Molten Phase Research Articles

Enhancement of Amorphous Celecoxib Stability by Mixing It with Octaacetylmaltose: The Molecular Dynamics Study

In this paper, we present a novel way of stabilization of amorphous celecoxib (CEL) against recrystallization by preparing binary amorphous celecoxib-octaacetylmaltose (CEL-acMAL) systems by quench-cooling of the molten phase. As far as we know this is the first application of carbohydrate derivatives with acetate groups to enhance the stability of an amorphous drug. We found that CEL in the amorphous mixture with acMAL is characterized by a much better solubility than pure CEL. We report very promising results of the long-term measurements of stability of the CEL-acMAL binary amorphous system with small amount of stabilizer during its storage at room temperature. Moreover, we examined the effect of adding acMAL on molecular dynamics of CEL in the wide temperature range in both the supercooled liquid and glassy states. We found that the molecular mobility of the mixture of CEL with 10 wt % acMAL in the glassy state is much more limited than that in the case of pure CEL, which correlates with the better stability of the amorphous binary system. By dielectric measurements and theoretical calculations within the framework of density functional theory (DFT), we studied the role of acMAL in enhancing the stability of amorphous CEL in mixtures and postulated which interactions between CEL and acMAL molecules can be responsible for preventing devitrification.

Dense and nanometric electronic excitations induced by swift heavy ions in an ionic CaF2crystal: Evidence for two thresholds of damage creation

CaF${}_{2}$ crystals as representatives of the class of ionic nonamorphizable insulators were irradiated with many different swift heavy ions of energy above 0.5 MeV/u providing a broad range of electronic energy losses (${S}_{e}$). Beam-induced modifications were characterized by Channeling Rutherford Backscattering Spectrometry (C-RBS) and x-ray diffraction (XRD), complemented by transmission electron microscopy (TEM). Results from C-RBS give evidence of significant damage appearing above a ${S}_{e}$ threshold of 5 $\ifmmode\pm\else\textpm\fi{}$ 2 keV/nm. A second critical ${S}_{e}$ appears around 18 $\ifmmode\pm\else\textpm\fi{}$ 3 keV/nm; below this value the damage as function of ion fluence saturates at 20$%$, while above this the damage saturation level increases with ${S}_{e}$, reaching $\ensuremath{\sim}$60$%$ for ions of ${S}_{e}$ $=$ 30 keV/nm. XRD measurements also show effects indicating two threshold values. Above 5 keV/nm, the widths of the XRD reflection peaks increase due to the formation of nanograins, as seen by TEM, while a significant decrease of the peak areas only occurs above 18 keV/nm. The track radii deduced from C-RBS measurements are in agreement with those extracted from the fluence evolution of the widths of the XRD peaks. Moreover, track radii deduced from the peak area analysis are slightly smaller but in agreement with previous track observations by high resolution electron microscopy. Calculations based on the inelastic thermal spike model suggest that the lower threshold at 5 keV/nm is linked to the quenching of the molten phase, whereas the threshold at 18 keV/nm can be interpreted as quenching of the boiling phase. The results of CaF${}_{2}$ are compared with other nonamorphizable materials such as LiF and UO${}_{2}$.

Solid State Bonding of Silicon Chips to Silver Buffer on Copper Substrates

A solid state bonding technique was designed and developed to bond silicon (Si) chips to silver (Ag) buffer layer electroplated on copper (Cu) substrates at a temperature (260°C ) far below the melting temperature of Ag (961°C ). This is possible because Ag atoms in Ag buffer are brought in contact with the gold (Au) atoms in Au layer coated on Si chip. The close proximity in atomic scale allows the Ag and Au atoms to see each other, to share electrons and to bond to each other. The possibility of mating between Au and Ag surfaces relies on the deformation of Ag. Ag, with proper annealing, is ductile and has low yield strength. Its surface can thus deform to conform to and to follow the surface of Au layer coated on Si chip. Its ductile property can also help accommodate large mismatch in coefficient of thermal expansion between Si (2.7×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> °C <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ) and Cu (17×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> °C <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ). Ag also has the highest electrical and thermal conductivities among all metals. It is the dream bonding medium. The challenge is to bond Ag to other materials at a temperature compatible to electronic packaging processes, 260°C. In this research, Si chips are coated with thin chromium (Cr) and Au layers and then bonded to Ag buffer plated on Cu substrates. To make Ag buffer ductile, Ag buffer together with Cu substrate is annealed to increase its grain size by a factor of 25. The bonding was performed at 260°C with 1000 psi (6.89 MPa) static pressure. There is no molten phase involved in the bonding process. Scanning electron microscope evaluations show that on the revealed cross section no voids or gaps are observed on Si/Ag and Ag/Cu bonding interfaces. The bonded structure consists of only Si/Cr/Au/Ag/Cu and can sustain at high operating temperature due to high melting temperature of Ag.

Welding of gold nanowires with different joining procedures

Welding of single-crystal metallic nanowires is likely to have an important role in the bottom-up fabrication of nanodevices. The welding effects of free ends of two single-crystal gold nanowires (Au–Au) were demonstrated by Monte Carlo simulations in this paper. The quantum corrected Sutton–Chen type many-body potential was used to model the metal–metal interactions. Metallic nanowires were first placed closely with head-to-head, head-to-side and side-to-side joining procedures. Two ends were successfully welded together to form a continuous nanowire by annealing at different temperatures. The welding effects of the different joining procedure with different temperatures were compared. Structures of the welded specimens were characterized by the common neighbor analysis technique. Variable atomic mobility, freedom and contacting angles may result in different bonding strengths in the three different configurations. The results showed that the joint structure welded at low temperature were similar to the cold welding without fusion meanwhile the molten phase was presented in the joint when processing at the high temperatures.

Alkylated organic cages: from porous crystals to neat liquids

Rigid organic iminospherand cages are rendered meltable by multiple alkylation; below their melting points they can take the form of permanently porous crystals, crystals unstable to desolvation or non-porous glassy solids depending on chain length and branching; melting points as low as 50 °C are observed and a fully Newtonian liquid phase is obtained above 80 °C. Thin glassy fibres can be drawn out from a molten phase.

Nanometric transformation of the matter by short and intense electronic excitation: Experimental data versus inelastic thermal spike model

Experimental investigations of ion tracks and sputtering phenomena with energetic heavy projectiles in the electronic energy loss regime are re-examined in metallic and insulating materials. An overview of track data such as the velocity dependence of the track size and the critical electronic energy loss for track formation is presented. Different physical characterizations of the material transformation are listed in order to deduce a track size which is independent of the observations. It will point out the differences of damage creation by electronic energy loss compared to nuclear energy loss. In the second part, we present a theoretical description of track formation based on the inelastic thermal spike model. This thermodynamic approach combines the initial size of the energy deposition with the subsequent diffusion process in the electronic and lattice subsystems of the target. The track size, resulting from the quench of a molten phase, is determined by the energy density deposited on the atoms around the ion path governed by the electron–phonon strength. Finally, we discuss the general validity of this model in metallic materials and its suitability to describe track formation in amorphizable and non-amorphizable insulators.

Low-Temperature Solid-State Silver Bonding of Silicon Chips to Alumina Substrates

Pure silver (Ag) is used as a bonding medium to bond silicon (Si) chips to alumina substrates. The bonding process is performed at 260°C, which is a typical peak reflow temperature of Sn3.5Ag solders used in electronic industries. A static pressure of 1000 psi (6.9 MPa) is applied. This is a solid-state bonding process without any molten phase involved. The Ag foil sandwiched between the Si chip and the alumina substrate is ductile enough to deform and mate with the gold (Au) layers coated, respectively, on the Si chip and on the alumina substrate. Ag and Au atoms on both sides of the bonding interfaces are brought within atomic distance, and bonding is thus achieved. The ductile Ag joints can accommodate significant mismatch in coefficient of thermal expansion between Si and alumina. Scanning electron microscopy evaluations show that nearly perfect joints are achieved and no voids are observed. A standard shear test is performed to assess the bonding strength. The shear strength measured far exceeds the requirement specified in MIL-STD-883G standard.

Ignition of an exothermal reaction by collision between Al and Ni crystals

Classical molecular dynamics methods have been used to investigate the atomic-scale dynamics of collisions between two Al and Ni crystals with rough surfaces. The crystals were approached along the direction perpendicular to the surfaces and simultaneously displaced along the direction parallel to them at relative velocities in the range between 1 and 10 nm ns−1. The mechanical stresses operating at collision determine a local deformation of Al and Ni lattices, accompanied by a significant temperature rise. As the Al melting point is reached, the Al crystal partially melts and Ni atoms start dissolving into the molten phase. The significant heat of mixing liberated further promotes the Al melting and the Ni dissolution processes. In the absence of neighboring Al-Ni interfaces, the heat dissipation processes and the limited rate of Ni dissolution gradually lead to the extinction of the reactive behavior. Conversely, the presence of Al-Ni interfaces in the vicinity of the Al-Ni one formed by collision permits the propagation of the high-temperature chemical reaction. It is shown that the ignition and propagation of the self-sustaining reaction is sensitive to the distance between Al-Ni interfaces and to their degree of chemical mixing.

40 μm Silver Flip-Chip Interconnect Technology With Solid-State Bonding

Formation of pure silver (Ag) flip-chip interconnect of silicon (Si) chips on copper (Cu) substrates is reported. Arrays of Ag columns, each 36 μm in height and 40 μm in diameter, are fabricated on 2-in. Si wafers which are first coated with chromium (Cr)/gold (Au) dual layers. The Si wafers are diced into 6 mm × 6 mm chips, each having 50 × 50 Ag columns. The Si chip with Ag columns is directly bonded to Cu substrate at 260 °C in 80 mTorr vacuum to inhibit oxidation. The static bonding pressure is as low as 680 psi (4.69 MPa), corresponding to a load of 0.021 oz (0.60 g) per column. During bonding, the Ag columns deform and conform to the Cu substrate. They are well bonded to the Cu. No molten phase is involved in the bonding process. The joints consist of pure Ag only. The ductile Ag joints are able to accommodate the thermal expansion mismatch between Si and Cu. It is well known that in nearly all soldering processes used in electronic industries, intermetallic compound (IMC) formation is essential to make a solder joint. In the pure Ag interconnect, no IMCs exist. Thus, reliability issues associated with IMCs are eliminated. Compared to tin-based lead-free solders, pure Ag joints have superior electrical and thermal properties.

Low Temperature Solid State Gold Bonding of Si Chips to Alumina Substrates

Pure gold (Au) is used as a bonding medium to bond silicon (Si) chips to alumina substrates. The bonding process is performed at 260 °C with only 150 psi (1.0 MPa) static pressure applied. This is a solid-state bonding without any molten phase involved. The Au layer plated on alumina is ductile enough to deform for its surface to mate with the thin Au layer coated on Si. Au atoms on both sides of the bond line are brought within atomic distance and bonding is achieved. The ductile Au joint also accommodates the significant mismatch in coefficient of thermal expansion (CTE) between Si and alumina. Scanning electron microscope (SEM) evaluations show that nearly perfect joints are achieved and no voids are observed. Five samples are shear tested. They all pass the MIL-STD-883G standard. This bonding technique can be applied to bonding any two objects that can be coated with smooth Au layers. The 260 °C bonding temperature is compatible with typical reflow temperature of Sn3.5Ag solders used in electronic industries.

Bed agglomeration characteristics of palm shell and corncob combustion in fluidized bed

Abstract Bed particle agglomeration was studied experimentally in an atmospheric laboratory scale fluidized bed combustor using quartz sand as bed material. Palm shell and corncob were tested. The objectives of the study were (i) to describe the contributions of the biomass ash properties and the operating conditions on the bed agglomeration tendency in term of the bed defluidization time (tdef) and the extent of potassium accumulation in the bed (K/Bed) and (ii) to further elucidate the ash inorganic behaviors and the governing bed agglomeration mechanisms. Defluidization caused by the bed agglomeration was experienced in all experiments during combustion of these biomasses, as a consequence of the presence of potassium in biomass. The experimental results indicated that biomass ash characteristics were the significant influence on the bed agglomeration. The increasing bed temperature, bed particle size and static bed height and the decreasing fluidizing air velocity enhanced the bed agglomeration tendency. The SEM/EDS analyses on the agglomerates confirmed that the agglomeration was attributed to the formation of potassium silicate liquid enriched on the surface of quartz sand particles in conjunction with the high surface temperature of the burning biomass char particles. Thermodynamic examination based on the phase diagram analysis confirmed that the molten phase formation was responsible for the agglomeration. In this study, the high molten ash fraction resulting from the high potassium content in biomass promoted the agglomeration and thus defluidization.

Improvement of high temperature oxidation resistance of AISI 316L stainless steel by incorporation of Ce–La elements using intense pulsed plasma beams

It is well documented that adding some active elements (such as Y, Ce, La, Er, others) to austenitic stainless steels helps to improve their high temperature oxidation resistance. In the present work we report preliminary results of incorporation of Ce and La into austenitic stainless steel AISI 316L using a new approach based on high intensity pulsed plasma beams. The characteristic feature of this approach relies upon the fact that incorporation of Ce–La occurs in the transiently melted near-surface layer of the substrate. Improving the near surface layer properties was connected with two facts: (i) material melting and cooling with very high rate and (ii) incorporation Ce–La to the molten phase. Modified samples were oxidized at 1000 °C for 80 h in air at atmospheric pressure. The obtained effects were similar to those observed in chromia-forming alloys doped by other conventional surface treatment techniques: oxide scales formed on the treated samples were more fine-grained, compact and adhered better than those formed on the un-treated samples, whereas the protective layer was produced closer to the surface.

Y thin films grown by pulsed laser ablation

The effects of laser fluence on the growth characteristics and surface morphology of yttrium films grown by pulsed laser deposition are investigated. The presence of droplets in the deposited films, which is the main drawback of pulsed laser deposition technique, was studied at different laser fluences. The morphology and the structure of the grown films were studied by scanning-electron microscopy and x-ray diffraction, respectively. Careful scanning-electron microscope investigations obtained by tilting the samples show that the droplets arrive to the substrate in the molten phase. The ablation rate measured at five different laser fluences (0.9–7.6 J/cm2) shows a nonlinear trend correlated with the presence of the plasma-shielding effect. The present interest in the deposition of yttrium thin films by laser ablation is due to the well-known photoemission characteristics of this metal. Depositing good-quality thin films with high adhension and low droplet density will improve the performance of photocathodes by reducing both the dark current and the thermal emittance of the photoemitted current.

Growth of graphene and graphite nanocrystals from a molten phase

We report on the development of a new technique for the growth of graphene and graphite nanocrystals from a metal–carbon melt. The process involves dissolving carbon inside molten nickel or copper at a specified temperature and then allowing the dissolved carbon to nucleate and grow on top of the melt at a lower temperature. The morphology of graphite forming inside nickel is shown to depend on the melt composition and cooling rate. The results indicate that the graphene layers contain a wrinkled structure due to thermal expansion coefficient mismatch between the grown layers and substrate. The graphene layers, however, preserve their continuity over these wrinkles. Although the formation of wrinkles was found not to depend on the type of metal substrate, the substrate had strong effect on the quality of grown layers. The films on copper contained quite a few defects including cracks and entrapped vacancies, whereas the films grown on nickel were pristine and defects free.

The effect of Cr on the properties and sintering of W skeleton as an activated element

Tungsten skeletons were fabricated by the activated sintering technique at 1050 °C, 1150 °C and 1300 °C. Cr and Cu elements with different mass ratios were selected to be the sintering activators. The interfaces of sintered W skeletons with Cr additives were characterized by EPMA attached to SEM. The compressive properties of sintered W skeleton were tested, and fracture morphologies of the porous W skeletons were examined by SEM. The results show that the addition of element Cr resulted in a W–Cr solid solution transition layer at the W interparticle grain boundaries. When the sintering temperature being above the melting point of copper, the molten phase acted as carrier and promoted the formation of sintering necks of tungsten skeleton, and the distribution of pores became more uniform with increasing the mass ratio of Cu to Cr. The compressive strength of W skeleton sintered at 1300 °C was enhanced to 283 MPa when the mass ratio of Cu to Cr was 60:1.

Synthesis of Platelet Carbon Nanofiber/Carbon Felt Composite on in Situ Generated Ni−Cu Nanoparticles

A platelet carbon nanofiber/carbon felt composite with a high surface area (>250 m2/g) is synthesized over a carbon felt supported Ni−Cu catalyst using ethane−hydrogen mixtures at 923 K. A uniform layer of carbon nanofibers is thus achieved via an in situ generation and dispersion of Ni−Cu nanoparticles induced by the surface reaction of hydrocarbon decomposition. This represents a cost-effective and flexible method without the requirement for presynthesis of uniformly distributed nanoparticles. Both thermodynamic analysis and experimental observation reveal that, in the initial growth period, formation of a molten phase of the nanosized Ni−Cu−C phase promotes the fragmentation of Ni−Cu parent particles of micrometer size. A unique octopus-like growth mode is observed during the formation of graphene layers from this liquid-like Ni−Cu−C system. A comparative study between Ni and Ni−Cu catalysts is performed. It is found that the addition of Cu in the Ni−Cu catalysts can enhance the fragmentation ability of the parent Ni−Cu particles when contacting with hydrocarbon. A treelike growth mode of carbon nanofibers grown on Ni−Cu catalysts is observed due to the further fragmentation of Ni−Cu particles during carbon nanofiber growth. In contrast to Ni−Cu catalysts, two different growth mechanisms on the Ni catalyst are found: a governing tip-growth mechanism by particles smaller than 50 nm and an octopus-like growth mechanism by larger particles (50−100 nm). The graphene sheet orientation in the carbon nanofibers depends on the composition of the metal particle; fishbone carbon nanofibers are obtained with carbon felt supported Ni catalysts, while platelet carbon nanofibers were obtained with Ni−Cu catalysts. The formation of platelet carbon nanofibers from the Ni−Cu catalysts is ascribed to a higher degree of interface wetting between the Ni−Cu particle and the produced graphene sheets.

ChemInform Abstract: One‐ and Two‐Dimensional Inorganic Crystals Inside Inorganic Nanotubes

AbstractReview: 74 refs.

Growth of large-area graphene films from metal-carbon melts

We have demonstrated a new method for the large-area graphene growth, which can lead to a scalable low-cost high-throughput production technology. The method is based on growing single layer or few-layer graphene films from a molten phase. The process involves dissolving carbon inside a molten metal at a specified temperature and then allowing the dissolved carbon to nucleate and grow on top of the melt at a lower temperature. The examined metals for the metal-carbon melt included copper and nickel. For the latter, the high-quality single layer graphene was grown successfully. The resulting graphene layers were subjected to detailed microscopic and Raman spectroscopic characterization. The deconvolution of the Raman 2D band was used to accurately determine the number of atomic planes in the resulting graphene layers and access their quality. The results indicate that our technology can provide bulk graphite films, few-layer graphene as well as high-quality single layer graphene on metals. Our approach can also be used for producing graphene-metal thermal interface materials for thermal management applications.

Superheater fireside corrosion mechanisms in MSWI plants: Lab‐scale study and on‐site results

AbstractCombustion of municipal waste generates highly corrosive gases (HCl, SO2, NaCl, KCl, and heavy metals chlorides) and ashes containing alkaline chlorides and sulfates. Currently, corrosion phenomena are particularly observed on superheater's tubes. Corrosion rates depend mainly on installation design, operating conditions i.e., gas and steam temperature and velocity of the flue gas containing ashes. This paper presents the results obtained using an innovative laboratory‐scale corrosion unit, which simulates MSWI boilers conditions characterized by a temperature gradient at the metal tube in the presence of corrosive gases and ashes. The presented corrosion tests were realized on carbon steel at fixed metal temperature (400 °C). The influence of the flue gas temperature, synthetic ashes composition, and flue gas flow pattern were investigated. After corrosion test, cross sections of tube samples were characterized to evaluate thickness loss and estimate corrosion rate while the elements present in corrosion layers were analyzed.Corrosion tests were carried out twice in order to validate the accuracy and reproducibility of results. First results highlight the key role of molten phase related to the ash composition and flue gas temperature as well as the deposit morphology, related to the flue gas flow pattern, on the mechanisms and corrosion rates.

Effects of Simulated Particle Deposition on Film Cooling

Diminishing natural gas resources has increased incentive to develop cleaner, more efficient combined-cycle power plants capable of burning alternative fuels such as coal-derived synthesis gas (syngas). Although syngas is typically filtered, particulate matter still exists in the hot gas path that has proven to be detrimental to the life of turbine components. Solid and molten particles deposit on film-cooled surfaces that can alter cooling dynamics and block cooling holes. To gain an understanding of the effects that particle deposits have on film cooling, a methodology was developed to simulate deposition in a low speed wind tunnel using a low melt wax, which can simulate solid and molten phases. A facility was constructed to simulate particle deposition on a flat plate with a row of film cooling holes. Infrared thermography was used to measure wall temperatures for quantifying spatially resolved adiabatic effectiveness values in the vicinity of the film cooling holes as deposition occurred. Results showed that deposition reduced cooling effectiveness by approximately 20% at momentum flux ratios of 0.23 and 0.5 and only 6% at a momentum flux ratio of 0.95.