structure symmetry. However the frequencies and pressure dependences of the four Raman modes remain unchanged under different nonhydrostatic stresses. Appearance of new Raman modes under nonhydrostatic stress and the linear relationship of the differences of lattice constants under hydrostatic and nonhydrostatic stresses with differential stress can be used to indicate state of stress in high pressure experiments. The effect of nonhydrostatic stress on materials under high pressure is complicated and our calculation would help to understanding state of stress at high pressure experiments.

Evolution of local lattice distortion under irradiation in medium- and high-entropy alloys

A major challenge for future nuclear reactors is to design nuclear materials that can sustain extremely prolonged radiation damage. Local lattice distortion, considered as a core effect of high-entropy alloys (HEAs), can suppress the growth of radiation defects to control radiation performance. However, local lattice distortion in HEAs is rarely quantitatively measured. Here we employed total scattering technique to study the local structure of the face-centered cubic (fcc) equiatomic FeCoNiCr MEA, and FeCoNiCrMn and FeCoNiCrPd HEAs before and after ion irradiation. Their local lattice distortions were quantitatively evaluated based on the difference of lattice constant between the local structure and the average structure. We revealed that the mean local lattice distortion in the pristine samples varies in the following order: FeCoNiCr < FeCoNiCrMn < FeCoNiCrPd. Density functional theory (DFT) calculations further unveiled that the fluctuation of local distortions in FeCoNiCr and FeCoNiCrMn is less than 5%, whereas the highest bond-length fluctuation in FeCoNiCrPd can approach to 8%. Under irradiation the mean local lattice distortion in FeCoNiCr and FeCoNiCrMn evolves differently from FeCoNiCrPd by showing a relaxation behavior at low dose. And the impact of local lattice distortion on dislocation loops after a prolonged ion-irradiation was investigated by transmission electron microscope (TEM) and the underlying mechanism was discussed.

Strain-engineered allotrope-like bismuth nanowires for enhanced thermoelectric performance

Allotropy is a fundamental concept that has been frequently studied since the mid-1800s. Although the bulk allotropy of elemental solids is fairly well understood, it remains challenging to reliably produce an allotrope at the nanoscale that has a different crystal structure and accompanies a change in physical properties for specific applications. Here, we demonstrate a "heterostructure" approach to produce allotrope-like bismuth nanowires, where it utilizes the lattice constant difference between bismuth and tellurium in core/shell structure. We find that the resultant strain of [100]-grown Bi nanowires increases the atomic linear density along the c-axis that has been predicted from theoretical considerations, enabling us to establish a design rule for strain-induced allotropic transformation. With our >400-nm-diameter nanowires, we measure a thermoelectric figure of merit ZT of 0.5 at room temperature with reduced thermal conductivity and enhanced Seebeck coefficient, which are primarily a result of the rough interface and the reduced band overlap according to our density-functional calculations.

(Invited) MOCVD of 2D Nanomaterials for Next-Generation Electronic and Optoelectronic Devices

The international road map of semiconductors (ITRS) lists 2D materials as possible future materials for electronic devices [ITRS]. Among them, the semiconducting transition metal dichalcogenides (TMDC) like MoS2 or WS2 are the most promising ones. As the miniaturization of electronic devices continues and the sub-5 nm gate limit is reached, direct source-to-drain tunneling will become a problem in Si devices with conventional architecture. 2D TMDC as channel material for FET might overcome this issue in digital CMOS. The larger carrier effective masses of most TMDC result in reduced direct source-to-drain tunneling, while they also yield a large density of states and hence an increased ballistic current in this extreme dimensions [Fiori]. Additionally, the lower in-plane dielectric constants of TMDC enable a better vertical electrostatic control over the channel [Desai]. Still, TMDC will have to compete against other approaches, e.g. multiple-gate transistors [Schwiertz]. Beyond that, other properties of TMDC turn them interesting for future devices, such as their strong absorbance across the visible spectrum for optoelectronic devices [Lotsch]. The first photodetectors, electromluminescent p-n diodes and photovoltaic cells have already been shown [Lopez, Cheng, Tsai]. The excellent mechanical properties additionally enable the realization of different kinds of flexible devices [Schwiertz]. And last but not least, the integration of 2D materials into conventional 3D semiconductor concepts would be another application. For example, the strain originating from the large difference in lattice constants in a 3D heterostructure is often a challenge. Inserting 2D materials might be a solution. Due to the fully terminated surface of a TMDC monolayer, the binding to another material is of van-der-Waals type. Hence, TMDC can be stacked with materials with huge differences in lattice constants without leading to considerable strain or to interface trap densities [Vogel]. First devices with a combination of 2D and 3D materials have already been investigated [Krishna, Lee]. Large-scale fabrication of TMDC is still a challenge to be overcome. Up to now, the most frequently used techniques are either exfoliation of natural layered crystals or the growth via chemical vapor deposition (CVD). Exfoliation is a very time-consuming process with relatively low yield and reproducibility. In CVD, different gaseous precursors decompose on a substrate and react with each other. Thermally evaporated MoO3 and S are commonly used as precursors for the fabrication of MoS2 monolayers via CVD. This technique allows the deposition of monolayered crystallites with a size in the range of micrometers [Dumcenco]. The process is carried out in small experimental reactors, and a uniform deposition on a whole wafer is very challenging. The use of metalorganic precursors is one possibility to enter an industrial scale of fabrication. The development of III/V and II/VI semiconductors has shown that metalorganic chemical vapor deposition (MOCVD) makes controllable and reproducible processes feasible, which are easily scalable and hence suitable for large-area deposition. Additionally, the reactors are well-developed, and a future integration of 2D materials and other semiconductors is within reach. First publications show very good results on the deposition of MoS2 and WSe2 via MOCVD [Kang, Eichfeld]. But despite these works and first simulations of the growth kinetics [Nie], little is known about the growth mechanism. For this reason, we have started to investigate the deposition of 2D MoS2 on an AIXTRON MOCVD reactor.The experiments are carried out in a horizontal hot-wall MOCVD reactor in a 10 × 2 inch configuration. Molybdenum hexacarbonyl (MCO) and Di-tert-butyl sulfide (DTBS) are used as Mo and S sources, respectively. In preliminary experiments, the carrier gas based transport of the precursors into the reactor and possible growth conditions are tested. Their results are used to develop an initial growth process. This process leads to a uniform, wafer-scale deposition of MoS2 on various substrate types such as sapphire (0001), Si (111), and AlN and GaN templates on sapphire substrates. The deposited films mainly consists of MoS2 bilayers and exhibit a very high initial nucleation density. Further investigations of the influence of the growth temperature, the carrier gas composition and the pretreatment of the substrates are carried out. With optimization of these growth parameters, a crystal growth process closer to thermodynamical equilibrium can be achieved. This results in the formation of triangular crystals on sapphire substrates which are also reported from CVD processes and exhibit higher crystal quality [Dumcenco]. Additional experiments are conducted to investigate the nucleation of the films and to further tune nucleation density and lateral growth rate in order to deposit wafer-scale monolayered MoS2 films.

Periodic Modulation of Graphene by a 2D-FeO/Ir(111) Moiré Interlayer

Ultrathin films of iron oxide form a two-dimensional (2D) FeO layer on Ir(111). Because of difference in lattice constant between 2D-FeO and Ir(111), a moire superstructure is formed. The 2D-FeO/Ir(111) structure is examined by soft X-ray photoelectron spectroscopy, X-ray photoemission diffraction, scanning tunneling microscopy, and low-energy electron diffraction. A 2D-FeO layer may also be grown by iron intercalation and subsequent oxidation underneath a graphene layer on Ir(111). Thus, the graphene can be decoupled from the metal by the 2D-FeO layer. Changes in the graphene C 1s binding energy can be mainly explained by shifts in the Fermi level of graphene as a consequence of interface band alignment for weak interactions between graphene and the substrate. A shift of C 1s to lower binding energy, for graphene supported on FeO/Ir(111), is a consequence of the dipole moment in the 2D-FeO layer normal to the Ir(111) surface. Broadening of the C 1s peak is consistent with a locally varying 2D-FeO dipole ...

Growth and Characterisation of Pulsed-Laser Deposited Tin Thin Films on Cube-Textured Copper at Different Temperatures

Abstract High-quality titanium nitride thin films have been grown on a cube-textured copper surface via pulsed laser deposition. The growth of TiN thin films has been very sensitive to pre-treatment procedure and substrate temperature. It is difficult to grow heteroexpitaxial TiN films directly on copper tape due to large differences in lattice constants, thermal expansion coefficients of the two materials as well as polycrystalline structure of substrate. The X-Ray diffraction measurement revealed presence of high peaks belonged to TiN(200) and TiN(111) thin films, depending on used etcher of copper surface. The electron diffraction patterns of TiN(200)/Cu films confirmed the single-crystal nature of the films with cube-on-cube epitaxy. The high-resolution microscopy on our films revealed sharp interfaces between copper and titanium nitride with no presence of interfacial reaction.

Synthesis of single‐phase ZnAl2O4 nanoparticles via a wet chemical approach and evaluation of crystal structure characteristics

Single‐phase ZnAl2O4 nanoparticles were hydrothermally synthesized utilizing improvements to the preparation of the precursor materials. This synthesis differs from more general syntheses of ZnAl2O4 nanoparticles in that it minimizes the presence of ZnO impurities. The higher purity of our ZnAl2O4 nanoparticles allowed for more comprehensive X‐ray diffraction analysis as well as more sensitive spectroscopic evaluation. Moreover, pH control of the precursor solution dramatically improved the reproducibility of the synthesis. The lattice constant of the ZnAl2O4 samples synthesized by our technique is different from what is observed in particles fabricated using more common solid state reactions. This difference in lattice constants indicates the occurrence of a site exchange phenomenon between Zn2+ and Al3+ and/or oxygen deficiency. This suggests the possibility of tailoring novel functional spinel oxides for a wide array of future applications.

Structure of some CoCrFeNi and CoCrFeNiPd multicomponent HEA alloys by diffraction techniques

The structure of CoCrFeyNi (y = 0, 0.8 and 1.2) and CoCrFeNi-Pdx (x = 0.0, 0.5, 0.8, 1.0, 1.2 and 1.5) High Entropy Alloys has been investigated by neutron and standard X-ray as well as by high-energy X-ray diffraction techniques. The alloys were produced by arc melting and afterwards heat treated under several different conditions. It has been concluded that the CoCrFeNi alloy in as-cast condition is, contrary to what is claimed in the literature, not single-phase but consists of at least two different phases, both of fcc type. The difference in lattice constant between the two phases is close to 0.001 Å. Diffraction patterns measured by X-ray and neutron diffraction have shown that the structure of the alloy is not affected by 3 h heat treatment up to 1100 °C. Changing the amount of Fe has no drastic effect on alloy structure. The Pd-containing alloys have also all been found not to be single-phase but to consist of at least four different phases, all being of fcc type. The lattice constants for all phases increase with Pd content. The relative amounts of the different phases depend on Pd concentration. Furthermore, heat treatments of 3 h duration at different temperatures have a significant effect on the alloy phase composition. It is suggested that HEAs should be considered as multicomponent alloys presenting “simple” diffraction patterns, e.g. consisting of one or several lattices of fcc, hcp or bcc type with very close lattice parameters.

Multiple role of dislocations in the heteroepitaxial growth of diamond: A brief review

In the synthesis of epitaxial heterostructures with appreciable lattice misfit, dislocations represent a crucial linear defect. Since device quality is typically deteriorated by their presence, minimizing the dislocation density by optimized growth strategies is a key challenge for R&amp;D. This especially holds also for the system diamond‐on‐iridium with its difference in lattice constants of more than 7%. As a consequence of this misfit, the initial dislocation density is very high. In this review we discuss different aspects of dislocations in heteroepitaxial diamond growth. They vary from mutual interaction mechanisms for an efficient reduction in density, over interaction with surface structures, their crucial role in the formation of intrinsic stress, their presumable role in formation of highly anisotropic stress and also their role as sink for the segregation of boron dopants. Finally, it is pointed out that the dislocation mediated stress formation may be used intentionally to strengthen devices by formation of compressively stressed surface layers. The present article is focused on recent studies from the authors’ research group.

Origin of differences in the excess volume of copper and nickel grain boundaries

The excess volume associated with grain boundaries is one of the primary factors driving defect segregation and diffusion which controls the electronic, mechanical and chemical properties of many polycrystalline materials. Experimental measurements of the grain boundary excess volume of fcc metals Cu and Ni have shown a difference of over 40%. The difference in lattice constant between Cu and Ni is only 3%, therefore this substantial difference is currently lacking explanation. In this article we employ a high throughput computational approach to determine the atomic structure, formation energy and excess volume of a large number of tilt grain boundaries in Cu and Ni. By considering 400 distinct grain boundary orientations we confirm that theoretically there is a systematic difference between the excess volumes in the two materials and we provide atomistic insight into the origin of the effect.

Anisotropic growth of PtFe nanoclusters induced by lattice-mismatch: Efficient catalysts for oxidation of biopolyols to carboxylic acid derivatives

By exploiting the large difference in lattice constants between Pt and Fe metals, anisotropic growth of bimetallic PtFe clusters with high index numbers has been prepared to investigate their catalytic performance in oxidation of biopolyols. Surface characterization using transmission electron microscopy shows that etching effects caused by Fe3+ and anisotropic growth induced by lattice mismatch contributed to the final nanocrystal geometry. The bimetallic PtFe heterocluster is shown to exhibit an unprecedented sixfold enhanced catalytic activity and threefold higher selectivity compared to monometallic Pt catalyst, in oxidation of biomass-derived polyols to dicarboxylic acids. A detailed study on structure-dependent kinetics of oxidation of glycerol, based on concentration–time profiles on Pt and PtFe catalysts, reveals that the presence of Fe in Pt catalysts significantly enhances the rate of oxidation and decreases the activation barriers for primary and secondary oxidation steps leading to enhanced catalytic activity and selectivity. The lattice mismatch methodology employed in this work provides an unique tool for designing high index catalytically active materials for various other industrial applications.

Diffraction tomography and Rietveld refinement of a hydroxyapatite bone phantom

A model sample consisting of two different hydroxyapatite (hAp) powders was used as a bone phantom to investigate the extent to which X-ray diffraction tomography could map differences in hAp lattice constants and crystallite size. The diffraction data were collected at beamline 1-ID, the Advanced Photon Source, using monochromatic 65 keV X-radiation, a 25 × 25 µm pinhole beam and translation/rotation data collection. The diffraction pattern was reconstructed for each volume element (voxel) in the sample, and Rietveld refinement was used to determine the hAp lattice constants. The crystallite size for each voxel was also determined from the 00.2 hAp diffraction peak width. The results clearly show that differences between hAp powders could be measured with diffraction tomography.

Magnetic Domains and Twin Microstructure of Single Crystal Ni–Mn–Ga Exhibiting Magnetic Shape Memory Effect

Magnetic Shape Memory Effect or more precisely magnetic-field-induced-structural reorientation (MIR) generate large strain (up to 12 %) and fast response (around 1 kHz) in a moderate magnetic field below 1 T. The strain is caused by twin microstructure reorientation in which the maximum generated strain is determined by difference of lattice constants of the pseudotetragonal structure a = b > c, i.e. e MAX = e 0 = 1−c/a. The reorientation is mediated by twin boundary motion. Very high mobility is, therefore, necessary condition for the existence of the effect. Recently considering more precise structural description of 10M martensite of Ni-Mn-Ga alloys as monoclinic, i.e. a > b > c and γ π 90, we showed that there are two different kinds of mobile a-c twin boundary, Type I and II with different microstructure [1]. The boundaries differ not only in magnitude of twinning stress needed to move twin boundary, i.e. in mobility, but also in temperature dependence of the mobility or twinning stress [2]. Despite of the simplicity of the moving interface the twinned structure is complex [3]. The model of the movable Type II twin boundary is shown in the figure. The twinned structure consists apart of the moving a-c twin boundary the modulation twinning bands and a-b twinning.

Effects of Lattice Misfit on the Growth of Coordination Polymer Heterostructures

The heterogeneous growth of a series of Prussian blue analogues on nickel hexacyanocobaltate seeds to produce coordination polymer heterostructues was investigated to determine the influence of lattice misfit over the growth mode. For small lattice misfits, the observation of a core–shell morphology indicates the growth of a pseudomorphic layer while larger lattice misfits result in the growth of islands. A structural study suggests an efficient mechanical coupling of the substrate and overlayer materials. The difference in lattice constants between the heterostructure components induces anisotropic strain that is relieved by switching to a different growth mode.

Experiment on alleviating the bending of CVD-grown heavily Al-doped 4H-SiC epiwafer by codoping of N

In order to alleviate the convex wafer bow that commonly occurs on a heavily Al-doped 4H-SiC epilayer grown on an n+ substrate, nitrogen gas was intentionally introduced, together with an Al dopant gas during CVD growth, that is Al–N codoping. It was found that, by introducing N2 at various flow rates, the concentration of incorporated N impurity increases along with the supplied flow rate, while the Al impurity remains at a similar concentration. With the codoping of N and Al in 4H-SiC, a marked reduction of wafer bow has been achieved. The crystal structural measurement suggests that the introduction of N impurity alleviates the large wafer bow owing to the reduced lattice constant differences between the p+ epilayer and the n+ substrate.