Limit Of Mechanical Stability Research Articles

Estimating metastable thermodynamic properties by isochoric extrapolation from stable states.

The description of metastable fluids, those in local but not global equilibrium, remains an important problem of thermodynamics, and it is crucial for many industrial applications and all first order phase transitions. One way to estimate their properties is by extrapolation from nearby stable states. This is often done isothermally, in terms of a virial expansion for gases or a Taylor expansion in density for liquids. This work presents evidence that an isochoric expansion of pressure at a given temperature is superior to an isothermal density expansion. Two different isochoric extrapolation strategies are evaluated, one best suited for vapors and one for liquids. Both are exact for important model systems, including the van der Waals equation of state. Moreover, we present a simple method to evaluate all the coefficients of the isochoric expansion directly from a simulation in the canonical ensemble. Using only the properties of stable states, the isochoric extrapolation methods reproduce simulation results with Lennard-Jones potentials, mostly within their uncertainties. The isochoric extrapolation methods are able to predict deeply metastable pressures accurately even from temperatures well above the critical. Isochoric extrapolation also predicts a mechanical stability limit, i.e., the thermodynamic spinodal. For water, the liquid spinodal pressure is predicted to be monotonically decreasing with decreasing temperature, in contrast to the re-entrant behavior predicted by the direct extension of the reference equation of state.

Water at negative pressure: nuclear quantum effects

Various condensed phases of water, spanning from the liquid state to multiple ice phases, have been systematically investigated under extreme conditions of pressure and temperature to delineate their stability boundaries. This study focuses on probing the mechanical stability of liquid water through path-integral molecular dynamics simulations, employing the q-TIP4P/F potential to model interatomic interactions in flexible water molecules. Temperature and pressure conditions ranging from 250 to 375 K and − 0.3 to 1 GPa, respectively, are considered. This comprehensive approach enables a thorough exploration of nuclear quantum effects on various physical properties of water through direct comparisons with classical molecular dynamics results employing the same potential model. Key properties such as molar volume, intramolecular bond length, H–O–H angle, internal and kinetic energy are analysed, with a specific focus on the effect of tensile stress. Particular attention is devoted to the liquid-gas spinodal pressure, representing the limit of mechanical stability for the liquid phase, at several temperatures. The quantum simulations reveal a spinodal pressure for water of − 286 and − 236 MPa at temperatures of 250 and 300 K, respectively. At these temperatures, the discernible shifts induced by nuclear quantum motion are quantified at 15 and 10 MPa, respectively. These findings contribute valuable insights into the interplay of quantum effects on the stability of liquid water under diverse thermodynamic conditions.

Preparation and physical properties of CeO2 doped and modified epoxy resin composites

AbstractEpoxy resin is a widely used polymer material, while its low mechanical properties and thermal stability limit its use in extreme environments such as high temperature and high pressure. In order to improve the performance of EP, this study assessed the enhancement of mechanical properties and thermal stability of EP by stearic acid‐modified n‐CeO2 and its mechanism. Laser particle size analyzers, transmission electron microscopes, and other instruments were used to characterize n‐CeO2. Universal testing machines, thermogravimetric analyzers, and other instruments were used to test n‐CeO2/EP nanocomposites. The results show that fatty acids effectively coated the surface of n‐CeO2, reducing its central particle size from 0.248 μm to 0.193 μm and improving its dispersion and compatibility. The tensile, flexural, impact strength, maximum decomposition temperature, and glass transition temperature (Tg) increased by 382%, 554%, 698%, 5%, and 10%, respectively with 2 wt% filler content. The scanning electron microscopy (SEM) images showed that the addition of n‐CeO2 induced plastic deformation and crack formation in the EP matrix, which enhanced the toughness of the composite material. This study provided the effective methods and basis for the preparation of high‐performance n‐CeO2/EP nanocomposites.Highlights Nano cerium dioxide was modified to improve its dispersion by using stearic acid. High‐performance nanocomposites were prepared by using modified nanoparticles. The stiffness and toughness of the epoxy resin were enhanced by adding n‐CeO2. Thermal stability and heat resistance of the epoxy resin were improved by doping n‐CeO2. SEM images of the impact section showed that n‐CeO2 can prevent crack propagation.

Auxetic polymer networks: The role of crosslinking, density, and disorder

Low-crosslinked polymer networks have recently been found to behave auxetically when subjected to small tensions, that is, their Poisson’s ratio ν becomes negative. In addition, for specific state points, numerical simulations revealed that diamond-like networks reach the limit of mechanical stability, exhibiting values of ν = −1, a condition that we define as hyper-auxeticity. This behavior is interesting per se for its consequences in materials science but is also appealing for fundamental physics because the mechanical instability is accompanied by evidence of criticality. In this work, we deepen our understanding of this phenomenon by performing a large set of equilibrium and stress–strain simulations in combination with phenomenological elasticity theory. The two approaches are found to be in good agreement, confirming the above results. We also extend our investigations to disordered polymer networks and find that the hyper-auxetic behavior also holds in this case, still manifesting a similar critical-like behavior as in the diamond one. Finally, we highlight the role of the number density, which is found to be a relevant control parameter determining the elastic properties of the system. The validity of the results under disordered conditions paves the way for an experimental investigation of this phenomenon in real systems, such as hydrogels.

How an electrical current can stabilize a molecular nanojunction.

The stability of molecular junctions under transport is of the utmost importance for the field of molecular electronics. This question is often addressed within the paradigm of current-induced heating of nuclear degrees of freedom or current-induced forces acting upon the nuclei. At the same time, an essential characteristic of the failure of a molecular electronic device is its changing conductance - typically from a finite value for the intact device to zero for a device that lost its functionality. In this publication, we focus on the current-induced changes in the molecular conductance, which are inherent to molecular junctions at the limit of mechanical stability. We employ a numerically exact framework based on the hierarchical equations of motion approach, which treats both electronic and nuclear degrees of freedom on an equal footing and does not impose additional assumptions. Studying generic model systems for molecular junctions with dissociative potentials for a wide range of parameters spanning the adiabatic and the nonadiabatic regime, we find that molecular junctions that exhibit a decrease in conductance upon dissociation are more stable than junctions that are more conducting in their dissociated state. This represents a new mechanism that stabilizes molecular junctions under current. Moreover, we identify characteristic signatures in the current of breaking junctions related to the interplay between changes in the conductance and the nuclear configuration and show how these are related to properties of the leads rather than characteristics of the molecule itself.

PSS-doped PANI nanoparticle/Ti3C2Tx composites for conductometric flexible ammonia gas sensors operated at room temperature

The detection of ammonia (NH3) is significant because it can seriously damage human health and atmospheric environment. However, the development of high-performance flexible NH3 gas sensors operated at room temperature is still challenging. Thus, a flexible NH3 film sensor based on poly(4-stryenesulfonate)-doped polyaniline nanoparticle (PANI:PSS)/Ti3C2Tx composite was developed by in-situ polymerization. This optimized PANI:PSS/Ti3C2Tx film displayed the high response of 0.57–1 ppm NH3, which was improved by 1.78-fold in comparison with PANI:PSS film. Besides, the response value of this film to NH3 was 1.46 times higher than to NO2, and this value was much higher than to other interfering gases (NO, toluene, ethanol, and acetone). Additionally, this film sensor possessed excellent mechanical stability and low detection limit (20 ppb) at room temperature. The abundant Schottky heterojunctions formed in PANI:PSS/Ti3C2Tx composite, the numerous -SO3H groups introduced by PSS anion chains, and the improved protonation degree of PANI component are responsible for the enhanced sensing properties. The monitoring experiment of shrimp spoilage has demonstrated the application feasibility of this film sensor. This research provides a novel and practical method for constructing high-performance polymer/MXene based gas sensors.

Interfacial Forces in Free-Standing Layers of Melted Polyethylene, from Critical to Nanoscopic Thicknesses.

Molecular dynamics simulations of ultrathin free-standing layers made of melted (373.15–673.15 K) polyethylene chains, which exhibit a lower melting temperature (compared to the bulk value), were carried out to investigate the dominant pressure forces that shape the conformation of chains at the interfacial and bulk liquid regions. We investigated layer thicknesses, , from the critical limit of mechanical stability up to lengths of tens of nm and found a normal distribution of bonds dominated by slightly stretched chains across the entire layer, even at large temperatures. In the bulk region, the contribution of bond vibrations to pressure was one order of magnitude larger than the contributions from interchain interactions, which changed from cohesive to noncohesive at larger temperatures just at a transition temperature that was found to be close to the experimentally derived onset temperature for thermal stability. The interchain interactions produced noncohesive interfacial regions at all temperatures in both directions (normal and lateral to the surface layer). Predictions for the value of the surface tension, , were consistent with experimental results and were independent of . However, the real interfacial thickness—measured from the outermost part of the interface up to the point where reached its maximum value—was found to be dependent on , located at a distance of 62 Å from the Gibbs dividing surface in the largest layer studied (1568 chains or 313,600 bins); this was ~4 times the length of the interfacial thickness measured in the density profiles.

Challenges and limits of mechanical stability in 3D direct laser writing

Direct laser writing is an effective technique for fabrication of complex 3D polymer networks using ultrashort laser pulses. Practically, it remains a challenge to design and fabricate high performance materials with different functions that possess a combination of high strength, substantial ductility, and tailored functionality, in particular for small feature sizes. To date, it is difficult to obtain a time-resolved microscopic picture of the printing process in operando. To close this gap, we herewith present a molecular dynamics simulation approach to model direct laser writing and investigate the effect of writing condition and aspect ratio on the mechanical properties of the printed polymer network. We show that writing conditions provide a possibility to tune the mechanical properties and an optimum writing condition can be applied to fabricate structures with improved mechanical properties. We reveal that beyond the writing parameters, aspect ratio plays an important role to tune the stiffness of the printed structures.

A Wearable Microneedle‐Based Extended Gate Transistor for Real‐Time Detection of Sodium in Interstitial Fluids (Adv. Mater. 10/2022)

Biosensors In article number 2108607, Miaomiao Yuan, Hossam Haick, and co-workers report an innovative, stretchable, skin-conformal, fast-response, microneedle, extended-gate field-effect transistor biosensor for real-time detection of sodium in interstitial fluids for minimally invasive health monitoring. High sensitivity, low limit of detection, excellent biocompatibility, and on-body mechanical stability are demonstrated. This platform can be integrated with a wireless-data transmitter and the Internet-of-Things cloud for real-time monitoring and long-term analysis.

A Wearable Microneedle-Based Extended Gate Transistor for Real-Time Detection of Sodium in Interstitial Fluids.

Sodium is a prominent prognostic biomarker for assessing health status, such as dysnatremia. As of now, detection and monitoring of sodium levels in the human body is carried out by means of laborious and bulky laboratory equipmentand/or by offline analysis of various body fluids. Herein, an innovative stretchable, skin-conformal and fast-response microneedle extended-gate FET biosensor is reported for real-time detection of sodium in interstitial fluids for minimally invasive health monitoring along with high sensitivity, low limit of detection, excellent biocompatibility, and on-body mechanical stability. The integration of the reported device with a wireless-data transmitter and the Internet-of-Things cloud for real-time monitoring and long-term analysis is reported and discussed. This platform would eventually help bringing unlimited possibilities for effecient medical care and accurate clinical decision-making.

Hydrophobic and stable MXene/ reduced graphene oxide/polymer hybrid materials pressure sensors with an ultrahigh sensitive and rapid response speed pressure sensor for health monitoring

Pressure sensors with highly sensitive, flexibility, biocompatibility and scalability have aroused widespread research interest in the field of smart skin and wearable electronics. Furthermore, to construct physiological signal monitoring equipment with best high-sensitivity, mechanical stability, low-cost, and ultra-low detection limit has always been a considerable challenge. In this paper, spin-coating process is used to prepare Mxene/rGO/P (VDF-TrFE) film and apply it to piezoresistive sensors. The lower detection limit, sensitivity, and response time of the sensor based on Mxene/rGO/PVDF is 8 Pa, 2298 kPa−1, and 10 ms respectively, and it has good stability in 10000 cycles. Piezoresistive sensors based on Mxene/rGO/P(VDF-TrFE) can easily capture signals below 8 Pa, so it can clearly test adult pulses randomly. The Mxene/rGO/P(VDF-TrFE) thin film sensor can monitor human physiological signals (such as finger touch and pulse) in real time. The potential applications of the sensor in measuring pressure distribution and human-computer interaction are discussed.

Perceptive Model Predictive Control for Continuous Mobile Manipulation

A mobile robot needs to be aware of its environment to interact with it safely. We propose a receding horizon control scheme for mobile manipulators that tracks task space reference trajectories. It uses visual information to avoid obstacles and haptic sensing to control interaction forces. Additional constraints for mechanical stability and joint limits are met. The proposed method is faster than state of the art sampling based planners, available as opensource and can be implemented on a broad class of robots. We validate the method both in simulation and through extensive hardware experiments with a multitude of mobile manipulation platforms. The resulting software package is released with this letter.

Artefact peaks of pore size distributions caused by unclosed sorption isotherm and tensile strength effect

The pore size distribution (PSD) is an important property of the porous materials, but an accurate and reliable PSD is very difficult to obtain. Quenched Solid Density Functional Theory (QSDFT) is widely used to analyze the pore size distribution. Here, a series of unclosed and closed isotherms of activated carbon were measured and compared. The experimental results revealed the appearance of an artefact peak in the PSD for the unclosed isotherms by employing QSDFT equilibrium model. Moreover, the location of the artificial peak is highly related to the end point of the desorption branch. In addition, tensile strength effect (TSE) illustrates the limit of mechanical stability of liquids in pores, and subsequent cavitation generates an artefact peak at around 2.8 nm. Interestingly, it was found that isotherm data associated with TSE can be used to calculate the total pore volume of all large cavities in which cavitation occurs.

Anisotropy in elasticity and physical properties of Ti2B under high pressure

The anisotropic elasticity and physical properties of Ti2B with I4/m symmetry are predicted by the first-principle method combined with the quasi-harmonic Debye model. The effects of pressure on elastic constants, polycrystalline elastic moduli, Poisson’s ratio, Vickers hardness, and fracture toughness are calculated. The high pressure limit of mechanical stability of Ti2B is ∼70 GPa. The calculated polycrystalline elastic moduli B, G, E, and fracture toughness increase with pressure. The Vickers hardness decreases with pressure, but it is still relatively hard. The values of G/B and Poisson’s ratio indicate the ductility of Ti2B enhanced with the increase of pressure. The elastic anisotropy is investigated via the various anisotropic indexes (AU, AB, AG, A1, A2 and A3), 3D surface constructions and projections of bulk and Young’s moduli. The opposite effects of pressure and temperature on the isothermal bulk modulus, thermal expansion coefficient, heat capacity, Debye temperature, and Grüneisen parameter of Ti2B have been found.

Phase diagram of ice polymorphs under negative pressure considering the limits of mechanical stability.

Thermodynamic and mechanical stabilities of various ultralow-density ices are examined using computer simulations to construct the phase diagram of ice under negative pressure. Some ultralow-density ices, which were predicted to be thermodynamically metastable under negative pressures on the basis of the quasi-harmonic approximation, can exist only in a narrow pressure range at very low temperatures because they are mechanically fragile due to the large distortion in the hydrogen bonding network. By contrast, relatively dense ices such as ice Ih and ice XVI withstand large negative pressure. Consequently, various ices appear one after another in the phase diagram. The phase diagram of ice under negative pressure exhibits a different complexity from that of positive pressure because of the mechanical instability.

Ionic liquid‐based organic–inorganic hybrid electrolytes: Impact of in situ obtained and dispersed silica

ABSTRACTOrganic–inorganic hybrid electrolytes based on PEO‐NaTFSI‐ionic liquid (HMIMTFSI)‐silica (in situ production via sol gel process) are being reported in this article. The variation in conductivity with ionic liquid (IL) addition has been explained on the basis of number of free TFSI anions evaluated using ATR‐IR data. The deconvolution of the IR spectra of these hybrid electrolytes has given evidence of ion‐pair formation which has been compared vis‐á‐vis the conductivity variation. The hybrid electrolyte with maximum conductivity comprises the highest number of free imide ions and has lowest glass transition temperature. FESEM has displayed a porous and layered surface morphology with dispersed silica nanoparticles. In addition, the optimized hybrid electrolyte has been compared with 5 wt% (limit of mechanical stability) ex situ silica added composite where the temperature cycling of conductivity has shown that the ex situ dispersed hybrid electrolytes do not retrace their conductivity path contrary to the in situ prepared hybrid electrolytes. This behavior has been explained to be due to the hindrance offered by the ex situ added silica in the recrystallization kinetics of PEO. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 207–218

Roll‐to‐Roll Fabrication of Flexible Orientated Graphene Transparent Electrodes by Shear Force and One‐Step Reducing Post‐Treatment

AbstractFlexible transparent electrodes (FTEs) are core sections for wearable photoelectric devices. However, the inevitable poor mechanical stability and expensive cost limit that the indium tin oxide FTEs are ineligible for future devices. In this work, the shear force of slot‐die printing during roll‐to‐roll process is creatively applied to form thin layer of reduced graphene oxide (RGO) for fabricating oriented poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate:RGO composite. The hydroiodic acid post‐treatment can remove insulating content of conductive polymer as well as reduce the graphene oxide to enhance the conductivity of FTEs to 1949 S cm−1 with a sheet resistance of 51 Ω □−1 and 82% transmittance. In addition, these FTEs demonstrate remarkable flexural endurance even under extreme bending situation. The FTEs possess a low cost of only 2.8 $ for per square meter due to the carbon materials and roll‐to‐roll technologies. Finally, the FTEs are faultlessly employed in polymer solar cells with a power conversion efficiency of 8.32% based on 1.01 cm2 effective area.

Structuring Methods of Plastic Substrates for Electroplating Applications

New approaches of packaging of integrated circuits show the need of flexible and cheap materials. Plastics like PET, PMMA and PC offer possibilities regarding flexibility and mass production possibilities in terms of roll to roll production for foil technology based substrates, as well the use of injection moulding for more complex three-dimensional housings, circuit and even sensor /actor arrays. [1] [2] [3] At the Institute of Micro Production Technology at the Leibniz Universität Hannover, we develop a new manufacturing strategy to produce prior structured wafer-substrates which can be used for electroplating processes without the need of photolithography. Important process steps are injection moulding, sputter deposition, electroplating, chemical mechanical polishing and micro milling. These manufacturing processes are not in the need of clean room technology and abstain from any lithography processes. Therefore this idea can be interesting for low cost production systems which neglect expensive clean room technology and expensive substrate materials without functionalized properties. At the IMPT structures were formed by an injection moulding process and followed by sputter deposition and electroplating to analyse the feasibility of common micro production technology in combination with plastics. After these process steps, CMP is performed and afterwards the structures will be covered by a following injection moulding or spin coating process. The flow of the manufacturing process can be seen in figure 1. Further developments will show the combination of these manufacturing processes with the use of a micro-milling machine to realise a transformer structure with an integrated electroplated magnetic core. Transformer-like structures are complex to manufacture by common micro production technologies due to the limited 3D-capability of electroplating technologies and the need of a closed magnetic core combined with helical copper coils [4]. This new combination of well-known fabrication processes and easy-to-handle substrate materials can offer a cost effective, reliable alternative kind of packaging of integrated circuit technology. At first structure size and height feasibility of the used injection moulding machine of the type BOY 55EV™ was performed. Structures of 100 µm height could easily be achieved and as well as structure sizes down to 50 µm diameter. Polycarbonate (PC) as a substrate material can be sputter deposited with Cu and NiFe up to 200 nm using a 50 nm adhesion layer of Cr (Figure 2). Temperatures during the sputter deposition process are critical and should not exceed 140°C as there will be the mechanical stability limit of the used PC. Electroplating was performed up to 5 µm on the first examined substrates. Mechanical dicing was used to create micrographs to analyse the electroplated structures and to examine the abrasive behaviour of the used polycarbonate. The CMP process is performed and should use slurry which does not solve or attack the polycarbonate. A neutral environment should be given. Figure 3 shows copper coated PC test structures after CMP process. The analysis of the process steps show feasibility of manufacturing of sensor arrays as well as transformer set-ups. This idea might open a new low cost fabrication process independent of clean room technology. [1] Alp Yaradanakul, Ali Yildiz, Donald P. Butler, Zeynep Celik-Butler, Fabrication of Micromachined Devices on Flexible Substrates, IEEE Emerging Technology Symposium, 2001 [2] Hung-Wei Yeha, Chih-Lung Tsengb, Po-Jui Chiang, Jau-Ji Joub, Nai-Hsiang Sunc a, Deposition of Homo-Multilayer Indium Tin Oxide Films on PET Substrates by Roll-to-Roll Sputtering, Next-Generation Electronics (ISNE) International Symposium, 2013 [3] Mitko Tanevski, Francesco Merli, Alexis Boegli, Anja K. Skrivervik, Jean-François Zürcher, Pierre-André Farine, RoSe: A Subgigahertz Wireless Sensor Platform With Housing-Integrated Overmolded Antenna, Transactions on Instruments and Measurements Vol. 61, Issue 11, 2012 [4] Enrico Macrelli, Ningning Wang, Saibal Roy, Michael Hayes, Rudi Paolo Paganelli, Marco Tartagni, Aldo Romani, Design and Fabrication of a 315 µH Bondwire Micro-Transformer for Ultra-Low Voltage Energy Harvesting, Design, Automation & Test in Europe Conference & Exhibition, 2014 Figure 1

A diffuse interface Lox/hydrogen transcritical flame model

We present a diffuse-interface all-pressure flame model that transitions smoothly between subcritical and supercritical conditions. The model involves a non-equilibrium liquid/gas diffuse interface of van der Waals/Korteweg type embedded into a non-ideal multicomponent reactive fluid. The multicomponent transport fluxes are evaluated in their thermodynamic form in order to avoid singularities at thermodynamic mechanical stability limits. The model also takes into account condensing liquid water in order to avoid thermodynamic chemical instabilities. The resulting equations are used to investigate the interface between cold dense and hot light oxygen as well as the structure of diffusion flames between cold dense oxygen and gaseous-like hydrogen at all pressures, either subcritical or supercritical.

The Limit of Mechanical Stability in Quantum Crystals: A Diffusion Monte Carlo Study of Solid $$^{4}$$ 4 He

We present a first-principles study of the energy and elastic properties of solid helium at pressures below the range in which it is energetically stable. We find that the limit of mechanical stability in hcp \(^{4}\)He is \(P_{s} = -33.8(1)\) bar, which lies significantly below the spinodal pressure found in the liquid phase (i.e., \(-9.6\) bar). Furthermore, we show that the pressure variation of the transverse and longitudinal sound velocities close to \(P_{s}\) does not follow a power law of the form \(\propto \left( P - P_{s} \right) ^{\gamma }\), in contrast to what is observed in the fluid.