High Sublimation Temperature Research Articles

Modeling snowline locations in protostars: The impact of the structure of protostellar cloud cores

Context. Snowlines during star and disk formation are responsible for a range of effects during the evolution of protostars, such as setting the chemical composition of the envelope and disk. This in turn influences the formation of planets by changing the elemental compositions of solids and affecting the collisional properties and outcomes of dust grains. Snowlines can also reveal echoes of past accretion bursts, providing insight into the formation process of stars. Aims. The objective is to identify which parameters (e.g., luminosity, gas density, and presence of disk) dictate the location of snowlines during the early, deeply embedded phase and to quantify how each parameter changes the observed snowline location. Methods. A numerical chemical network coupled with a grid of cylindrical-symmetric physical models was used to identify what physical parameters alter the CO and H2O snowline locations. The investigated parameters are the initial molecular abundances, binding energies of CO and H2O, heating source, cloud core density, outflow cavity opening angle, and disk geometry. Simulated molecular line emission maps were used to quantify the change in the snowline location with each parameter. Results. The snowline radius of molecules with low sublimation temperatures (≲30 K), such as CO, shift outward on the order of 103 AU with an order of magnitude increase in protostellar luminosity. An order of magnitude decrease in cloud core density also shifts the CO snowline position outward by a few 103 AU. The presence of disk(-like) structures cause inward shifts by a factor of a few, and mainly along the disk mid-plane. For molecules that sublimate at higher temperatures, such as H2O, increasing the protostellar luminosity or decreasing the cloud core density by an order of magnitude shifts the snowline position outward by a factor of a few. The presence of a disk concentrates molecules with high sublimation temperatures to compact regions (a few 10 AU) around the protostar by limiting the outward shift of snowline positions. Successful observational measurements of snowline locations are strongly dependent on spatial resolution, the presence or lack thereof of disk(-like) structures, and the inclination of the disk(-like) structure. Conclusions. The CO and H2O snowline locations do not occur at a single, well-defined temperature as is commonly assumed. Instead, the snowline position depends on luminosity, cloud core density, and whether a disk is present or not. Inclination and spatial resolution affect the observability and successful measurement of snowline locations. We note that N2H+ and HCO+ emission serve as good observational tracers of CO and H2O snowline locations. However, constraints on whether or not a disk is present, the observation of additional molecular tracers, and estimating envelope density will help in accurately determining the cause of the observed snowline position. Plots of the N2H+ and HCO+ peak emission radius versus luminosity are provided to compare the models with observations of deeply embedded protostars aiming to measure the CO and H2O snowline locations.

Experimental Investigation of Technological Indicators and Surface Roughness of Hastelloy C-22 after Electrical Discharge Machining Using POCO Graphite Electrodes.

Modern industry is focused on looking for new and effective technologies to manufacture complex shapes from alloys based on nickel and chromium. One of the materials widely used in the chemical and aerospace industry is Hastelloy C-22. This material is difficult to machine by conventional methods, and in many cases, unconventional methods are used to manufacture it, such as electrical discharge machining (EDM). In the EDM process, the material is removed by electrical discharges between a workpiece and a tool electrode. The physical and mechanical properties of the tool electrodes have a direct impact on the process efficiency, machining accuracy, and surface roughness. Currently, there has been a significant increase in the use of graphite as a material for tool electrodes due to the low purchase cost of the raw material, good machinability, and high sublimation temperature. In this work, an experimental investigation of the influence of the grain size of the graphite tool electrode on material removal rate (MRR), tool wear rate (TWR), and surface roughness (Ra) of Hastelloy C-22 was carried out. Two POCO graphite tool electrodes with a grain size of 1 µm (AF-5) and 10 µm (S-180) were used. Based on the experimental studies, empirical models describing the influence of machining parameters on technological indicators and the condition of the surface texture were determined. The research indicates that graphite with a larger grain provides higher process efficiency with high relative wear of the tool electrode. The lowest surface roughness was obtained for graphite with a smaller grain size (AF-5). The analysis of the machining parameters proves that the discharge current and pulse duration are the main factors determining the MRR and Ra values for both AF-5 and S-180 graphite. The time interval is the dominant parameter with regard to the relative wear of the graphite electrode.

Evidence of ammonium salts in comet 67P as explanation for the nitrogen depletion in cometary comae

Cometary comae are generally depleted in nitrogen. The main carriers for volatile nitrogen in comets are NH3 and HCN. It is known that ammonia readily combines with many acids like e.g. HCN, HNCO, HCOOH, etc. encountered in the interstellar medium as well as in cometary ice to form ammonium salts (NH4+X-) at low temperatures. Ammonium salts, which can play a significant role in prebiotic chemistry, are hard to detect in space as they are unstable in the gas phase and their infrared signature is often hidden by thermal radiation or by e.g. OH in minerals. Here we report the presence of all possible sublimation products of five different ammonium salts at comet 67P/Churyumov-Gerasimenko measured by the ROSINA instrument on Rosetta. The relatively high sublimation temperatures of the salts leads to an apparent lack of volatile nitrogen in the coma. This then also explains the observed trend of higher NH3/H2O ratios with decreasing perihelion distances in comets.

Fabrication of free-standing vertically aligned carbon nanotubes structures by pyrolysis of ferrocene in conditions of low to high local ‘carbon-to metal’ ratio

We report the observation of selective fabrication of carbon nano-onions (CNOs) and carbon nanotubes (CNTs) films in an advanced chemical vapour deposition (CVD) approach. Selective formation of these nanostructures is triggered by a residence-time induced variation of carbon to metal ratios in the atomic species of the pyrolyzed ferrocene vapour. The use of high sublimation temperatures is found to play a crucial role in reducing such residence time. Due to this effect, free standing vertically aligned CNTs films could be nucleated in the outer zone of the CVD reactor. The characteristics of these structures were investigated using multiple characterization techniques, namely: TEM, SEM, XRD, UV–vis and Raman spectroscopy.

Plasma corrosion resistance of aluminosilicate glasses containing Ca, Y and B under fluorocarbon plasma with Ar+

In this study, the corrosion resistance of alumino-silicate glass containing Ca, Y and B is assessed against CF4/O2/Ar plasma, with the results compared to those of poly-crystalline and single-crystalline alumina. All of the alumino-silicate glasses containing Ca, Y and B were more resistant than sintered alumina to a plasma-etching gas. Glass samples with boron oxide were less corroded by plasma than those without B2O3, and their surfaces were contaminated by the CFx generated by the plasma reaction.CaO-Y2O3-Al2O3-SiO2 (CYAS) showed a lower etch rate than sapphire and retained a smooth surface microstructure, unlike sintered alumina. The plasma resistance improved with an increase in the calcium oxide content. The improved plasma resistance of CYAS glass is thought to be due to the high sublimation temperature of the calcium fluoride and the strong atomic binding energy of the calcium ions in the glass.

Plasma corrosion resistance of RO-Al2O3-SiO2 (R: Alkaline earth) under fluorocarbon plasma with Ar+: Ⅱ. Plasma resistant glass

High-density plasma dry etching was performed with a mixture of CF4/O2/Ar gases, and the plasma resistant properties of RO-Al2O3-SiO2 (R: alkaline earth) glass were analyzed. RAS had etch rates below 25㎚/min, which are lower than that of alumina and sapphire. CAS showed an improvement in the etch rate, which corresponds to 72% of that recorded by sapphire. The plasma resistant properties of RAS glass are more significantly influenced by the sublimation temperature with fluoride than atomic binding energy. The difference in the etch rates of the RAS glass was smaller at higher sublimation temperatures.

Carbon nanotubes or carbon globules: Optimization of the pyrolytic synthesis parameters and study of the magnetic properties

We report the optimized parameters necessary for the synthesis of carbon nanotubes (CNTs) and carbon globules during the pyrolysis of benzene in a closed reactor. The precursor concentration and sublimation temperature were controlled to tune the growth of carbon nanostructures. A low sublimation temperature leads to the formation of globular carbon aggregates while a high sublimation temperature (∼ 600 °C) favours the growth of carbon nanotubes. A set of optimized parameters allows the growth of CNTs or carbon globules, selectively or simultaneously. The carbon globules and the CNTs contain embedded magnetic nanoparticles, which makes them suitable electronic and magnetic material for various applications.

Modification of adhered dust on plasma-facing surfaces due to exposure to ELMy H-mode plasma in DIII-D

Transient heat load tests have been conducted in the lower divertor of DIII-D using DiMES manipulator in order to study the behavior of dust on tungsten Plasma Facing Components (PFCs) during ELMy H-mode discharges. Samples with pre-adhered, pre-characterized dust have been exposed at the outer strike point (OSP) in a series of discharges with varied intra-(inter-) ELM heat fluxes. We used C dust because of its high sublimation temperature and non-metal properties. Al dust as a surrogate for Be and W dust were employed as relevant to that in the ITER divertor. The poor initial thermal contact between the substrate and the particles led to overheating, sublimation and shrinking of the carbon dust, and wetting induced coagulation of Al dust. Little modification of the W dust was observed. An enhanced surface adhesion and improvement of the thermal contact of C and Al dust were the result of exposure. A post mortem “adhesive tape” sampling showed that 70% of Al, <5% of W and C particles could not be removed from the surface owing to the improved adhesion. Al and C but not W particles that could be lifted had W inclusions indicating damage to the substrate. This suggests that non destructive methods may be inefficient for removal of dust in ITER.

Adsorption, desorption, and film formation of quinacridone and its thermal cracking product indigo on clean and carbon-covered silicon dioxide surfaces.

The evaporation of quinacridone from a stainless steel Knudsen cell leads to the partial decomposition of this molecule in the cell, due to its comparably high sublimation temperature. At least one additional type of molecules, namely indigo, could be detected in the effusion flux. Thermal desorption spectroscopy and atomic force microscopy have been used to study the co-deposition of these molecules on sputter-cleaned and carbon-covered silicon dioxide surfaces. Desorption of indigo appears at temperatures of about 400 K, while quinacridone desorbs at around 510 K. For quinacridone, a desorption energy of 2.1 eV and a frequency factor for desorption of 1 × 10(19) s(-1) were calculated, which in this magnitude is typical for large organic molecules. A fraction of the adsorbed quinacridone molecules (∼5%) decomposes during heating, nearly independent of the adsorbed amount, resulting in a surface composed of small carbon islands. The sticking coefficients of indigo and quinacridone were found to be close to unity on a carbon covered SiO2 surface but significantly smaller on a sputter-cleaned substrate. The reason for the latter can be attributed to insufficient energy dissipation for unfavorably oriented impinging molecules. However, due to adsorption via a hot-precursor state, the sticking probability is increased on the surface covered with carbon islands, which act as accommodation centers.

Inventory of the volatiles on comet 67P/Churyumov-Gerasimenko from Rosetta/ROSINA

The ESA Rosetta spacecraft (S/C) is tracking comet 67P/Churyumov-Gerasimenko in close vicinity. This prolonged en- counter enables studying the evolution of the volatile coma composition. Aims. Our work aims at comparing the diversity of the coma of 67P/Churyumov-Gerasimenko at large heliocentric distance to study the evolution of the comet during its passage around the Sun and at trying to classify it relative to other comets. Methods. We used the Double Focussing Mass Spectrometer (DFMS) of the ROSINA experiment on ESA’s Rosetta mission to determine relative abundances of major and minor volatile species. This study is restricted to species that have previously been detected elsewhere. Results. We detect almost all species currently known to be present in cometary coma with ROSINA DFMS. As DFMS measured the composition locally, we cannot derive a global abundance, but we compare measurements from the summer and the winter hemisphere with known abundances from other comets. Differences between relative abundances between summer and winter hemispheres are large, which points to a possible evolution of the cometary surface. This comet appears to be very rich in CO2 and ethane. Heavy oxygenated compounds such as ethylene glycol are underabundant at 3 AU, probably due to their high sublimation temperatures, but nevertheless, their presence proves that Kuiper belt comets also contain complex organic molecules.

Idiosyncrasies of Physical Vapor Deposition Processes from Various Knudsen Cells for Quinacridone Thin Film Growth on Silicon Dioxide.

Thin films of quinacridone deposited by physical vapor deposition on silicon dioxide were investigated by thermal desorption spectroscopy (TDS), mass spectrometry (MS), atomic force microscopy (AFM), specular and grazing incidence X-ray diffraction (XRD, GIXD), and Raman spectroscopy. Using a stainless steel Knudsen cell did not allow the preparation of a pure quinacridone film. TDS and MS unambiguously showed that in addition to quinacridone, desorbing at about 500 K (γ-peak), significant amounts of indigo desorbed at about 420 K (β-peak). The existence of these two species on the surface was verified by XRD, GIXD, and Raman spectroscopy. The latter spectroscopies revealed that additional species are contained in the films, not detected by TDS. In the film mainly composed of indigo a species was identified which we tentatively attribute to carbazole. The film consisting of mainly quinacridone contained in addition p-sexiphenyl. The reason for the various decomposition species effusing from the metal Knudsen cell is the comparably high sublimation temperature of the hydrogen bonded quinacridone. With special experimental methods and by using glass Knudsen-type cells we were able to prepare films which exclusively consist of molecules either corresponding to the β-peak or the γ-peak. These findings are of relevance for choosing the proper deposition techniques in the preparation of quinacridone films in the context of organic electronic devices.

Preparation of NiCl2 Nanorods with Enhanced Electrochemical Properties in Thermal Batteries

Modern sophisticated weapon systems such as missiles, rockets and space aircrafts require thermal batteries of high energy and high power density, which makes NiCl2 a choice to replace disulfide as cathode material due to its high open circuit voltage and large discharge current density. Compared with bulk materials, nanomaterials are known to show superior electrochemical performance (1, 2) when they are pressed or sprayed onto the current collector acting as the electrodes. Ming (3) has synthesized nanostructured FeS2 power as the cathode material for Li-Si/FeS2 thermal batteries by a high-energy ball milling process. The results show that the nanostructured cathode pellets are 23% thinner than the micronscale counterparts leading to a 31% increase of pellet density, and the energy density of the nanostructured thermal batteries can reach 109 J g-1, two times higher than the conventional bulk materials (58 J g-1). Furthermore, nanorods such as NiO (4) have been successfully synthesized for other electronic applications exhibiting excellent electrochemical properties. However, nano-NiCl2 materials have rarely been reported due to the difficult preparing technology and relatively high sublimation temperature. Besides, anhydrous NiCl2 powder is sensitive to moisture, which makes it agglomerate easily. It is noted that the electrochemical performances of the cathode materials for thermal batteries are strongly dependent on their sizes, morphologies and structures (5). Therefore, efforts must be devoted to synthesizing novel structure of NiCl2to improve the discharge properties and meet various requirements in possible application fields. In our present study, highly uniform NiCl2 nanorods (see the inset in Figure 1(a)) with the diameter of approximately 100 nm and the length of 700 nm have been synthesized via a recrystallization method at ambient temperature and subsequent calcination at 300 oC for 4 h. In a typical procedure, crystalline NiCl2·6H2O was employed as the raw material, anhydrous ethanol as the solvent and acetone as anti-solvent. Compared with JCPDS card (No. 01-1134, space group: R-3m (166)), the characteristic peaks in the X-ray diffraction (XRD) pattern, as shown in Figure 1(a), can be assigned to (003), (101), (104), (009), (018), (113) and (205) planes, respectively. No other peaks can be found, indicating that the as-synthesized NiCl2nanorods are of high purity with a rhombohedral structure (see Figure 1(b)). To evaluate the electrochemical performance of as-prepared NiCl2 nanorods, the cathode pellets were fabricated by compressing a mixture of 85 wt% NiCl2 nanorods and 15 wt% conductive carbon. The single-cell thermal batteries in sandwiched-structure were assembled in an Ar-filled glove-box, and then the discharge tests were carried out. For comparison, bulk NiCl2 materials were abtained by direct calcination of raw NiCl2·6H2O for the following measurements. Figure 2 shows that the battery with NiCl2 nanorods cathode can maintain 338 s at the current density of 500 mA g-1 until the voltage falls to 0.8 V delivering a capacity of 47 mAh g-1 and an energy density of 184 J g-1, while the battery of bulk NiCl2 cathode can only last for 265 s with lower voltage under the same conditons. With the current density incresing to 1000 mA g-1, the discharge time for the two cathodes are 150 s and 118 s, respectively. The reason of the voltage rise at the beginning of discharge has been investigating.

Optimization of Freeze-Drying Cycle for Tert-Butanol–Based Formulations of Ibuprofen

We carried out the formulation set-up and thermodynamics as well as morphological characterization of two systems: an aqueous-based formulation and an organic-solvent-based formulation with tert-butanol (TBA) at eutectic composition (20% TBA + water 80% by mass) containing ibuprofen as the model pharmaceutical active component. The goal was to carry out the optimization of a soft freeze-drying cycle for heat-sensitive drugs. The organic-based ibuprofen formulation had a glass transition temperature significantly higher compared to that of the aqueous-based ibuprofen formulation, which allowed much higher sublimation temperatures for these systems in comparison with the pure water-based ibuprofen formulations. The morphological study of the frozen organic-based formulation showed unexpected behavior in comparison with previous literature data reported for water-based formulations. For the same freezing rates, the mean diameters of the solvent crystals of organic-based formulations were much larger than that of ice crystals of aqueous-based formulation, which led to a freeze-dried matrix of higher permeability. In contrast to what was observed with the aqueous-based formulations, the freezing rates, in the range of classical industrial values investigated, had no significant effect on the supercooling of the liquid ibuprofen formulations tested as well as on the nucleation temperatures and on the morphology of the organic solvent crystals. All these singular characteristics, with lower sublimation enthalpies and higher equilibrium vapor pressure values previously measured with the same formulations in our laboratory, explain the large reduction of sublimation times observed during the soft freeze-drying processes for heat-sensitive drugs with organic (TBA)-based formulations.

Metal–Sulfur Valence Orbital Interaction Energies in Metal–Dithiolene Complexes: Determination of Charge and Overlap Interaction Energies by Comparison of Core and Valence Ionization Energy Shifts

The electronic interactions between metals and dithiolenes are important in the biological processes of many metalloenzymes as well as in diverse chemical and material applications. Of special note is the ability of the dithiolene ligand to support metal centers in multiple coordination environments and oxidation states. To better understand the nature of metal-dithiolene electronic interactions, new capabilities in gas-phase core photoelectron spectroscopy for molecules with high sublimation temperatures have been developed and applied to a series of molecules of the type Cp(2)M(bdt) (Cp = η(5)-cyclopentadienyl, M = Ti, V, Mo, and bdt = benzenedithiolato). Comparison of the gas-phase core and valence ionization energy shifts provides a unique quantitative energy measure of valence orbital overlap interactions between the metal and the sulfur orbitals that is separated from the effects of charge redistribution. The results explain the large amount of sulfur character in the redox-active orbitals and the 'leveling' of oxidation state energies in metal-dithiolene systems. The experimentally determined orbital interaction energies reveal a previously unidentified overlap interaction of the predominantly sulfur HOMO of the bdt ligand with filled π orbitals of the Cp ligands, suggesting that direct dithiolene interactions with other ligands bound to the metal could be significant for other metal-dithiolene systems in chemistry and biology.

Sublimation temperature of circumstellar dust particles and its importance for dust ring formation

Dust particles in orbit around a star drift toward the central star by the Poynting-Robertson effect and pile up by sublimation. We analytically derive the pile-up magnitude, adopting a simple model for optical cross sections. As a result, we find that the sublimation temperature of drifting dust particles plays the most important role in the pile-up rather than their optical property does. Dust particles with high sublimation temperature form a significant dust ring, which could be found in the vicinity of the sun through in-situ spacecraft measurements. While the existence of such a ring in a debris disk could not be identified in the spectral energy distribution (SED), the size of a dust-free zone shapes the SED. Since we analytically obtain the location and temperature of sublimation, these analytical formulae are useful to find such sublimation evidences.

New set-up for high-quality soft-X-ray absorption spectroscopy of large organic molecules in the gas phase

We present a new experimental set-up for the investigation of large (>128 amu) organic molecules in the gas-phase by means of near-edge X-ray absorption fine structure spectroscopy in the soft X-ray range. Our approach uses a gas cell, which is sealed off against the surrounding vacuum and which can be heated above the sublimation temperature of the respective molecular compound. Using a confined volume rather than a molecular beam yields short acquisition times and intense signals due to the high molecular density, which can be tuned by the container temperature. In turn, the resulting spectra are of very high quality with respect to signal-to-noise ratio and energy resolution, which are the essential aspects for the analysis of fine spectroscopic details. Using the examples of ANQ, NTCDA, and PTCDA, specific challenges of gas phase measurements on large organic molecules with high sublimation temperatures are addressed in detail with respect to the presented set-up and possible ways to tackle them are outlined.

Thin films of porphyrin-perylene molecular array fabricated by electrophoresis methodology

Thin solid films of organic conjugated molecules are at the center of organic electronics. Low solubility and high sublimation temperature of porphyrin-perylene arrays make it impossible to fabricate uniform solid films with spin-coating and vacuum deposition methodology, though these arrays have important applications in the area of opto-electronics. Here we show that high quality thin films of a porphyrin-perylene array can be prepared by electro-chemical deposition, a facile and widely used film-forming technique. The electrophoretic species are protonated porphyrin-perylene molecules, which allow us to grow molecular array films on electrodes. By annealing in ammonia atmosphere or in vacuum at elevated temperature, the protons coordinated with molecular arrays on the deposited films can be eliminated and the porphyrin-perylene arrays recovered to their pristine state.

Etching of sub-micron high aspect ratio holes in oxides and polymers

Sub-micron holes have been successfully etched into: PMMA/DR-1 on Teflon, BCB, Ta2O5, Nb2O5 and SiO2, with the aim of forming photonic crystal structures. Pattern transfer from the NiCr masks into the material is achieved in an electron cyclotron resonance high-density plasma system, using a combination of the gases: CHF3, CF4, O2, Ar and SF6, and an RF bias on the sample. The etching of Ta2O5 and Nb2O5 is hindered by the high sublimation temperature, approximately 230 °C, of TaF5 and NbF5. This problem was resolved by heating the sample. For these materials aspect ratios of 5 were achieved, with holes of down to 200-nm wide being etched. Wall roughness was typically 30 nm, peak to trough. For SiO2, aspect ratios of 8 were achieved and the wall roughness was about 10 nm, peak to trough. For the polymers: PMMA/DR-1 on Teflon and BCB aspect ratios of 16 are possible for 250-nm wide holes, along with high, 1 μm/min etching rates.

Etching of sub-micron high aspect ratio holes in oxides and polymers

Sub-micron holes have been successfully etched into: PMMA/DR-1 on Teflon, BCB, Ta 2O 5, Nb 2O 5 and SiO 2, with the aim of forming photonic crystal structures. Pattern transfer from the NiCr masks into the material is achieved in an electron cyclotron resonance high-density plasma system, using a combination of the gases: CHF 3, CF 4, O 2, Ar and SF 6, and an RF bias on the sample. The etching of Ta 2O 5 and Nb 2O 5 is hindered by the high sublimation temperature, approximately 230 °C, of TaF 5 and NbF 5. This problem was resolved by heating the sample. For these materials aspect ratios of 5 were achieved, with holes of down to 200-nm wide being etched. Wall roughness was typically 30 nm, peak to trough. For SiO 2, aspect ratios of 8 were achieved and the wall roughness was about 10 nm, peak to trough. For the polymers: PMMA/DR-1 on Teflon and BCB aspect ratios of 16 are possible for 250-nm wide holes, along with high, 1 μm/min etching rates.

Etching of submicron holes in SiO2, Ta2O5, and Nb2O5

We have etched submicron holes into SiO2, Ta2O5, and Nb2O5. The etching process has produced walls with low roughness, less than &amp;lt;30 nm, for Ta2O5 and Nb2O5, and &amp;lt;10 nm for SiO2, which compares well to the NiCr mask hole edge roughness of 5–10 nm. Three-inch wafers with real photonic-crystal structures have been etched. The problems associated with the high sublimation temperatures of TaF5 and NbF5 have been solved by increasing the substrate surface temperature to above 120 °C. However, for the narrowest holes, about 250 nm across, a pulsed electron-cyclotron-resonance method has had to be used in addition, in order to see an etching effect.