Andrea Natale Tallarico

The effect of crystallization on the rheology and dynamics of lava flows

Abstract The dynamics of a lava flow is studied by a two-dimensional model describing a viscous fluid with Bingham rheology, flowing down a slope. The temperature in the flow is calculated assuming that heat is transferred through the plug by conduction and is lost by radiation to the atmosphere at the top of the flow. Taken into account is that the increasing crystallization takes place in the flow as a consequence of cooling. The lava viscosity and yield stress are expressed as a function of crystallization degree as well as of temperature: in particular it is assumed that yield stress reaches a maximum value above the solidus temperature, according to experimental data. Dynamical variables, such as velocity and thickness of the flow, are calculated for different values of the maximum crystallization degree and the flow rate. The model shows how the lava flow dynamics is affected by cooling and crystallization. The cooling of the flow is controlled by the increase of yield stress, which produces a thicker plug and makes the heat loss slower. The increasing crystallization has two opposing effects on viscosity: it produces an increase of viscosity, but at the same time produces an increase of yield stress and hence reduces the heat loss and keeps the internal temperature high. As a consequence, lava flows are significantly affected by the dependence of yield stress on temperature and scarcely by the maximum crystallization degree.

A model for the formation of lava tubes by roofing over a channel

The formation of lava tubes is a common phenomenon on some basaltic volcanoes, such as Etna. A model for tube formation by roofing of a channel is proposed and involves first describing lava as a Bingham liquid flowing down a slope. It is further assumed that lava flows in a channel with rectangular cross section: as a result of heat loss into the atmosphere, a crust is gradually formed on the upper surface of the flow and this crust eventually welds to the channel levees. We assume that a lava tube is formed when such a crust is sufficiently thick to resist the drag of the underlying flow and to sustain itself under its own weight. The minimum thickness of the crust satisfying such conditions depends on the tensile strength and shear strength of the crust itself. Assuming that the growth of the crust produces a downflow linear increase of the shear stress at the interface between flowing lava and the crust, the distance is evaluated between the eruption vent and the point where the tube is formed. The model predicts that if the flow rate is constant, the thickness of the flow increases as the crust fragments grow and weld to each other, and the velocity of the crust decreases to zero. Once the lava tube is formed, the initial flow rate can be achieved by a flow thickness smaller than the vertical size of the tube, with the same viscous dissipation: this may explain why under steady state conditions, the lava level inside a tube is frequently lower than the roof of the tube itself.

A three‐dimensional Bingham model for channeled lava flows

We propose a three-dimensional (3-D) Bingham model for channeled lava flow. Unlike from the 3-D Newtonian models, this model can be applied also far from the vent where the Bingham rheology cannot be neglected as a consequence of the lava cooling. We assume the lava to be an isothermal Bingham liquid flowing in a rectangular channel down a constant slope. The flow velocity is calculated by solving semianalytically the steady state Navier-Stokes equation together with the 3-D Bingham constitutive equation. The flow vorticity is evaluated and used to define the plug shape and position for different flows: a completely filled conduit, a partially filled conduit, and an open channel. Each component of the flow vorticity vector satisfies the Laplace equation and has been evaluated by using the relaxation method. The mass flow rate is evaluated for different values of the yield stress; it appears that the Bingham rheology causes a significant reduction in flow rate as the yield stress increases. For the highest yield stress values the plug in the center of the flow welds with the plugs in the flow corners, suggesting a possible rheological mechanism for the lava tube formation.

Longitudinal deformation of a lava flow : the influence of Bingham rheology

Abstract The behaviour of a lava flow is reproduced by a two-dimensional model of a Bingham liquid flowing down a slope. The liquid is described by two parameters, viscosity and yield stress, both strongly temperature dependent. Assuming liquidus temperature at the eruption vent, the temperature decrease due to the heat loss by radiation produces changes in the rheological parameters and, consequently, in velocity, strain and strain rate along the flow. Velocity, compressive strain, strain rate and stress along the flow direction are computed as functions of the distance from the vent and of time, for different kinds of lava flows (basic and acidic) and are compared with corresponding results for a Newtonian liquid. The model shows that, in connection with the pronounced velocity decrease occurring at a certain distance from the vent, the compressive strain, strain rate and stress also show a strong variation. A greater compressive strain is, in fact, induced in a Bingham flow cooling by radiation, than in a Newtonian flow. This behaviour may explain the presence of folds which are commonly observed at the surface of cooled lava flows.

A model for the opening of ephemeral vents in a stationary lava flow

Abstract A model is proposed for the opening of ephemeral vents in the solid front of a stationary lava flow. The front is considered to be a viscoelastic shell which deforms due to the pressure exerted on it by the inner fluid lava. The vent opens when the normal stress in the front overcomes the tensile strength of solid lava. Cooling of the front is considered and the increase in crust thickness, as the isothermal surface at the solidus temperature deepens into the lava body, is calculated. The thermal effect is negligible, if the timescale for the opening of ephemeral vents is on the order of days, as is often observed. Considering stationary flow lengths of a few hundred meters, this timescale corresponds to an average viscosity of the crust of 10 12 Pa s, assuming reasonable values of the crust strength and initial thickness. Observation of ephemeral vents on Mount Etna is found to be consistent with the model predictions.

Impact of the Substrate Orientation on CHC Reliability in n-FinFETs—Separation of the Various Contributions

In this paper, we perform a comparative analysis of the degradation induced by a channel hot carrier (CHC) in bulk n-FinFETs with rotated (100) and nonrotated (110) sidewall surfaces. CHC degradation includes several components, and separating each contribution is necessary for identifying different mechanisms in devices with different surface orientations. First, the permanent and recoverable components are separated based on the recovery phenomenon after the CHC stress, ascribed to the electron detrapping from the oxide bulk defects. Then, the contribution of generated interface states to the permanent component is quantified by charge pumping measurements. The nonrotated bulk FinFETs showed higher threshold voltage degradation at the maximum bulk current stress condition due to the higher interface precursor defect density compared with the rotated devices. On the other hand, the rotated bulk FinFETs showed higher threshold voltage degradation at the stress condition of VG = VD due to higher bulk defect trapping, ascribed to the lower physical oxide thickness.

Investigation of the p-GaN Gate Breakdown in Forward-Biased GaN-Based Power HEMTs

In this letter, we report a detailed experimental investigation of the time-dependent breakdown induced by forward gate stress in GaN-based power HEMTs with a p-type gate, controlled by a Schottky metal/p-GaN junction. When a high stress voltage is applied on the gate, a large voltage drop and an electric field occur in the depletion region of the p-GaN close to the metal interface, promoting the formation of a percolation path. We have investigated the mechanisms underlying the gate breakdown by adopting different stress conditions, analyzing the influence of the temperature, and investigating the activation energy of the traps. In addition, thanks to this approach, the device lifetime has been evaluated and an original empirical model, representing the relationship between the gate leakage current and the time to failure, has been proposed.

Statistical Analysis of the Impact of Anode Recess on the Electrical Characteristics of AlGaN/GaN Schottky Diodes With Gated Edge Termination

In this paper, we have extensively investigated the impact of anode recess on the reverse leakage current, forward voltage (VF), and dynamic characteristics of Au-free AlGaN/GaN Schottky barrier diodes with a gated edge termination (GET-SBDs) on 200-mm silicon substrates. By increasing the number of atomic layer etching (ALE) cycles for anode recessing, we have found that: 1) the reverse leakage current is strongly suppressed due to a better electrostatic control for pinching off the channel in the GET region; a median leakage current of ~1 nA/mm and an ION/IOFF ratio higher than 108 have been achieved in GET-SBDs with six ALE cycles; 2) the forward voltage (~1.3 V) is almost independent of the ALE cycles, taking into account its statistical distribution across the wafers; 3) when the remaining AlGaN barrier starts to be very thin (in the case of six ALE cycles), a spread of the ON-resistance, mainly attributed to the GET region, can occur due to the difficult control of the remaining AlGaN thickness and surface quality; and 4) the dynamic forward voltage of GET-SBDs shows a mild dependence on the ALE process in pulsed I-V characterization, and a more ALE-dependent dynamic ON-resistance is observed.

Negative Bias Temperature Stress Reliability in Trench-Gated P-Channel Power MOSFETs

In this paper, we present the results of an experimental analysis of the degradation induced by negative-bias temperature stress (NBTS) in trench-gated p-channel power MOSFETs. Threshold voltage and carrier mobility are affected by hole trapping in bulk oxide and interface-state generation due to oxide electric field effects. A fast recovery phase occurs when gate bias is removed or reduced in order to measure the threshold voltage. Hence, various techniques for evaluating threshold voltage shift are adopted in order to highlight the differences in the dynamics of degradation. We investigate the influence of gate bias levels during the stress. Moreover, with the help of recovery studies, we try to distinguish the impact of interface-state generation and charge trapping on the NBTS degradation.

Interaction between seismic and aseismic slip along a transcurrent plate boundary : a model for seismic sequences

Abstract A model is proposed which may explain the multiple occurrence of earthquakes which is often observed in the same seismogenic region. Such earthquake sequences or swarms may consist of events several tens of kilometres apart in space and a few days to months apart in time. The model considers a long, vertical strike-slip fault embedded in an elastic half-space. The fault is heterogeneous with respect to strength and to slip style (seismic or aseismic) and is subject to a slowly increasing ambient shear stress. We assume that conditions for stable aseismic slip are present in a part of the fault plane together with unstable asperities. The failure of an asperity, producing one earthquake of the sequence, is assumed to produce the onset or the acceleration of a seismic slip on a distant area of the fault, in proximity to another asperity. The conditions under which the latter asperity may fail, producing another earthquake of similar magnitude, are studied as functions of ambient stress rate, asperity strength, amount of aseismic slip, and aseismic slip area. It is found that the effect of aseismic slip can be remarkable in anticipating the occurrence of the second earthquake.