Steady-state Rate Research Articles

Linear and nonlinear integrate-and-fire neurons driven by synaptic shot noise with reversal potentials.

The steady-state firing rate and firing-rate response of the leaky and exponential integrate-and-fire models receiving synaptic shot noise with excitatory and inhibitory reversal potentials is examined. For the particular case where the underlying synaptic conductances are exponentially distributed, it is shown that the master equationfor a population of such model neurons can be reduced from an integrodifferential form to a more tractable set of three differential equations. The system is nevertheless more challenging analytically than for current-based synapses: where possible, analytical results are provided with an efficient numerical scheme and code provided for other quantities. The increased tractability of the framework developed supports an ongoing critical comparison between models in which synapses are treated with and without reversal potentials, such as recently in the context of networks with balanced excitatory and inhibitory conductances.

Distinct Site Motifs Activate O2 and H2 on Supported Au Nanoparticles in Liquid Water.

Au nanoparticles catalyze the activation and conversion of small molecules with rates and kinetic barriers that depend on the dimensions of the nanoparticle, composition of the support, and presence of catalytically culpable water molecules that solvate these interfaces. Here, molecular interpretations of steady-state rate measurements, kinetic isotope effects, and structural characterizations reveal how the interface of Au nanoparticles, liquid water, and metal oxide supports mediate the kinetically relevant activation of H2 and sequential reduction of O2-derived intermediates during the formation of H2O2 and H2O. Rates of H2 consumption are 10-100 fold greater on Au nanoparticles supported on metal oxides (e.g., titania) compared to more inert and hydrophobic materials (carbon, boron nitride). Similarly, Au nanoparticles on reducible and Lewis acidic supports (e.g., lanthana) bind dioxygen intermediates more strongly and present lower barriers (<22 kJ mol-1) for O-O bond dissociation than inert interfaces formed with silica (>70 kJ mol-1). Selectivities for H2O2 formation increase significantly as the diameters of the Au nanoparticles increase because differences in nanoparticle size change the relative fractions of exposed sites that exist at Au-support interfaces. In contrast, site-normalized rates and barriers for H2 activation depend weakly on the size of Au nanoparticles and the associated differences in active site motifs. These findings suggest that H2O aids the activation of H2 at sites present across all surface Au atoms when nanoparticles are solvated by water. However, molecular O2 preferentially binds and dissociates at Au-support interfaces, leading to greater structure sensitivity for barriers of O-O dissociation across different support identities and sizes of Au nanoparticles. These insights differ from prior knowledge from studies of gas-phase reactions of H2 and O2 upon Au nanoparticle catalysts within dilute vapor pressures of water (10-4 to 0.1 kPa H2O), in which catalysis occurs at the perimeter of the Au-support interface. In contrast, contacting Au catalysts with liquid water (55.5 M H2O) expands catalysis to all surface Au atoms and enables appreciable H2O2 formation.

Ring Growth Mechanism in the Reaction between Fulvenallenyl and Cyclopentadienyl Radicals.

Recombination between resonance-stabilized hydrocarbon radicals is an important class of reactions that contribute to molecular growth chemistry in combustion. In the present study, the ring growth mechanism in the reaction between fulvenallenyl (C7H5) and cyclopentadienyl (C5H5) radicals is investigated computationally. The reaction pathways are explored by quantum chemical calculations, and the phenomenological and steady-state rate constants are determined by solving the multiple-well master equations. The primary reaction routes following the recombination between the two radicals are found to be as follows: formation of the adducts, isomerization by hydrogen shift reactions, cyclization to form tricyclic compounds, and their isomerization and dissociation reactions, leading to the formation of acenaphthylene. The overall process can be approximately represented as C7H5 + C5H5 → acenaphthylene + 2H with the bimolecular rate constant of about 4 × 10-12 cm3 molecule-1 s-1. A reaction mechanism consisting of 20 reactions, including the formation, isomerization, and dissociation processes of major intermediate species, is proposed for use in kinetic modeling.

Sensory adaptation in a continuum model of bacterial chemotaxis—working range, cost-accuracy relation, and coupled systems

Sensory adaptation enables organisms to adjust their perception in a changing environment. A paradigm is bacterial chemotaxis, where the output activity of chemoreceptors is adapted to different baseline concentrations via receptor methylation. The range of internal receptor states limits the stimulus magnitude to which these systems can adapt. Here, we employ a highly idealized, Langevin-equation based model to study how the finite range of state variables affects the adaptation accuracy and the energy dissipation in individual and coupled systems. Maintaining an adaptive state requires constant energy dissipation. We show that the steady-state dissipation rate increases approximately linearly with the adaptation accuracy for varying stimulus magnitudes in the so-called perfect adaptation limit. This result complements the well-known logarithmic cost-accuracy relationship for varying chemical driving. Next, we study linearly coupled pairs of sensory units. We find that the interaction reduces the dissipation rate per unit and affects the overall cost-accuracy relationship. A coupling of the slow methylation variables results in a better accuracy than a coupling of activities. Overall, the findings highlight the significance of both the working range and collective operation mode as crucial design factors that impact the accuracy and energy expenditure of molecular adaptation networks.

Dynamic effects of tourism shocks on innovation in an open-economy Schumpeterian growth model

International travel restrictions during the COVID-19 pandemic drastically reduced the number of tourists. This study explores the dynamic effects of tourism shocks in an open-economy Schumpeterian model with endogenous market structure. A tourism shock affects the economy via a reallocation effect and an employment effect. A positive tourism shock increases employment, which raises production and innovation in the short run. However, a positive tourism shock also reallocates labor from production to service for tourists, which reduces production and innovation. If leisure preference is weak, the reallocation effect dominates, and the short-run effect of positive tourism shocks on innovation is monotonically negative. If leisure preference is strong, the employment effect dominates initially, and the short-run effect of tourism shocks on innovation becomes inverted-U, which is consistent with the stylized facts that we document using cross-country data. Finally, permanent tourism shocks do not affect the steady-state innovation rate in our scale-invariant model.

Unveiling the electronic structure of GaSb/AlGaSb quantum dots emitting in the third telecom window

Epitaxially-grown semiconductor quantum dots (QDs) provide an attractive platform for the development of deterministic sources of high-quality quantum states of light. Such non-classical light sources are essential for quantum information processing and quantum communication. QDs emitting in the telecom wavelengths are especially important for ensuring compatibility with optical fiber systems required to implement quantum communication networks. To this end, GaSb QDs fabricated by filling local-droplet etched nanoholes are emerging as a viable approach, yet the electronic properties of such nanostructures have not been studied in detail. In this article, an insight into the electronic structure and carrier dynamics in GaSb/AlGaSb QDs is provided through a systematic experimental analysis of their temperature-dependent photoluminescence behavior. A steady-state rate equation model is used to reveal the relevant energy barriers for thermally activated carrier capture and escape processes. Furthermore, results of detailed theoretical simulations of quantum-confined energy states using the multi-band k·p model and the effective mass method are presented. The purpose of the simulations is to reveal the direct and indirect energy states, carrier wavefunctions, and allowed optical transitions for GaSb QDs with different physical dimensions.

No yield stress required: Stress-activated flow in simple yield-stress fluids

An elastoviscoplastic constitutive equation is proposed to describe both the elastic and rate-dependent plastic deformation behavior of Carbopol® dispersions, commonly used to study yield-stress fluids. The model, a variant of the nonlinear Maxwell model with stress-dependent relaxation time, eliminates the need for a separate Herschel–Bulkley yield stress. The stress dependence of the viscosity was determined experimentally by evaluating the steady-state flow stress at a constant applied shear rate and by measuring the steady-state creep rate at constant applied shear stress. Experimentally, the viscosity’s stress-dependence was confirmed to follow the Ree–Eyring model. Furthermore, it is shown that the Carbopol® dispersions used here obey time-stress superposition, indicating that all relaxation times experience the same stress dependence. This was demonstrated by building a compliance mastercurve using horizontal shifting on a logarithmic time axis of creep curves measured at different stress levels and by constructing mastercurves of the storage- and loss-modulus curves determined independently by orthogonal superposition measurements at different applied constant shear stresses. Overall, the key feature of the proposed constitutive equation is its incorporation of a nonlinear stress-activated change in relaxation time, which enables a smooth transition from elastic to viscous behavior during start-up flow experiments. This approach bypasses the need for a distinct Herschel–Bulkley yield stress as a separate material characteristic. Additionally, the model successfully replicates the observed steady-state flow stress in transient-flow scenarios and the steady-state flow rate in creep experiments, underlining its effectiveness in capturing the material’s dynamic response. Finally, the one-dimensional description is readily extended to a full three-dimensional finite-strain elastoviscoplastic constitutive equation.

The mechanism of ceftazidime and cefiderocol hydrolysis by D179Y variants of KPC carbapenemases is similar and involves the formation of a long-lived covalent intermediate.

Klebsiella pneumoniae carbapenemase (KPC) variants have been described that confer resistance to both ceftazidime-avibactam and cefiderocol. Of these, KPC-33 and KPC-31 are D179Y-containing variants derived from KPC-2 and KPC-3, respectively. To better understand this atypical phenotype, the catalytic mechanism of ceftazidime and cefiderocol hydrolysis by KPC-33 and KPC-31 as well as the ancestral KPC-2 and KPC-3 enzymes was studied. Steady-state kinetics showed that the D179Y substitution in either KPC-2 or KPC-3 is associated with a large decrease in both kcat and KM such that kcat/KM values were largely unchanged for both ceftazidime and cefiderocol substrates. A decrease in both kcat and KM is consistent with a decreased and rate-limiting deacylation step. We explored this hypothesis by performing pre-steady-state kinetics and showed that the acylation step is rate-limiting for KPC-2 and KPC-3 for both ceftazidime and cefiderocol hydrolysis. In contrast, we observed a burst of acyl-enzyme formation followed by a slow steady-state rate for the D179Y variants of KPC-2 and KPC-3 with either ceftazidime or cefiderocol, indicating that deacylation of the covalent intermediate is the rate-limiting step for catalysis. Finally, we show that the low KM value for ceftazidime or cefiderocol hydrolysis of the D179Y variants is not an indication of tight binding affinity for the substrates but rather is a reflection of the deacylation reaction becoming rate-limiting. Thus, the hydrolysis mechanism of ceftazidime and cefiderocol by the D179Y variants is very similar and involves the formation of a long-lived covalent intermediate that is associated with resistance to the drugs.

An experimental, theoretical, and kinetic modeling study of post-flame oxidation of ammonia

The post-flame oxidation rate of ammonia was investigated in a novel atmospheric pressure flow reactor at temperatures of 1280 ± 16 K and as a function of residence time and mixture composition (1-10% O2, dry and moist). The experimental results, as well as selected data from literature, were analyzed using an updated detailed chemical kinetic model. The medium temperature, very lean conditions enhance the importance of reactions of the nitroxyl (HNO) intermediate. High-level theory was used to calculate the rate constant for HNO + NH2, indicating that this step is significantly faster than values used in literature. Furthermore, a trajectory based approach was used to determine collision efficiencies for selected bath gases for HNO + M. The experimental results show that the NH3 oxidation rate increases with temperature and O2 concentration, while the presence of water vapor slightly inhibits reaction. Formation of NO and N2O was strongly promoted at higher levels of O2. Modeling results agreed well with the measurements, except at the lowest level of O2. The predicted oxidation rate of NH3 was shown to result from a delicate balance between chain branching and terminating steps involving NH2, H2NO, and HNO. Recent theoretical work on reactions of these species by Klippenstein and coworkers and Stagni et al. was instrumental in improving modeling predictions. After initiation, NO reached a pseudo-steady-state level, where the pathways to NO were largely balanced by the NH2 + NO reaction. Nitric oxide was partly oxidized to NO2, with the NH2 + NO2 reaction responsible for most of the N2O formation. Novelty and significance statement:This study provides the first detailed kinetic analysis of the lean post-flame oxidation of ammonia, based on time-resolved flow reactor data in a novel reactor. In addition to the post-flame oxidation rate of ammonia, data for formation of NO and N2O were compared with modeling predictions. The medium temperature, very lean conditions enhance the importance of reactions of the HNO and H2NO intermediates. Inclusion in the model of results from recent high-level theoretical work, including present calculations for HNO + NH2 and HNO + M, was crucial for capturing the observed behavior. It is argued that the post-initiation steady-state NH3 oxidation rates constitute important data for model validation, along with ignition delays and laminar flame speeds.

Study on Reliability of PACSs with Integrated Consideration of Both Basic and Mission Reliability

Protective relays play a fundamental role in power systems, whose functions consist of three parts, namely protection, automation, and control, thus called the PAC altogether. It turns out that the architecture of a PACS (protection, automation, and control system) may exert a direct impact on the reliability of protective relay systems, previous studies of which are often conducted on the assumption that intelligent electronic devices (IEDs) have a constant fault rate with only mission reliability of the systems considered. To obtain a more accurate evaluation of system reliability bearing system maintenance in mind, this article proposes a novel method to appraise the reliability of PACSs with integrated consideration of basic and mission reliability. Firstly, the basic reliability of IEDs is calculated by parts count prediction, which will provide a more precise fault rate of IEDs. Secondly, the reliability of PACSs is analyzed by the reliability diagram and Markov model. By using the parts count prediction method to calculate the failure rate of the device, the steady-state probability rate will be accurate more than 0.0107%. At last, a quantitative criterion is finalized for selecting a suitable architecture easily.

Market-Based Estimates of the Natural Real Rate: Evidence from Latin American Bond Markets

We provide market-based estimates of the natural real rate, that is, the steady-state short-term real interest rate, for Brazil, Chile, and Mexico. Our approach uses a dynamic term structure finance model estimated directly on the prices of individual inflation indexed bonds with adjustments for bond-specific liquidity and real term premia. First, we find that inflation-indexed bond liquidity premia in all three countries are sizable with significant variation. Second, we find large differences in their estimated equilibrium real rates: Brazil’s is large and volatile, Mexico’s is stable but elevated, while Chile’s is low and has fallen persistently. Although uncertain, our estimates could have important implications for the conduct of monetary policy in these three countries.

Microstructural characterisation of grain size effect on creep properties of 304HCu SS

The influence of grain size, in the range of 10 to 180 μm, on creep properties of 304HCu stainless steel (SS) was evaluated at 180 MPa and 923 K. A 30 μm grain size showed a peak in creep life and minimum in steady-state creep rate with low damage potential. Detailed microstructural investigations revealed an increase in the tendency to intergranular fracture at grain sizes above 30 μm, whereas ‘transgranular fracture coupled with isolated intergranular cracks/cavities’ have been observed with decreasing grain size from 30 to 10 μm.

Econometric issues in the estimation of the natural rate of interest

The natural rate of interest is a key benchmark steady-state rate of return that is used by central banks to measure the stance of monetary policy. Asset managers rely on estimates of the natural rate to make long-term investment decisions for their clients. This paper outlines a number of important econometric issues in the estimation of the natural rate of interest from a widely used structural model. Using a simulation experiment as well as empirical data for the U.S., I show that one of the key parameters of the structural model is overestimated and leads to a spuriously amplified downward trend in the natural rate. The paper shows how the overestimation can be remedied and provides alternative estimates of the natural rate. Various other issues are discussed that policy makers should be aware of when utilizing this model’s natural rate estimates for policy decision.

Low-temperature oxidative removal of benzene from the air using titanium carbide (MXene)-Supported platinum catalysts

MXenes are an emerging class of two-dimensional (2D) inorganic materials with great potential for versatile applications such as adsorption and catalysis. Here, we describe the synthesis of a platinized titanium carbide MXene (Pt@Ti3C2) catalyst with varying amounts of platinum (0.1%–2 wt.%) for the low-temperature oxidation of benzene, an aromatic volatile organic compound often found in industrial flue gas. A 1% formulation of Pt@Ti3C2–R allowed near-complete (97%) oxidation of benzene to CO2 at 225 °C with a steady-state reaction rate (r) of 0.119 mol g−1·h−1. This low-temperature catalytic oxidation reaction was promoted by an increase in the lattice oxygen (O*)/Pt2+ species (active sites) of 1%Pt@Ti3C2–R from 45.3/34.6% to 71.0/61.1% through pre-thermal reduction under H2 flow, as revealed by X-ray photoelectron spectroscopy, temperature-programmed reduction, and in situ diffuse reflectance infrared Fourier transform spectroscopy analyses. The cataltyic activity of 1% Pt@Ti3C2–R against benzene was assessed under the control of the key process variables (e.g., catalyst mass, flow rate, benzene concentration, relative humidity, and time-on-stream) to help optimize the oxidation reaction process. The results provide new insights into the use of platinum-based 2D MXene catalysts for low-temperature oxidative removal of benzene from the air.

Energy transfer mechanism and luminescence properties of green phosphors Sr2YF7: Dy3+, Tb3+

In this study, color-adjustable Sr2YF7: xDy3+, and yTb3+ light-emitting phosphors were synthesized through the hydrothermal method. Systematic experiments were performed to determine the optimal doping concentration for Sr2YF7: xDy3+ phosphor with x = 0.03, resulting in a distinctive light blue emission band attributed to the 4F9/2 → 6H15/2 transition. The energy transfer within the double-doped system of Sr2YF7: Dy3+, yTb3+ (y = 0, 0.06–0.1) was examined by analyzing spectral overlap and reductions in emission lifetimes. Moreover, the mechanism governing this energy transfer phenomenon was investigated. The concentration proportion of Dy3+/Tb3+ in Sr2YF7 phosphors was modulated to achieve color modulation, transitioning from light blue to vibrant green emissions in Sr2YF7: Dy3+, Tb3+ samples. The intensity of green emission increased with increasing Tb3+ ion concentrations because of the dynamics of energy migration from Dy3+ to Tb3+ levels (4F9/2 + 7F6 → 6H15/2 + 5D4). Based on calculations using steady-state rate equations, Tb3+ ion doping effectively reduced ionic spacing and consequently amplified the green emission. Furthermore, the results indicated that the luminous intensity ratio of Sr2YF7: 0.03Dy3+, 0.07 Tb3+ phosphors at 423/303K was notably high at 93.59 %, indicating high thermal stability. Thus, Sr2YF7 phosphors co-doped with Dy3+ and Tb3+ emerge as potential candidates for application in advanced illumination technologies.

Output Stream from the AQM Queue with BMAP Arrivals

We analyse the output stream from a packet buffer governed by the policy that incoming packets are dropped with a probability related to the buffer occupancy. The results include formulas for the number of packets departing the buffer in a specific time, for the time-dependent output rate and for the steady-state output rate. The latter is the key performance measure of the buffering mechanism, as it reflects its ability to process a specific number of packets in a time unit. To ensure broad applicability of the results in various networks and traffic types, a powerful and versatile model of the input stream is used, i.e., a BMAP. Numeric examples are provided, with several parameterisations of the BMAP, dropping probabilities and loads of the system.

Market-based estimates of the natural real rate: evidence from latin american bond markets

We provide market-based estimates of the natural real rate, that is, the steady-state short-term real interest rate, for Brazil, Chile, and Mexico. Our approach uses a dynamic term structure finance model estimated directly on the prices of individual inflation- indexed bonds with adjustments for real term and bond-specific liquidity premia. First, we find that inflation-indexed bond liquidity premia in all three countries are sizable with significant variation. Second, we find large differences in their estimated equilibrium real rates with Brazil’s being large and volatile, Mexico’s stable, but elevated, while Chile’s is low and has fallen persistently.

Multi-dimensional tailoring of the thermometric behaviors of Er3+/Yb3+-codoped Y2W3O12 polychromatic upconverting microparticles for multi-mode visual optical thermometry

To combat the thermal quenching and inadequate sensitivities of luminous materials, the Y1.98–2xW3O12:0.02Er3+/2xYb3+ (YWO:Er3+/2xYb3+; 0.01 ≤ x ≤ 0.11) microparticles with splendid negative thermal expansion (NTE) effect were designed. Excited at 980 nm, the produced samples emit polychromatic upconversion (UC) emissions triggered by the energy back transfer from Er3+ to Yb3+ and it has been systematically discussed via steady-state rate theory. The thermally enhanced UC emissions are accomplished in YWO:Er3+/2xYb3+ microparticles due to the distinctive NTE properties. Moreover, according to the temperature-associated UC emissions of Er3+ arising from its thermally coupled levels (TCLs) and non-thermally coupled levels (non-TCLs), the thermometric properties of synthesized microparticles are investigated. When non-TCLs are adopted, the absolute and relative sensitivities of resultant microparticles are 0.0129% and 1.7875% K−1, respectively. Furthermore, through manipulating doping content and spatial mode, the temperature sensing capacities of YWO:Er3+/2xYb3+ microparticles are multi-dimensionally regulated. In addition, contactless temperature recognition can also be realized through analyzing the thermochromic features of final products and its relative sensitivity is 2.1604% K−1. Ultimately, visual optical temperature detection is obtained in designed microparticles via its temperature-related emitting colour. Our findings may provide an important insight into the design of luminescent materials with high sensitivities for multi-mode visual optical thermometry.

RELATIVE AND ABSOLUTE METABOLIC COST OF DAILY PHYSICAL ACTIVITIES ACROSS THE LIFESPAN

Abstract Americans spend over 90% of their activity related energy expenditure performing common daily activities. However, there is a gap in knowledge pertaining to potential age-related differences in the metabolic cost of performing these activities which diminishes proper physical activity prescription. We hypothesized age differences in both the absolute and relative metabolic cost of performing common daily physical. Participants (n=248, age: 20-94 yrs) performed 33 activities (e.g. dressing, walking, making a bed, gardening, computer work) in a standardized setting. A portable indirect calorimeter was worn for approximately 6-10 minutes to achieve steady-state metabolic rate. A graded exercise treadmill test was performed to quantify peak metabolic rate. A metabolic equivalent (MET as a function of 3.5 milliliter•min-1•kg-1) and percentage of peak were used to express absolute and relative metabolic rate, respectively. Watching television (1.0+/-0.36 METs; 15.6+/-5.7% of peak) produced the lowest and climbing stairs (6.2+/-2.2 METs; 84.8+/-17.0% of peak) elicited the highest metabolic rate. Results from mixed effect regression models adjusting for health conditions, demographics and anthropometry showed no association between absolute MET and age. However, when expressed relative to peak, adjusted models showed a strong association with age (0.66+/-0.33% per year, p=0.006). As an example, adults &amp;lt; 40 yrs old worked at 34.1+/-14.8% of peak during a leisurely walk while those 80+ years old worked at 64.2+/-16.3%. Aging is associated with a substantial increase in the relative metabolic cost of performing common daily physical activities. These results could have implications on prescribing physical activity older adults to achieve higher fitness.

A Retrospective Comparison of Electrocardiographic Parameters in Ketamine and Tiletamine-Zolazepam Anesthetized Indian Rhesus Monkeys (Macaca mulatta).

Electrocardiographic evaluation is performed in rhesus monkeys to establish the cardiovascular safety of candidate molecules before progressing to clinical trials. These animals are usually immobilized chemically by ketamine (KTM) and tiletamine-zolazepam (TZ) to obtain a steady-state heart rate and to ensure adequate human safety. The present study aimed to evaluate the effect of these anesthetic regimens on different electrocardiographic parameters. Statistically significant lower HR and higher P-wave duration, RR, QRS, and QT intervals were observed in the KTM-anesthetized group in comparison to TZ-anesthetized animals. No significant changes were noticed in the PR interval and p-wave amplitude. Sex-based significance amongst these parameters was observed in male and female animals of TZ- and KTM-anesthetized groups. Regression analysis of four QTc formulas in TZ-anesthetized rhesus monkeys revealed that QTcNAK (Nakayama) better corrected the QT interval than QTcHAS (Hassimoto), QTcBZT (Bazett), and QTcFRD (Fridericia) formulas. QTcNAK exhibited the least correlation with the RR interval (slope closest to zero and r = .01) and displayed no statistical significance between male and female animals. These data will prove useful in the selection of anesthetic regimens for chemical restraint of rhesus monkeys in nonclinical safety evaluation studies.