Sublimation Rate Research Articles

Crystal Growth, HOMO–LUMO Engineering, and Charge Transfer Degree in Perylene-FxTCNQ (x = 1, 2, 4) Organic Charge Transfer Binary Compounds

The methodologies of searching for novel organic charge transfer binary compounds and large-size crystal growth, in the case that only the two starting organic substances are known but the phase diagram is not known, the thermodynamic data of the binary compound are not known, and even the existence of new binary compounds is not known, were studied. Centimeter-long crystals of novel perylene-F1TCNQ, perylene-F2TCNQ, and perylene-F4TCNQ charge transfer binary compounds are obtained from the gas phase. Kinetically lowering the sublimation rate is the key factor for growing large-size charge transfer compound single crystals. Changing the number of fluorine atoms in FxTCNQ results in the variation of the electron affinity, which further changes the HOMO–LUMO of acceptor. Charge transfer degree is increased with increasing of fluorine atoms in the perylene-FxTCNQ system. Therefore, the structure, stoichiometry, and kind of donor and acceptor enable HOMO–LUMO engineering of the charge transfer compound and tu...

Effect of Freezing on Lyophilization Process Performance and Drug Product Cake Appearance

This study highlights the significance of the freezing step and the critical role it can play in modulating process performance and product quality during freeze-drying. For the model protein formulation evaluated, the mechanism of freezing had a significant impact on cake appearance, a potential critical product quality attribute for a lyophilized drug product. Contrary to common knowledge, a freezing step with annealing resulted in 20% increase in primary drying time compared to without annealing. In addition, annealing resulted in poor cake appearance with shrinkage, cracks, and formation of a distinct skin at the top surface of the cake. Finally, higher product resistance (7.5 cm2.Torr.hr/g) was observed in the case of annealing compared to when annealing was not included (5 cm2.Torr.hr/g), which explains the longer primary drying time due to reduced sublimation rates. An alternative freezing option using controlled ice nucleation resulted in reduced primary drying time (i.e., 30% reduction compared to annealing) and a more homogenous batch with elegant uniform (i.e., significantly improved) cake appearance. Here, a mechanistic understanding of the distinct differences in cake appearance as a function of freezing mechanism is proposed within the context of ice nucleation temperature, ice crystal growth, and presumed solute distribution within the frozen matrix.

LIMA (v1.0): A quasi two-moment microphysical scheme driven by a multimodal population of cloud condensation and ice freezing nuclei

Abstract. The paper describes the LIMA (Liquid Ice Multiple Aerosols) quasi two-moment microphysical scheme, which relies on the prognostic evolution of an aerosol population, and the careful description of the nucleating properties that enable cloud droplets and pristine ice crystals to form from aerosols. Several modes of cloud condensation nuclei (CCN) and ice freezing nuclei (IFN) are considered individually. A special class of partially soluble IFN is also introduced. These "aged" IFN act first as CCN and then as IFN by immersion nucleation at low temperatures. All the CCN modes are in competition with each other, as expressed by the single equation of maximum supersaturation. The IFN are insoluble aerosols that nucleate ice in several ways (condensation, deposition and immersion freezing) assuming the singular hypothesis. The scheme also includes the homogeneous freezing of cloud droplets, the Hallett–Mossop ice multiplication process and the freezing of haze at very low temperatures. LIMA assumes that water vapour is in thermodynamic equilibrium with the population of cloud droplets (adjustment to saturation in warm clouds). In ice clouds, the prediction of the number concentration of the pristine ice crystals is used to compute explicit deposition and sublimation rates (leading to free under/supersaturation over ice). The autoconversion, accretion and self-collection processes shape the raindrop spectra. The initiation of the large crystals and aggregates category is the result of the depositional growth of large crystals beyond a critical size. Aggregation and riming are computed explicitly. Heavily rimed crystals (graupel) can experience a dry or wet growth mode. An advanced version of the scheme includes a separate hail category of particles forming and growing exclusively in the wet growth mode. The sedimentation of all particle types is included. The LIMA scheme is inserted into the Meso-NH cloud-resolving mesoscale model. The flexibility of LIMA is illustrated by two 2-D experiments. The first one highlights the sensitivity of orographic ice clouds to IFN types and IFN concentrations. Then a squall line case discusses the microstructure of a mixed-phase cloud and the impacts of pure CCN and IFN polluting plumes. The experiments show that LIMA responds well to the complex nature of aerosol–cloud interactions, leading to different pathways for cloud and precipitation formation.

In Situ Observation on Dislocation-Controlled Sublimation of Mg Nanoparticles.

Sublimation is an important endothermic phase transition in which the atoms break away from their neighbors in the crystal lattice and are removed into the gas phase. Such debonding process may be significantly influenced by dislocations, the crystal defect that changes the bonding environment of local atoms. By performing systematic defects characterization and in situ transmission electron microscopy (TEM) tests on a core--shell MgO-Mg system, which enables us to "modulate" the internal dislocation density, we investigated the role of dislocations on materials' sublimation with particular focus on the sublimation kinetics and mechanism. It was observed that the sublimation rate increases significantly with dislocation density. As the density of screw dislocations is high, the intersection of screw dislocation spirals creates a large number of monatomic ledges, resulting in a "liquid-like" motion of solid-gas interface, which significantly deviates from the theoretically predicted sublimation plane. Our calculation based on density functional theory demonstrated that the remarkable change of sublimation rate with dislocation density is due to the dramatic reduction in binding energy of the monatomic ledges. This study provides direct observation to improve our understanding on this fundamental phase transition as well as to shed light on tuning materials' sublimation by "engineering" dislocation density in applications.

Improvement of the stability of Hg/AC catalysts by CsCl for the high-temperature hydrochlorination of acetylene

Activated carbon-supported mercuric chloride (HgCl2) is used as an industrial catalyst for acetylene hydrochlorination. However, the characteristic of easy sublimation of HgCl2 leads to the deactivation of the catalyst. Here, we showed that the thermal stability of the Hg/AC catalyst can be evidently improved when CsCl is added into the Hg/AC catalyst. Compared with the pure Hg/AC catalyst, the sublimation rate of HgCl2 from the Hg–Cs/AC catalyst decreased significantly and the Hg–Cs/AC catalyst showed better catalytic activity and stability in the reaction. This promoting effect is related to the existence of cesium mercuric chlorides (CsxHgyClx+2y) highlighted by XRD, HR-TEM and EDX analyses. Thus, reacting HgCl2 with alkali chlorides to form alkali-mercuric chlorides may be a key to design highly efficient and thermally stable mercuric chloride catalyst for hydrochlorination reactions.

Controlling Gaussian and mean curvatures at microscale by sublimation and condensation of smectic liquid crystals.

Soft materials with layered structure such as membranes, block copolymers and smectics exhibit intriguing morphologies with nontrivial curvatures. Here, we report restructuring the Gaussian and mean curvatures of smectic A films with free surface in the process of sintering, that is, reshaping at elevated temperatures. The pattern of alternating patches of negative, zero and positive mean curvature of the air–smectic interface has a profound effect on the rate of sublimation. As a result of sublimation, condensation and restructuring, initially equilibrium smectic films with negative and zero Gaussian curvature are transformed into structures with pronounced positive Gaussian curvature of layers packing, which are rare in the samples obtained by cooling from the isotropic melt. The observed relationship between the curvatures, bulk elastic behaviour and interfacial geometries in sintering of smectic liquid crystals might pave the way for new approaches to control soft morphologies at micron and submicron scales.

Evaporation Behavior and Characterization of Eutectic Solvent and Ibuprofen Eutectic Solution.

Liquid eutectic system of menthol and camphor has been reported as solvent and co-solvent for some drug delivery systems. However, surprisingly, the phase diagram of menthol-camphor eutectic has not been reported previously. The evaporation behavior, physicochemical, and thermal properties of this liquid eutectic and ibuprofen eutectic solution were characterized in this study. Differential scanning calorimetry (DSC) analysis indicated that a eutectic point of this system was near to 1:1 menthol/camphor and its eutectic temperature was -1°C. The solubility of ibuprofen in this eutectic was 282.11 ± 6.67mgmL(-1) and increased the drug aqueous solubility fourfold. The shift of wave number from Fourier transform infrared spectroscopy (FTIR) indicated the hydrogen bonding of each compound in eutectic mixture. The weight loss from thermogravimetric analysis of menthol and camphor related to the evaporation and sublimation, respectively. Menthol demonstrated a lower apparent sublimation rate than camphor, and the evaporation rate of eutectic solvent was lower than the sublimation rate of camphor but higher than the evaporation of menthol. The evaporation rate of the ibuprofen eutectic solution was lower than that of the eutectic solvent because ibuprofen did not sublimate. This eutectic solvent prolonged the ibuprofen release with diffusion control. Thus, the beneficial information for thermal behavior and related properties of eutectic solvent comprising menthol-camphor and ibuprofen eutectic solution was attained successfully. The rather low evaporation of eutectic mixture will be beneficial for investigation and tracking the mechanism of transformation from nanoemulsion into nanosuspension in the further study using eutectic as oil phase.

Influence of ultrasound-assisted nucleation on freeze-drying of carrots

ABSTRACTUltrasound has been often used to assist nucleation process in liquid samples, while solid samples were barely taken into account. In this study, direct-contact ultrasound was used to realize the sonocrystallization for solid sample and the influence of ultrasound-assisted nucleation on freeze-drying process was investigated. First, a microscopic observation was performed to exam the effectiveness of direct-contact ultrasound on inducing nucleation for carrot slice at different supercooling degrees. Second, an ultrasonic vibrating plate was used to integrate the contact ultrasound into freeze-drying process and its influence on ice crystals size and sublimation rate was evaluated. The results showed that direct-contact ultrasound is applicable to stimulate nucleation in carrot slice at lower supercooling degree and obtain bigger ice crystals. Compared to the control sample, the sublimation time of the samples under sonocrystallization was significantly reduced, by 21.80%. Therefore, direct-contact ultrasound maybe a promising method to be applied in solid-sample freeze-drying process.

What drives the dust activity of comet 67P/Churyumov-Gerasimenko?

We use the gravitational instability formation scenario of cometesimals to derive the aggregate size that can be released by the gas pressure from the nucleus of comet 67P/Churyumov-Gerasimenko for different heliocentric distances and different volatile ices. To derive the ejected aggregate sizes, we developed a model based on the assumption that the entire heat absorbed by the surface is consumed by the sublimation process of one volatile species. The calculations were performed for the three most prominent volatile materials in comets, namely, H_20 ice, CO_2 ice, and CO ice. We find that the size range of the dust aggregates able to escape from the nucleus into space widens when the comet approaches the Sun and narrows with increasing heliocentric distance, because the tensile strength of the aggregates decreases with increasing aggregate size. The activity of CO ice in comparison to H_20 ice is capable to detach aggregates smaller by approximately one order of magnitude from the surface. As a result of the higher sublimation rate of CO ice, larger aggregates are additionally able to escape from the gravity field of the nucleus. Our model can explain the large grains (ranging from 2 cm to 1 m in radius) in the inner coma of comet 67P/Churyumov-Gerasimenko that have been observed by the OSIRIS camera at heliocentric distances between 3.4 AU and 3.7 AU. Furthermore, the model predicts the release of decimeter-sized aggregates (trail particles) close to the heliocentric distance at which the gas-driven dust activity vanishes. However, the gas-driven dust activity cannot explain the presence of particles smaller than ~1 mm in the coma because the high tensile strength required to detach these particles from the surface cannot be provided by evaporation of volatile ices. These smaller particles can be produced for instance by spin-up and centrifugal mass loss of ejected larger aggregates.

A theoretical approach to the sublimation separation of zirconium and hafnium in the tetrafluoride form

Synopsis The separation of zirconium and hafnium is essential in the nuclear industry, since zirconium alloys for this application require hafnium concentrations of less than 100 ppm. The separation is, however, very difficult due to the numerous similarities in the chemical and physical properties of these two elements. Traditional methods for separation of zirconium and hafnium rely predominantly on wet chemical techniques, e.g. solvent extraction. In contrast to the traditional aqueous chloride systems, the AMI zirconium metal process developed by Necsa focuses on dry fluoride-based processes. Dry processes have the advantage of producing much less hazardous chemical waste. In the proposed AMI process, separation is effected by selective sublimation of the two tetrafluorides in an inert atmosphere under controlled conditions, and subsequent selective desublimation. Estimates are made for the sublimation rates of the two tetrafluorides based on the equilibrium vapour pressures. A sublimation model, based on the sublimation rates, was developed to determine if the concept of separation by sublimation and subsequent desublimation is theoretically possible.

A macroscale mixture theory analysis of deposition and sublimation rates during heat and mass transfer in dry snow

Abstract. The microstructure of a dry alpine snowpack is a dynamic environment where microstructural evolution is driven by seasonal density profiles and weather conditions. Notably, temperature gradients on the order of 10–20 K m−1, or larger, are known to produce a faceted snow microstructure exhibiting little strength. However, while strong temperature gradients are widely accepted as the primary driver for kinetic growth, they do not fully account for the range of experimental observations. An additional factor influencing snow metamorphism is believed to be the rate of mass transfer at the macroscale. We develop a mixture theory capable of predicting macroscale deposition and/or sublimation in a snow cover under temperature gradient conditions. Temperature gradients and mass exchange are tracked over periods ranging from 1 to 10 days. Interesting heat and mass transfer behavior is observed near the ground, near the surface, as well as immediately above and below dense ice crusts. Information about deposition (condensation) and sublimation rates may help explain snow metamorphism phenomena that cannot be accounted for by temperature gradients alone. The macroscale heat and mass transfer analysis requires accurate representations of the effective thermal conductivity and the effective mass diffusion coefficient for snow. We develop analytical models for these parameters based on first principles at the microscale. The expressions derived contain no empirical adjustments, and further, provide self consistent values for effective thermal conductivity and the effective diffusion coefficient for the limiting cases of air and solid ice. The predicted values for these macroscale material parameters are also in excellent agreement with numerical results based on microscale finite element analyses of representative volume elements generated from X-ray tomography.

Improved thermal stability and reduced sublimation rate of pentaerythritol tetranitrate through doping graphene oxide

Various graphene oxide (GO)–pentaerythritol tetranitrate (PETN) micro-composites were prepared in this study to investigate the influence of nanomaterials on the stability of PETN. Differential scanning calorimetry analysis showed that PETN microcrystals become more stable after doping with GO. Additionally, thermogravimetric analysis indicated that sublimation rates and vapor pressures of various GO–PETN micro-composites were significantly reduced compared to pure PETN microcrystals. Scanning electron microscopy images indicated that PETN microcrystals were at least partially covered with GO, which may explain the increased thermal stability and reduced sublimation rate.

Characterization of the Sublimation and Vapor Pressure of 2-(2-Nitrovinyl) Furan (G-0) Using Thermogravimetric Analysis: Effects of Complexation with Cyclodextrins.

In the present work, the sublimation of crystalline solid 2-(2-nitrovinyl) furan (G-0) in the temperature range of 35 to 60 °C (below the melting point of the drug) was studied using thermogravimetric analysis (TGA). The sublimated product was characterized using Fourier-transformed-infrared spectroscopy (FT-IR) and thin layer chromatography (TLC). The sublimation rate at each temperature was obtained using the slope of the linear regression model and followed apparent zero-order kinetics. The sublimation enthalpy from 35 to 60 °C was obtained from the Eyring equation. The Gückel method was used to estimate the sublimation rate and vapor pressure at 25 °C. Physical mixtures, kneaded and freeze-dried complexes were prepared with 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) and sulfobutyl ether-β-cyclodextrin (SBE-β-CD) and analyzed using isothermal TGA at 50 °C. The complexation contributed to reducing the sublimation process. The best results were achieved using freeze-dried complexes with both cyclodextrins.

A study of turbulent fluxes and their measurement errors for different wind regimes over the tropical Zongo Glacier (16° S) during the dry season

Abstract. Over glaciers in the outer tropics, during the dry winter season, turbulent fluxes are an important sink of melt energy due to high sublimation rates, but measurements in stable surface layers in remote and complex terrains remain challenging. Eddy-covariance (EC) and bulk-aerodynamic (BA) methods were used to estimate surface turbulent heat fluxes of sensible (H) and latent heat (LE) in the ablation zone of the tropical Zongo Glacier, Bolivia (16° S, 5080 m a.s.l.), from 22 July to 1 September 2007. We studied the turbulent fluxes and their associated random and systematic measurement errors under the three most frequent wind regimes. For nightly, density-driven katabatic flows, and for strong downslope flows related to large-scale forcing, H generally heats the surface (i.e. is positive), while LE cools it down (i.e. is negative). On average, both fluxes exhibit similar magnitudes and cancel each other out. Most energy losses through turbulence occur for daytime upslope flows, when H is weak due to small temperature gradients and LE is strongly negative due to very dry air. Mean random errors of the BA method (6 % on net H + LE fluxes) originated mainly from large uncertainties in roughness lengths. For EC fluxes, mean random errors were due mainly to poor statistical sampling of large-scale outer-layer eddies (12 %). The BA method is highly sensitive to the method used to derive surface temperature from longwave radiation measurements and underestimates fluxes due to vertical flux divergence at low heights and nonstationarity of turbulent flow. The EC method also probably underestimates the fluxes, albeit to a lesser extent, due to underestimation of vertical wind speed and to vertical flux divergence. For both methods, when H and LE compensate each other in downslope fluxes, biases tend to cancel each other out or remain small. When the net turbulent fluxes (H + LE) are the largest in upslope flows, nonstationarity effects and underestimations of the vertical wind speed do not compensate, and surface temperature errors are important, so that large biases on H + LE are expected when using both the EC and the BA method.

An enhanced model of the contemporary and long‐term (200 ka) sublimation of the massive subsurface ice in Beacon Valley, Antarctica

AbstractA massive ice body buried under several decimeters of dry regolith in Beacon Valley, Antarctica, is believed to be more than 1 Ma old and perhaps over 8.1 Ma; however, vapor diffusion models suggest that subsurface ice in this region is not stable under current climate conditions. To better understand the controls on sublimation rates and stability of this massive ice, we have modeled vapor diffusion using 12 years of climate and soil temperature data from 1999 to 2011, including field measurements of episodic snow cover and snowmelt events that have not been represented in previous models of ground ice sublimation. The model is then extended to reconstruct the sublimation history over the last 200 ka using paleotemperatures estimated from ice core data from nearby Taylor Dome and a relationship between atmospheric temperature and humidity derived from our meteorological records. The model quantifies the impact of episodic snow events; they account for a nearly 30% reduction in the massive ice loss. The sublimation rate of ground ice averages 0.11 mm a−1 between 1999 and 2011 in Beacon Valley. Parameterized with past environmental conditions and assuming the same regolith thickness, the modeled sublimation rate of ground ice in Beacon Valley averages 0.09 mm a−1 for the last 200 ka, comparable to the long‐term average rate estimated independently from various studies based on cosmogenic isotopes. This study provides a realistic estimate of the long‐term sublimation history and supports the inference that the buried ice in Beacon Valley is older than 1 Ma.

The changing rotation period of comet 67P/Churyumov-Gerasimenko controlled by its activity

Context. The ESA Rosetta spacecraft has been orbiting the nucleus of comet 67P/Churyumov-Gerasimenko since August 2014. The rotation axis of the irregularly shaped nucleus has a large obliquity (52 degrees) and is oriented such that the southern hemisphere is insolated during perihelion. Aims. We calculate the change in the rotation period as a function of the cometary orbital position due to forces exerted by cometary activity. Methods. We used a detailed shape model of 67P with \textgreater 10(5) facets. We calculated the efficiency of the facets to exert a torque based on their radial distance from the center of mass and their orientation. We applied our thermal model to calculate the diurnal water-ice sublimation rate from each facet. The reaction force per facet combined with its torque efficiency creates a torque and changes the angular momentum. The component of the torque parallel to the spin axis changes the rotation period. Results. Our model shows that the rotation period increases slightly during the approach of the comet to the Sun. It reaches a maximum shortly before equinox and drops rapidly during perihelion passage. The magnitude of the change depends on the actual sublimation rates. The change in sign mainly depends on the shape of the nucleus and not much on the sublimation variation. The roughness of the nucleus has little influence. Conclusions. For the given geometry of the rotation axis, the change in the rotation period is mainly influenced by the sublimation activity of the irregular shape of the nucleus. The rotation period increases until shortly before equinox in early May 2015, in good agreement with observations, and will then become shorter rapidly.

Mechanism for the separation of organic semiconductors via thermal gradient sublimation

Two common hole transporting materials, N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPD) and tris(4-carbazoyl-9-ylphenyl)amine (TCTA) are separated and purified using thermal gradient sublimation. Their sublimation rates are proportional to the tube diameter squared and inversely proportional to the distance from the source boat to the deposition region. The deposition of these materials exponentially decays with distance down the tube. A theory predicting these results is derived. This theory suggests that diffusion and surface capture are the dominant steps in this purification, not processes in the source boat. It provides a rationale for improving the separation between multiple components and suggests routes for producing larger quantities of these materials.

Evaluation of spin freezing versus conventional freezing as part of a continuous pharmaceutical freeze-drying concept for unit doses

Spin-freezing as alternative freezing approach was evaluated as part of an innovative continuous pharmaceutical freeze-drying concept for unit doses. The aim of this paper was to compare the sublimation rate of spin-frozen vials versus traditionally frozen vials in a batch freeze-dryer, and its impact on total drying time.Five different formulations, each having a different dry cake resistance, were tested.After freezing, the traditionally frozen vials were placed on the shelves while the spin-frozen vials were placed in aluminum vial holders providing radial energy supply during drying. Different primary drying conditions and chamber pressures were evaluated.After 2h of primary drying, the amount of sublimed ice was determined in each vial. Each formulation was monitored in-line using NIR spectroscopy during drying to determine the sublimation endpoint and the influence of drying conditions upon total drying time.For all tested formulations and applied freeze-drying conditions, there was a significant higher sublimation rate in the spin-frozen vials. This can be explained by the larger product surface and the lower importance of product resistance because of the much thinner product layers in the spin frozen vials. The in-line NIR measurements allowed evaluating the influence of applied drying conditions on the drying trajectories.

Pulsed-laser deposition of superconducting LiTi2O4 ultrathin films

We report epitaxial growth, structural characterizations and thickness-dependent superconducting properties of LiTi2O4 films grown on (111) MgAl2O4 substrates by using pulsed-laser deposition. The variations of growth temperature (Tg) and excess Li composition of target materials were found to be crucial for improving the crystallinity, as verified by X-ray diffraction rocking curve and the in-plane domain configuration. The crystallinity of the desired LiTi2O4 phase improved with increasing Tg, whereas the inclusion of Li-deficient phases became more significant due to high sublimation rate of Li species. The use of the excess Li targets allowed us to elevate Tg up to 750°C (Li/Ti=1.0) with keeping high crystallinity and phase purity. For films grown under the best condition, the superconducting transition was observed near 12K even when thickness was as thin as 9nm.

Modeling laser-induced incandescence of soot: a new approach based on the use of inverse techniques

Two LII models derived from the literature have been tested to simulate signals provided in a recently published extensive set of experimental data collected in a non-smoking laminar diffusion flame of ethylene. The first model classically accounts for particle heating by absorption and cooling by radiation, sublimation and conduction. The second one also integrates an alternative absorption term that accounts for saturation of the linear, single-photon and multi-photon absorption leading to C2-photodesorption at high fluence, a heating flux attributable to oxidation and a cooling process based on thermionic emission. Obtained results illustrate that both models fail to reproduce the LII signals experimentally monitored on a wide range of fluences (up to ~1 J cm−2) regardless of the value implemented for the main parameters involved in the energy- and mass-balance equations. We therefore originally proposed a new modeling approach based on the use of inverse techniques to gain information about the specific terms that should be integrated into the calculation. The inverse procedure allows inferring the temporal evolution of the soot diameter as well as the temporal and fluence dependence of additional energy rates that have to be considered to fulfill the particle energy and mass balances while providing a complete fit with experimental data. Conclusions issued from the present work indicate that modeling soot LII using only absorption, radiation, conduction and sublimation rates (as these fluxes are generally expressed and computed in the literature) is inadequate to correctly simulate the soot heating and cooling mechanisms over a wide range of fluences. The inverse modeling procedure also gave insights concerning the relevance of integrating photolytic mechanisms such as multi-photon absorption and carbon cluster photodesorption as previously proposed by Michelsen. Additional calculations performed using a new model formulation integrating such processes finally led to predictions merging on a single curve with experimental data. Additional works should be undertaken, however, to complete this first-approach analysis especially to address the large uncertainties existing in the input parameters and equations accounting for photolytic processes that are likely to significantly impact soot LII.