Computation of charge distribution and electrostatic potential in silicates with the use of chemical potential equalization models

New parameters for the electronegativity equalization model (EEM) and the split-charge equilibration (SQE) model are calibrated for silicate materials, based on an extensive training set of representative isolated systems. In total, four calibrations are carried out, two for each model, either using iterative Hirshfeld (HI) charges or ESP grid data computed with density functional theory (DFT) as a reference. Both the static (ground state) reference quantities and their responses to uniform electric fields are included in the fitting procedure. The EEM model fails to describe the response data, whereas the SQE model quantitatively reproduces all of the training data. For the ESP-based parameters, we found that the reference ESP data are only useful at those grid points where the electron density is lower than 0.001 a.u. The density value correlates with a distance criterion used for selecting grid points in common ESP fitting schemes. All parameters are validated with DFT computations on an independent set of isolated systems (similar to the training set), and on a set of periodic systems including dense and microporous crystalline silica structures, zirconia, and zirconium silicate. Although the transferability of the parameters to new isolated systems poses no difficulties, the atomic hardness parameters in the HI-based models must be corrected to obtain accurate results for periodic systems. The SQE/ESP model permits the calculation of the ESP with similar accuracy in both isolated and periodic systems

On the viscous dissipation caused by randomly rough indenters in smooth sliding motion

The viscous dissipation between rigid, randomly rough indenters and linearly elastic counter bodies sliding past them is investigated using Green’s function molecular dynamics. The study encompasses a variety of models differing in the height spectra properties of the rigid indenter, in the viscoelasticity of the elastomer, and in their interaction. All systems reveal the expected damping linear in sliding velocity at small and a pronounced maximum at intermediate . Persson’s theory of rubber friction, which is adopted to the studied model systems, reflects all observed trends. However, close quantitative agreement is only found up to intermediate sliding velocities. Relative errors in the friction force become significant once the contact area is substantially reduced by sliding

Stress Anisotropy Severely Affects Zinc Phosphate Network Formation

Using density-functional theory based simulations, we study how initially disconnected zinc phosphate molecules respond to diferent externally imposed deformations. Hybridization changes are observed in all cases, in which the coordination of zinc atoms changes irreversibly from tetrahedral to seesaw and square pyramidal, whereby the system stifens substantially. The point at which stif networks are formed does not only depend on the hydrostatic pressure. Stress anisotropy generally reduces the required hydrostatic network formation pressure. Moreover, networks obtained under isotropic deformations turn out stifer, elastically more isotropic, and lower in energy after decompression than those produced under anisotropic stresses. We also fnd that the observed stress-memory efects are encoded to a signifcant degree in the arrangement of atoms in the second neighbor shell of the zinc atoms. These fndings refne previously formulated conjectures of pressure-assisted cross-linking in zinc phosphate-based anti-wear flms

Mitigating Malicious Firmware by Detecting the Removal of a Storage Device

User equipment (UE), such as smartphones, notebooks, laptops, and the like, require protection from malicious software and firmware. To this end, device manufacturers install security chips on their devices, which are often located on the motherboard of the device. This publication describes hardware designs and methods to detect the removal of a storage device, such as a solid-state drive (SSD), a non-volatile memory (NVM), a non-volatile dual in-line memory module (NVDIMM), an embedded multimedia card (eMMC), and other types of non-volatile memory. After the UE detects the removal of the storage device, the UE sends a signal to a security chip (root of trust (ROT)). The ROT notifies a user that the storage device was removed, later re-installed or replaced, and that a full verification of the firmware of the storage device needs to occur. Full verification of firmware of the storage device has limitations, such as a limited access speed to the storage device. To this end, at times, the UE performs verification of critical OS areas (files). The verification process changes depending on the power state of the UE and the removal detection of the storage device. A cold boot, or the removal detection of the storage device, triggers a full verification of the storage device, whereas, a warm boot, without an indication that the storage device was removed, triggers verification of critical OS areas (files)

Plasma Torch Working Gas Selection Rationale for the Production Technology of Ultrapure White Corundum

This paper presents the results of one of the stages in the development of a plasma technology for producing ultrapure white corundum. This technology involves the melting of alumina in a reactor under the influence of plasma. To create the plasma, a plasma-forming gas is needed; in other words, the plasma torch working gas. To implement this technology, the chosen working gas must meet certain requirements. More precisely: 1) the gas should not form explosive or toxic compounds either with the reactor materials or the material of the electrodes; 2) it should not accelerate their erosion. An important requirement that a plasma torch working gas must have is minimal interaction with surrounding substances, even at high temperatures. This will reduce the likelihood of contamination of the melt with materials of the reactor itself. We consider using hydrogen, nitrogen and argon. Taking into account the requirements for the working gas, we opt for the mixture of nitrogen and argon at a concentration of 25÷30% N2 – 70÷75% Ar. Keywords: plasma torch, corundum, plasma technology, ultrapure material

Supply Chain Verification of Hardware Components Using Open-Source Root of Trust

This publication describes techniques and apparatuses for an open-source silicon-based Root of Trust (RoT) (a security chip, a RoT chip) solution for supporting supply chain verification of hardware components embedded in or on a circuit board (e.g., motherboard). In aspects, an open-source read-only (RO) Component Verifier, which is part of an RO Firmware, is provided to establish trust at all levels, such as a field-modifiable or read-write (RW) Firmware, an operating system (OS), cloud computing, component manufacturers, and so forth. The security chip includes two classes of non-volatile memory (storage): RO non-volatile memory (e.g., RO NVRAM), which is accessible only by the RO Firmware, and RW non-volatile memory (e.g., NVRAM), which is accessible by the RO Firmware and the RW Firmware. The security chip stores private keys (e.g., EK, AIK) used in for provisioning component Secrets in the RO NVRAM, preventing access to the RW Firmware. Without sharing any details, the RO Component Verifier of the RO Firmware is able to access the contents of the RO NVRAM. Such an intrinsically secure communication channel between the RO Firmware and the RO NVRAM, as well as preventing access to the RO NVRAM by the RW Firmware, is guaranteed by the hardware design of the security chip. This solution enables component manufacturers to use computationally inexpensive common protocols for identity verification (e.g., MARS, DICE) to communicate the Secrets associated with the components, while entrusting the open-source security chip to protect these secret keys

Mechanochemical Ionization: Differentiating Pressure-, Shear-, and Temperature-Induced Reactions in a Model Phosphate

Using density-functional theory-based molecular dynamics simulations, we study stress and temperature-induced chemical reactions in bulk systems containing triphosphoric acid and zinc phosphate molecules. The nature of the products depends sensitively on the imposed conditions, e.g., isotropic and even more so shear stress create (zwitter-) ionic products. Free ions also emerge from thermal cycles, but the reactions are endothermic rather than exothermic as for stress-induced transitions and zinc atoms remain four-coordinated. Hydrostatic stresses required for reactions to occur lie well below those typical for tribological micro-contacts of stiff solids and are further reduced by shear. Before zinc atoms change their coordination under stress, proton mobility increases, i.e., hydrogen atoms start to change the oxygen atom they are bonded to within 10 ps time scales. The hydrostatic stress for this to occur is reduced with increasing shear. Our finding suggests that materials for which number, nature, and mobility of ions are stress sensitive cannot have a well-defined position in the triboelectric series, since local contact stresses generally depend on the stiffness of the counter body. Moreover, our simulations do not support the idea that chemical reactions in a tribo-contact are commonly those that would be obtained through heating alone

Mechanochemical Ionization: Differentiating Pressure-, Shear-, and Temperature-Induced Reactions in a Model Phosphate

Using density-functional theory-based molecular dynamics simulations, we study stress and temperature-induced chemical reactions in bulk systems containing triphosphoric acid and zinc phosphate molecules. The nature of the products depends sensitively on the imposed conditions, e.g., isotropic and even more so shear stress create (zwitter-) ionic products. Free ions also emerge from thermal cycles, but the reactions are endothermic rather than exothermic as for stress-induced transitions and zinc atoms remain four-coordinated. Hydrostatic stresses required for reactions to occur lie well below those typical for tribological micro-contacts of stiff solids and are further reduced by shear. Before zinc atoms change their coordination under stress, proton mobility increases, i.e., hydrogen atoms start to change the oxygen atom they are bonded to within 10 ps time scales. The hydrostatic stress for this to occur is reduced with increasing shear. Our finding suggests that materials for which number, nature, and mobility of ions are stress sensitive cannot have a well-defined position in the triboelectric series, since local contact stresses generally depend on the stiffness of the counter body. Moreover, our simulations do not support the idea that chemical reactions in a tribo-contact are commonly those that would be obtained through heating alone

Interatomic potentials: Achievements and challenges

Interactions between atoms can be formally expanded into two-body, three-body, and higher-order contributions. Unfortunately, this expansion is slowly converging for most systems of practical interest making it inexpedient for molecular simulations. This is why effective descriptions are needed for the accurate simulation of many-atom systems. This article reviews potentials designed towards this end with a focus on empirical interatomic potentials not necessitating a-priori knowledge of what pairs of atoms are bonded to each other, i.e., on potentials meant to describe defects and chemical reactions from bond breaking and formation to redox reactions. The classes of discussed potentials include popular two-body potentials, embedded-atom models for metals, bond-order potentials for covalently bonded systems, polarizable potentials including charge-transfer approaches for ionic systems and quantum-Drude oscillator models mimicking higher-order and many-body dispersion. Particular emphasis is laid on the question what constraints on materials properties ensue from the functional form of a potential, e.g., in what way Cauchy relations for elastic tensor elements can be violated and what this entails for the ratio of defect and cohesive energies. The review is meant to be pedagogical rather than encyclopedic. This is why we highlight potentials with functional forms that are sufficiently simple to remain amenable to analytical treatments, whereby qualitative questions can be answered, such as, why the ratio of boiling to melting temperature tends to be large for potentials describing metals but small for pair potentials. However, we abstain for the most part from discussing specific parametrizations. Our main aim is to provide a stimulus for how existing approaches can be advanced or meaningfully combined to extent the scope of simulations based on empirical potentials