Decreasing Sample Thickness Research Articles

Enhancement of thermal conductivity in PDMS/CFP composites using a dredging-plugging thermal network

In the preparation of thermally conductive polymer composites (TCPC), it is necessary to incorporate a filler network in the polymer matrix. Further enhancement of the thermal conductivity (TC) of TCPCs is challenging due to the exceptional heat absorption of the polymer matrix, which results in heat flow losses in the network. To address this issue, a novel procedure, termed thermal network dredging-plugging, has been proposed. In this study, polydimethylsiloxane (PDMS) composites with a carbon fiber powder (CFP) network and a CFP/glass ball (GB) network enhanced by dredging-plugging were prepared by a coating method. The TC of the PDMS/CFP system increased with an increase in CFP loading from 0 to 12 wt% and a decrease in sample thickness from 0.4 to 0.2 mm. At a 12 wt% CFP loading, the PDMS/CFP composites exhibited a maximum TC of 4.687 W/(mK). With the addition of GBs ranging from 0 to 2 wt%, the dredging-plugging CFP/GB network increased the maximum TC by a factor of 1.26 relative to the dredged CFP network. In conjunction with the thermal resistance method, a mathematical model that considers both the plugging and dilution effects of GBs was constructed. The calculated TCs were validated by using the prepared PDMS/CFP/GB composites, with average absolute relative errors ranging from 0.5 % to 4.19 %. In addition to TC enhancement, the mechanical properties and thermal management of the PDMS composites were evaluated.

Thickness-dependent creep rate caused by climbing control of a superdislocation in a Ni-based single crystal superalloy

Creep tests are carried out on Ni-based single crystal superalloy sheet samples with different thicknesses at 1100 °C/130 MPa. The results indicate that the secondary creep rate at constant stress exhibits a strong thickness dependence and increases with the decrease in sample thickness. Under load in [001] direction, two types of a<100> family superdislocations with Burgers vectors a[100] and a[010] enter γ′ rafts in thin samples, whereas only the type of a[100] superdislocation enter γ′ rafts in thicker samples. The density of a<100> superdislocations is higher in thin sheets than in thick ones. It is proved that the movement mode of a<100> superdislocations in γ′ rafts is pure climb according to non-compact dislocation core structure. In addition, the climb velocity of the a<100> superdislocations in thin samples is determined to be greater than that in thick ones caused by the fact that the a<100> superdislocations in thin samples experience a higher driving force. On these bases, it is revealed that more types of Burgers vectors, higher density and greater climb velocity of a<100> superdislocations in γ′ rafts contributes to higher secondary creep rate in thinner samples.

Controllable CVD Growth of 2D Cr5Te8 Nanosheets with Thickness-Dependent Magnetic Domains.

As a unique 2D magnetic material with self-intercalated structure, Cr5Te8 exhibits many intriguing magnetic properties. While its ferromagnetism of Cr5Te8 has been previously reported, the research on its magnetic domain remains unexplored. Herein, we have successfully fabricated 2D Cr5Te8 nanosheets with controlled thickness and lateral size by chemical vapor deposition (CVD). Then magnetic property measurement system revealed Cr5Te8 nanosheets exhibiting intense out-of-plane ferromagnetism with a Curie temperature (TC) of 176 K. Significantly, we reported for the first time two magnetic domains: magnetic bubbles and thickness-dependent maze-like magnetic domains in our Cr5Te8 nanosheets by cryogenic magnetic force microscopy (MFM). The domain width of the maze-like magnetic domains increases rapidly with decreasing sample thickness; meanwhile, the domain contrast decreases. This indicates the dominant role of ferromagnetism shifts from dipolar interactions to magnetic anisotropy. Our research not only establishes a pathway for the controllable growth of 2D magnetic materials but also points toward novel avenues for regulating magnetic phases and methodically tuning domain characteristics.

Inhomogeneous Superconductivity Onset in FeSe Studied by Transport Properties

Heterogeneous superconductivity onset is a common phenomenon in high-Tc superconductors of both the cuprate and iron-based families. It is manifested by a fairly wide transition from the metallic to zero-resistance states. Usually, in these strongly anisotropic materials, superconductivity (SC) first appears as isolated domains. This leads to anisotropic excess conductivity above Tc, and the transport measurements provide valuable information about the SC domain structure deep within the sample. In bulk samples, this anisotropic SC onset gives an approximate average shape of SC grains, while in thin samples, it also indicates the average size of SC grains. In this work, both interlayer and intralayer resistivity were measured as a function of temperature in FeSe samples of various thicknesses. To measure the interlayer resistivity, FeSe mesa structures oriented across the layers were fabricated using FIB. As the sample thickness decreases, a significant increase in superconducting transition temperature Tc is observed: Tc raises from 8 K in bulk material to 12 K in microbridges of thickness ∼40 nm. We applied analytical and numerical calculations to analyze these and earlier data and find the aspect ratio and size of the SC domains in FeSe consistent with our resistivity and diamagnetic response measurements. We propose a simple and fairly accurate method for estimating the aspect ratio of SC domains from Tc anisotropy in samples of various small thicknesses. The relationship between nematic and superconducting domains in FeSe is discussed. We also generalize the analytical formulas for conductivity in heterogeneous anisotropic superconductors to the case of elongated SC domains of two perpendicular orientations with equal volume fractions, corresponding to the nematic domain structure in various Fe-based superconductors.

Ultrasonic assisted far infrared drying characteristics and energy consumption of ginger slices.

Three drying methods, including far infrared drying, infrared convection drying, and ultrasonic pretreatment assisted far infrared drying, were adopted in the drying of ginger slices. The effects of main parameters (ultrasonic pretreatment power and time, far infrared temperature and power, sample thickness, infrared convection temperature) on the drying kinetics, energy consumption, and color change were investigated and discussed in detail. The results showed that the drying process of ginger slices was controlled by falling rate period. For far infrared drying, the drying rate increased with the increase of infrared temperature and decrease of sample thickness, while the infrared power had no obvious effect on the drying process. The infrared convection drying showed the fastest drying rate and the smallest color change, however, the energy consumption was the highest. For ultrasonic pretreatment assisted far infrared drying, an appropriate ultrasonic pretreatment time and power would promote the far infrared drying process and the energy consumption was only slightly increased. However, the color change was relatively large. The ultrasound technology showed its greatest potential to enhance the drying rate at the early stage of drying and increasing ultrasonic power was more effective than prolonging the pretreatment time in promoting far infrared drying.

Stress state mechanism of thickness debit effect in creep performances of a Ni-based single crystal superalloy

Weight reduction and advanced cooling schemes to improve aircraft engine efficiency drive the design of turbine blades toward thin-walled structures. The decrease of wall thickness leads to a decrease of creep rupture life and/or the increase of creep rate, which is termed as the thickness debit effect. To capture the nature of this effect and improve the reliability of turbine blades, creep tests at 1100 °C/130 MPa are carried out on sheet samples of a Ni-based single crystal superalloy with thicknesses from 0.60 to 2.17 mm. The creep rupture life is found to decrease with the decrease of sample thickness linearly, exhibiting an obvious thickness debit effect. The analysis of the microstructure evolution demonstrates that the rafting mechanism of the γ' phase of all samples remains the same (i.e. stress-induced, diffusion-controlled process) and is independent of the sample thickness. The fracture morphology reveals that the fracture mechanism of thin sample is a mixed fracture including micro voids and cleavage-like planes, while that of thick sample is micro voids-dominated. Environmental degradation suggests that surface damage due to oxidation and nitridation is not the primary causes for thickness debit effect, yet promotes the effect integrally. Finite element analyses and the observed dislocation configurations reveal that the thin sample is in a plane stress state, that activates six slip systems of the type {111}<110> and one slip system of the type {111}<112> during creep, while the thick sample is close to the ideal uniaxial stress state which activates all possible eight slip systems of the type {111}<110> and two slip systems of the type {111}<112>. Less slip systems activated in thin sample induce less work hardening, which results in heterogeneous deformation and shorter rupture life.

Effect of thickness on magnetic properties of single domain GdBCO bulk superconductors

To study the influence of thickness on the magnetic properties of ReBCO (Re = Y, Gd, Sm, Nd, etc.) bulk superconductors, a single domain gadolinium barium copper oxide (GdBCO) bulk superconductor fabricated by the Re + 011 top seeded infiltration growth (Re + 011 TSIG) method was continuously sliced along the bottom to obtain samples of different thickness. The levitation force and attractive force of these samples were tested at 77 K in the zero-field-cooled (ZFC) state. It is found that as the sample thickness decreases, the levitation force decreases gradually whereas the attractive force increases. This is related to the varied ability to resist the penetration of magnetic field occasioned by varying sample thickness, which are deeply revealed by combining with the characteristics of the non-ideal type-II superconductor. Further, the levitation force exhibits a trend of slow initial change followed by rapid change, which may be attributed to the growth of the sample. Measurement of the trapped field shows that a similar distribution of trapped field at the top and bottom surfaces can be achieved by removing some materials from the bottom of the bulk. These results provide a reference for meeting the actual requirements of ReBCO bulks of different thicknesses and greatly contribute to practical designs and applications.

The beneficial effect of rolling on the fracture toughness and R-curve behavior of pure tungsten

Selected microstructural states of pure tungsten sheets with thicknesses of 1, 0.5, 0.2 and 0.1 mm were tested at room-temperature (RT) and 200 °C regarding their fracture properties with focus on the occurrence of an R-curve (crack growth resistance curve) behavior. Through thermomechanical rolling and the resulting increase of deformation a pronounced refinement of microstructure and strengthening of the rotated cube texture was induced with decreasing sheet thickness. The fracture experiments exhibited large variations of the fracture behavior depending on the testing temperature, material thickness and microstructure: While at RT, the 1 mm material did not exhibit R-curve behavior, it was found at 200 °C irrespective of the sample thickness. With decreasing sample thickness and grain-size the RT fracture-behavior changed and showed the onset of an extended R-curve behavior for the 0.5 mm followed by an increasing extent of this characteristic for the thinner material states. The change of the fracture behavior at RT is associated with a transition of the failure behavior from transcrystalline fracture for the thicker samples to a mixture of transcrystalline with delamination fracture to uniform delamination failure for the thinnest material. These observations suggest a continuous shift of the ductile to brittle transition temperature from 200 °C to RT and even below RT with decreasing sample thickness. The fracture toughness in terms of the maximum stress intensity, Kmax, increased at RT from approximately 20 MPam1/2 for the 1 mm samples to values up to 60 MPam1/2 for the thinnest material states, whereas at 200 °C this increase was less pronounced.

Galvanomagnetic properties and thermoelectric power of ultrathin films of the bismuth-antimony system on a mica substrate

The study of the electronic properties of ultrathin films of pure bismuth and bismuth-antimony alloys is of interest, since an increase in conductivity with decreasing sample thickness was found. This paper presents the results of an experimental study of the structure, electrical, galvanomagnetic and thermoelectric properties of pure bismuth and Bi1-xSbx thin films (x=0.05 and 0.12) on a mica substrate in the thickness range of 10-30 nm. An increase in the conductivity with a decrease in the thickness of the samples was found. It may be due to the presence of topologically protected surface states. It is shown that the features of the manifestation of this effect are significantly influenced by the alloys band structure. The form of the temperature dependences of the Seebeck coefficient casts doubt on the fact that surface states have a positive effect on the thermoelectric efficiency of thin bismuth-antimony films. However, the detection of a positive thermoelectric power in Bi0.88Sb0.12 samples can become an important factor for searching for the possibility of creating a p-branch of thermoelectric converters. Keywords: bismuth, antimony, thin films, surface states, thermoelectric power.

Гальваномагнитные свойства и термоэдс ультратонких пленок системы висмут--сурьма на подложке из слюды

The study of the electronic properties of ultrathin films of pure bismuth and bismuth-antimony alloys is of interest, since an increase in conductivity with decreasing sample thickness was found. This paper presents the results of an experimental study of the structure, electrical, galvanomagnetic and thermoelectric properties of pure bismuth and Bi1−x Sbx thin films (x = 0.05 and 0.12) on a mica substrate in the thickness range of 10−30 nm. An increase in the conductivity with a decrease in the thickness of the samples was found. It may be due to the presence of topologically protected surface states. It is shown that the features of the manifestation of this effect are significantly influenced by the alloys band structure. The form of the temperature dependences of the Seebeck coefficient casts doubt on the fact that surface states have a positive effect on the thermoelectric efficiency of thin bismuth-antimony films. However, the detection of a positive thermoelectric power in Bi0.88Sb0.12 samples can become an important factor for searching for the possibility of creating a p-branch of thermoelectric converters.

Inverse size effects in un-notched and notched metallic glass thin films

The sample size effect on the deformation behaviors of metallic glasses (MGs) attracts a lot of interest. Here, a series of molecular dynamics simulations have been performed on un-notched and notched MG thin films to fully investigate the influence of sample thickness on their fracture strengths and failure mechanisms. With decreasing sample thickness, the failure strength of un-notched MG thin films decreases slightly and the failure mode changes from shear banding to homogeneous deformation. In contrast to this brittle-to-ductile transition observed in un-notched MG thin films, shear banding prevails for notched MG thin films regardless of sample thickness. Concurrently, a notch strengthening effect is observed in the notched MG thin films when the sample thickness falls close to the shear band (SB) thickness. This significant notch strengthening is due to the failure mode transition from homogeneous deformation in un-notched MGs to shear banding in notched MGs. Based on the energetic model, a theoretical analysis is proposed to rationalize the size effect on failure mechanism transitions of both un-notched and notched MG thin films. The present study offers an understanding of size effect on the un-notched and notched MG thin films, as well as useful guidelines for the design, testing, and engineering of MG for microelectromechanical applications.

Hematite at its thinnest limit

Motivated by the recent synthesis of two-dimensional -Fe2O3 (Balan et al 2018 Nat. Nanotechnol. 13 602), we analyze the structural, vibrational, electronic and magnetic properties of single- and few-layer -Fe2O3 compared to bulk, by ab initio and Monte-Carlo simulations. We reveal how monolayer -Fe2O3 (hematene) can be distinguished from the few-layer structures, and how they all differ from bulk through observable Raman spectra. The optical spectra exhibit gradual shift of the prominent peak to higher energy, as well as additional features at lower energy when -Fe2O3 is thinned down to a monolayer. Both optical and electronic properties have strong spin asymmetry, meaning that lower-energy optical and electronic activities are allowed for the single-spin state. Finally, our considerations of magnetic properties reveal that 2D hematite has anti-ferromagnetic ground state for all thicknesses, but the critical temperature for Morin transition increases with decreasing sample thickness. On all accounts, the link to available experimental data is made, and further measurements are prompted.

An Investigation of Short Range Residual Stress Fields in Ferrous Lath Martensite

A novel experiment is reported where the X-ray line broadening from martensite has been measured as a function of the specimen thickness. The diffraction lines become narrower as the sample thickness decreases, which is attributed to the relaxation of local residual stresses. By comparison with results of microstructural examination, it is concluded that the length scales of the stress fields correspond with the grain size or, more specifically, with the sub-block size of the martensite structure.

Molecular dynamics simulation of Surface-Adsorbed-Hydrogen-Induced Dislocation Motion in a thin film

Elucidating the effects of hydrogen on dislocation activities is important for revealing hydrogen embrittlement mechanisms. In situ transmission electron microscopy observations have shown that dislocation motion is enhanced or activated when hydrogen gas is introduced into the sample chamber, supporting the hydrogen-enhanced localized-plasticity mechanism. However, adsorbed hydrogen atoms affect dislocation activities in thin films. Here we analyzed the effects of hydrogen and sample thickness on the motion of screw dislocations through molecular dynamics simulations. The surface effect becomes significant with decreasing sample thickness, and the change in surface energy upon hydrogen adsorption induces motion of screw dislocations in ultrathin films.

Flame Spread Over Ultra-thin Solids: Effect of Area Density and Concurrent-Opposed Spread Reversal Phenomenon

There are no existing experimental studies of flame spread rate trends for ultra-thin solid samples. Previous theory has predicted that for concurrent flame in kinetic regime, the flame spread rate decreases as the sample thickness decreases and there is a critical thickness below which burning is not possible. To test this hypothesis, a series of microgravity experiments of concurrent-flow flame spread over samples of ultra-low area densities are conducted using NASA Glenn Research Center’s Zero Gravity Research Facility (the 5.18 s drop tower). The tested samples are cellulose-based materials of various area densities, ranging from 0.2 mg/cm2 to 13 mg/cm2, as low as one order of magnitude less than those ever tested before. Each sample is 30 cm long by 5 cm wide and is burned in a low-speed concurrent air flow (5 to 30 cm/s). The results show that the concurrent flame spread rate is proportional to the flow velocity relative to the flame and is inversely proportional to the sample area density. A theoretical formulation, provided in this work, suggests that the flame length has a linear relationship with the relative flow speed and has no direct dependency on the sample area density. The experimental data supports this conclusion. From the images recorded in the experiments, a unique flame base tubular structure directed upstream away from the burnout zone is observed for thin samples. This structure is suspected to be due to flame stretching and localized blowoff caused by the oxidative pyrolysis Stefan flows at the sample burnout. This can be an indication that the chemical time becomes comparable to the flow time of the Stefan flow and the tested samples are approaching the kinetically-limited thickness. For the thinnest tested sample (0.2 mg/cm2), flames with concurrent and opposed dual natures are observed when the air flow rate is low (< 20 cm/s). At the lowest tested flow rate (5 cm/s), the flame spread rate exceeds the air flow rate and the flame transits to an opposed flame in the concurrent flow. The dual nature and flame transition are presented and discussed. This study provides detailed examination through high-resolution images of the transition between the concurrent to opposed flame spread modes.

Separation behavior of metal-resin composite film using high frequency induction heating

Waste flexible printed circuit (WFPC) contains a number of precious resources such as base and noble metals. In this research, we propose a new separation / recovery technology using high frequency induction heating as a recycling process of WFPC. The peeling strength of the sample heated by high frequency induction heating was quantitatively evaluated using a tensile tester, and the effect of sample dimensions and heating conditions on adhesive properties was experimentally investigated. As a result of the high frequency induction heating experiment, it became clear that the coil current when the FPC simulated sample is peeled decrease with increase in heating area and decrease in sample thickness. As a result of the tensile test, there was a difference in peeling behavior between the unheated sample and the heated sample. It was found that the peeling tensile force of the heated sample increased with increase in the coil current. In addition, the peeling tensile force showed a tendency to decrease after reaching the maximum. Ultimately, the peeling tensile force of the sample heated at the critical current value was decrease than that of the unheated sample.

The effect of light penetration depth on the LCST phase behavior of a thermo‐ and photoresponsive statistical copolymer in an ionic liquid

ABSTRACTA reflection cloud point technique allows for rapid screening of light‐dependent phase separation temperatures of thermo‐ and photoresponsive polymer/ionic liquid solutions as a function of sample thickness, molecular weight, and copolymer composition. We systematically investigate the lower critical solution temperature (LCST) phase behavior of poly(benzyl methacrylate‐stat‐(4‐phenylazophenyl methacrylate)). Under UV light, the photoresponsive azobenzene‐based repeat unit becomes more polar as the cis form dominates, increasing its solubility in the ionic liquids 1‐ethyl‐3‐methyl imidazolium and 1‐butyl‐3‐methyl imidazolium bis(trifluoromethanesulfonyl)imide. This light‐dependent polarity change leads to two phase separation temperatures, depending on the illumination wavelength. Under visible light, which drives the azobenzene moiety into the trans ground state, the LCST shows no sample thickness dependence. Under UV light, however, sample thickness plays a significant role. Samples of around 1 mm thickness show no apparent difference under UV and visible light, whereas thinner samples show an increasing difference between the phase separation temperatures with decreasing sample thickness. Neither phase separation temperature exhibits a significant dependence on molecular weight. Increasing the photoresponsive monomer content did not lead to an increase in the difference between the phase separation temperatures at fixed thickness, due to a concomitant increase in UV light absorbed at the sample surface. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 281–287

Magnetotransport in Ultrathin 2-D Superconducting Mo2C Crystals

Ultrathin transition metal carbides are a class of emerging 2-D materials with many intriguing properties and applications. Here, we report the transport measurements on high-quality ultrathin Mo2C crystals, which are synthesized by a chemical vapor deposition method. We demonstrate 2-D nature of the observed superconductivity in ultrathin Mo2C crystals, being consistent with a Berezinskii–Kosterlitz–Thouless transition. As the sample thickness decreases, the Mo2C crystals exhibit negative magnetoresistance behavior at low magnetic fields deep in the superconducting state. We attribute these results to strong phase fluctuations of the superconducting order parameters near the superconductor–insulator transition. Our results demonstrate that these high-quality ultrathin Mo2C crystals provide an appealing platform to gain insights into 2-D superconductivity in a clean system.

On-axis TKD for orientation mapping of nanocrystalline materials in SEM

A new configuration for Transmission Kikuchi Diffraction (TKD) in SEM was recently developed, with a scintillator perpendicular to the electron beam. This configuration, “on-axis” TKD, makes grains and twins below 10nm well visible in orientation maps and with high indexation rate, as demonstrated with an electrodeposited nanocrystalline Ni. This high lateral spatial resolution is achieved by combining high accelerating voltage and low sample thickness to reduce the interaction volume. Among the advantages of the on-axis TKD, orientation mapping of particularly thin samples can be realized thanks to the advantageous axial position of the detector. Indeed, the reduction of the solid angle of the transmitted intensity around the beam axis with decreasing sample thickness is much less of a problem with an axial detector than with an off-axis detector. With an on-axis detector, the only condition for production of an orientation map is that the sample must be thick enough to produce Kikuchi diffraction in addition to spots.

Narrowband noise study of sliding charge density waves in NbSe3 nanoribbons

Transport properties (dc electrical resistivity, threshold electric field, and narrow-band noise) are reported for nanoribbon specimens of NbSe3 with thicknesses as low as 18 nm. As the sample thickness decreases, the resistive anomalies characteristic of the charge density wave (CDW) state are suppressed and the threshold fields for nonlinear CDW conduction apparently diverge. Narrow-band noise measurements allow determination of the concentration of carriers condensed in the CDW state nc, reflective of the CDW order parameter Δ. Although the CDW transition temperatures are relatively independent of sample thickness, in the lower CDW state Δ decreases dramatically with decreasing sample thickness.