Analytical solution for heat conduction due to a moving Gaussian heat flux with piecewise constant parameters

We provide an analytical solution of the heat equation in the half-space subject to a moving Gaussian heat flux with piecewise constant parameters. The solution is of interest in powder bed fusion applications where these parameters can be used to control the conduction of heat due to a scanning beam of concentrated energy. The analytical solution is written in a dimensionless form as a sum of integrals over (dimensionless) time. For the numerical computation of these integrals we suggest a quadrature scheme that utilizes pre-calculated look-up tables for the required quadrature orders. Such a scheme is efficient because the required quadrature orders are strongly dependent on the parameters in the heat flux. The possibilities of using the obtained computational technique for the control and optimization of powder bed fusion processes are discussed

A greedy algorithm for optimal heating in powder-bed-based additive manufacturing

Powder-bed-based additive manufacturing involves melting of a powder bed using a moving laser or electron beam as a heat source. In this paper, we formulate an optimization scheme that aims to control this type of melting. The goal consists of tracking maximum temperatures on lines that run along the beam path. Time-dependent beam parameters (more specifically, beam power, spot size, and speed) act as control functions. The scheme is greedy in the sense that it exploits local properties of the melt pool in order to divide a large optimization problem into several small ones. As illustrated by numerical examples, the scheme can resolve heat conduction issues such as concentrated heat accumulation at turning points and non-uniform melt depths

A greedy algorithm for optimal heating in powder-bed-based additive manufacturing

Powder-bed-based additive manufacturing involves melting of a powder bed using a moving laser or electron beam as a heat source. In this paper, we formulate an optimization scheme that aims to control this type of melting. The goal consists of tracking maximum temperatures on lines that run along the beam path. Time-dependent beam parameters (more specifically, beam power, spot size, and speed) act as control functions. The scheme is greedy in the sense that it exploits local properties of the melt pool in order to divide a large optimization problem into several small ones. As illustrated by numerical examples, the scheme can resolve heat conduction issues such as concentrated heat accumulation at turning points and non-uniform melt depths

MECHANISMS FOR MOLECULAR-OXYGEN DESORPTION FROM THE CAO(100) SURFACE

Electronic structure calculations have been employed to study the associative desorption of preadsorbed atomic oxygen from a calcium oxide (100) surface. Two reaction mechanisms were considered, corresponding to symmetric and asymmetric reaction paths. The energy barrier of the latter was found to be the lower of the two. The barrier for the asymmetric reaction was found to be sensitive both to the location of the nearest calcium ions and to the distance between the surface oxygen ions, and is argued to be strongly affected by surface temperature. The desorption mechanism is shown to be conceptually similar to the previously addressed N2O decomposition and CO2 formation reactions. Catalytical aspects are stressed as oxygen desorption from CaO(100) is compared to those of nickel and platinum

N2O2, N2O2- and N2O22-: structures, energetics and N-N bonding

The density functional theory and the ab initio quantum chemistry methods, Becke3LYP and CASPT2, have been employed to determine structures and energetics of neutral and anionic N2O2 species. When going from a neutral NO dimer to an anionic species the N---N bond lengths decrease drastically and the corresponding N---N frequencies increase. The relative stabilities of the different N2O2− isomers suggest an ONNO structure in a Trans configuration to be the most stable mono valent anion. The formation energies of 1.4–1.7 eV relative to the NO + NO− asymptote are in agreement with experiments. Excitation energies are determined and specific formation channels for three N2O2− isomers are discussed. The hyponitrite ion, N2O22−, is also studied. Its total energy is 2.7–2.8 eV above the total energy of two NO−. In order to connect to an experimental study of NO adsorption on MgO the ability of two NO molecules to form a complex with Mg and Mg+ is investigated

Effective dynamic correlation in multiconfigurational wave-function calculations on atoms and molecules

An intuitive understanding of dynamic correlation in terms of a regularized electron repulsion expression is outlined. Expressions for cusp kinetic energy corrected regularized electron repulsion integrals are deduced and implemented in a multiconfigurational wave-function framework. A regularized complete active space self-consistent field (reg-CASSCF) technique is suggested and tested on atomic total energies, molecular structures and binding energies

N2O2, N2O2- and N2O22-: structures, energetics and N-N bonding

The density functional theory and the ab initio quantum chemistry methods, Becke3LYP and CASPT2, have been employed to determine structures and energetics of neutral and anionic N2O2 species. When going from a neutral NO dimer to an anionic species the N---N bond lengths decrease drastically and the corresponding N---N frequencies increase. The relative stabilities of the different N2O2− isomers suggest an ONNO structure in a Trans configuration to be the most stable mono valent anion. The formation energies of 1.4–1.7 eV relative to the NO + NO− asymptote are in agreement with experiments. Excitation energies are determined and specific formation channels for three N2O2− isomers are discussed. The hyponitrite ion, N2O22−, is also studied. Its total energy is 2.7–2.8 eV above the total energy of two NO−. In order to connect to an experimental study of NO adsorption on MgO the ability of two NO molecules to form a complex with Mg and Mg+ is investigated

Structures and stabilities of some neutral and anionic (NO)(n)H-m based compounds (n=2-8, m=0-2)

Structures and stabilities of various neutral and anionic (NO)nHm (n = 2-8, m =0-2) based molecules which display diaza bonds are determined. By taking the oxygen biradical resonance in ON=NO as the chemical building unit, some understanding of the bindings in these systems is obtained. The calculations employ the gradient corrected hybrid density functional theory approach Becke3LYP. For some of the species single point correlated ab initio electronic structure calculations were performed by means of the CASPT2 method. Connections are made to experimental findings at surfaces, where (NO)n clusters appear as either stable or metastable intermediates

Surface complexes of nitric oxide: adsorption of anionic NO and N2O2 on CaO

Chemical reactions involving nitric oxide molecules and a calcium oxide surface are addressed by means of quantum chemistry. The regularised complete active space self-consistent field method (reg-CASSCF) is employed to determine structures and vibrational frequency bands of the monomeric and dimeric surface species. Stabilities are computed by the CASPT2 method. Reactivity trends for the adsorbates are estimated for various surface sites, including bonding to the Ca and O surface atoms, to an oxygen vacancy and to two different edge sites. Implications of excess surface electrons to heterogeneous NO reduction processes at oxide catalysts are discussed