Activation Energy For Sublimation Research Articles

Sublimation and Diffusion Kinetics of 2,4,6-Trinitrotoluene (TNT) Single Crystals by Atomic Force Microscopy (AFM).

In this article, we report the in-situ nanoscale experimental measurement of sublimation rates, activation energy of sublimation, and diffusion coefficients of 2,4,6-trinitrotoluene (TNT) single crystals in air using atomic force microscopy (AFM). The crystals were prepared by slow evaporation at 5 °C using acetone-dissolved TNT. The mass loss was calculated by monitoring the shrinkage of the surface area of layered islands formed on the surface of the TNT crystals due to sublimation upon isothermal heating at temperatures below the melting point. The results suggest the sublimation process occurs via two-dimensional detachment of TNT molecules from the non-prominent facets on the crystal surface which imitates the nucleation and crystal growth process. Sublimation rates are one order of magnitude smaller than previously reported values. However, the calculated activation energy (112.15 ± 3.2 kJ/mol) and temperature-dependent sublimation rates agree well with the reported values for TNT thin films and microcrystals determined by UV-vis absorbance spectroscopy and quartz crystal microscopy (QCM) (90–141 kJ/mol). The average diffusion coefficient is (4.35 × 10–6 m2/s) which is within the range of the reported theoretical values with an average of 5.59 × 10–6 m2/s, and about 25% less than that determined using thermogravimetric analysis for powder TNT.

Non-Isothermal Sublimation Kinetics of 2,4,6-Trinitrotoluene (TNT) Nanofilms.

Non-isothermal sublimation kinetics of low-volatile materials is more favorable over isothermal data when time is a crucial factor to be considered, especially in the subject of detecting explosives. In this article, we report on the in-situ measurements of the sublimation activation energy for 2,4,6-trinitrotoluene (TNT) continuous nanofilms in air using rising-temperature UV-Vis absorbance spectroscopy at different heating rates. The TNT films were prepared by the spin coating deposition technique. For the first time, the most widely used procedure to determine sublimation rates using thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC) was followed in this work using UV-Vis absorbance spectroscopy. The sublimation kinetics were analyzed using three well-established calculating techniques. The non-isothermal based activation energy values using the Ozawa, Flynn–Wall, and Kissinger models were 105.9 ± 1.4 kJ mol−1, 102.1 ± 2.7 kJ mol−1, and 105.8 ± 1.6 kJ mol−1, respectively. The calculated activation energy agreed well with our previously reported isothermally-measured value for TNT nanofilms using UV-Vis absorbance spectroscopy. The results show that the well-established non-isothermal analytical techniques can be successfully applied at a nanoscale to determine sublimation kinetics using absorbance spectroscopy.

Thermal decomposition of ammonium perchlorate—A TGA-FTIR-MS study: Part II

Thermogravimetric analysis (TGA), coupled with Fourier transform infrared (FTIR) spectroscopy and mass spectrometry (MS), has been utilized to study the thermal decomposition of ammonium perchlorate (AP). The effect of sample size and that of confinement using a perforated lid on crucibles was studied. Kinetic parameters were extracted from TGA data using isoconversional methods and kinetic compensation effect. Among several interesting trends that emerged, the increase in the decomposition rates with increase in particle size, in the first phase the presence of NO among the gases detected only during the tests conducted in crucibles with a pierced lid (CPLs), and the activation energy for the tests conducted in CPLs being close to the activation energy of sublimation of AP, were the prominent ones. The species detected by FTIR spectroscopy were NH3, N2O, HCl, NO2, H2O, NO, and HNO3, while those detected by MS were Cl2, O2, HOCl, ClO, and NH4Cl.

Pressure dependence of the interfacial structure of potassium chloride films on iron

Potassium chloride films on a clean iron surface are used as a model system to explore the interfacial structure of the films and the dependence of that structure on film thickness and pressure. The interfacial structure of one-, two-, three- and four-layer films is measured experimentally using low-energy electron diffraction. Those findings are then complemented by molecular dynamics simulations in which the atomic interaction between the film and substrate is tuned to match film thickness-dependent sublimation activation energy obtained from temperature-programmed desorption measurements. The resultant simulation reliably predicts the structure of thicker films and is then used to study the effect of pressure on the distribution of the lattice constant within and between each layer of the potassium chloride films. Findings indicate that both film thickness and pressure affect the structure within the films as well as the degree of registry between the film and adjacent substrate.

The Surface Kinetics of MBE-Grown CdTe (211)B During In Situ Cyclic Annealing

The surface kinetics of CdTe (211)B grown by molecular beam epitaxy (MBE) is investigated using spectroscopic ellipsometry (SE) during in situ cyclic annealing. A method of measuring sublimation rates from high-index surfaces without use of reflection high-energy electron diffraction is presented. The effect of Te2 overpressure on the activation energy of sublimation for the CdTe (211)B surface is reported. The sensitivity of SE to surface temperature and film thickness was leveraged to monitor sublimation rates of CdTe stabilized by a Te2 overpressure. The sublimation activation energy was found to increase from 0.45 eV to 2.94 eV under the Te2 beam pressure regime investigated.

Thermodynamic parameters of Rhodamine B in powder and nanofilms forms

We report the use of UV–Vis absorbance spectroscopy in determining thermodynamic properties of materials based on their absorbance in the visible region of the electromagnetic spectrum. Rhodamine B (RhB) dye which has always been considered to be a non-volatile material, was chosen for the validation of this method because of its well-defined and strong absorbance in this region. Continuous films of RhB suitable for absorbance spectroscopic study were prepared by spin coating in a 0.2 M ethanol solution to determine the sublimation rates and the activation energy of RhB in the temperature range 65–100 °C. For comparison, the same thermodynamic parameters of powder RhB were also determined using thermogravimetric analysis, at higher temperatures (100–180 °C). The sublimation of RhB was determined to be a first-order process by both methods. The activation energies of sublimation calculated by both techniques are in very good agreement. However, the sublimation rates calculated using UV–Vis absorbance spectroscopy are two orders of magnitude smaller than those calculated using TG which is explained by the inaccurate calculation of the powder surface area due to its small needle-like morphology. The results indicate that RhB sublimates at relatively low temperatures as well as the ability of UV–Vis absorbance spectroscopy to determine such parameters, accurately, in the nanometer scale at relatively lower temperatures.

Sublimation Kinetic Studies of the Zr(tmhd) 4 Complex

The thermal behaviour of tetrakis(2,2,6,6‐tetramethyl‐3,5‐heptanedionato)zirconium(IV), [Zr(tmhd)4] was investigated by nonisothermal and isothermal thermogravimetric methods in a high pure nitrogen atmosphere. The influence of the heating rate in dynamic measurements (6, 8, 10, and 12°C/min) on activation energy was also studied. The nonisothermal sublimation activation energy values determined following the procedures of Arrhenius, Coats and Redfern, Kissinger, and Flynn‐Wall yielded 76 ± 5, 92 ± 2, 81 ± 8, and 72 ± 7 kJ/mol, respectively, and the isothermal sublimation activation energy was found to be 87 ± 4 kJ/mol over the temperature range of 411–462 K. Different reaction mechanisms were used to compare with this value. Analysis of the experimental results suggested that the actual reaction mechanism was an Rn deceleration type.

Thermo-optical determination of vapor pressures of TNT and RDX nanofilms

Accurate thermodynamic parameters of thin films of explosives are important for understanding their behavior in the nanometer scale as well as in standoff detection. Using UV-absorbance spectroscopy technique, accurate thermodynamic parameters such as activation energies of sublimation, sublimation rates, and vapor pressures of the explosives cyclotrimethylenetrinitramine (RDX) and 2,4,6-trinitrotoluene (TNT) were determined. The values of these parameters are in excellent agreement with those reported using traditional experiments based on gravimetry. In terms of the Clapeyron equation, the dependence of RDX and TNT vapor pressures on temperature can be described by the relations LnP (Pa)=39.6−15459/T (K) and LnP (Pa)=34.9−12058/T (K), respectively. Heats of sublimation of RDX and TNT were also determined to be 128kJ/mol and 100.2kJ/mol, respectively.

Stabilization of ammonium azide particles through its microencapsulation with some organic coating agents

This work is devoted to reduce spontaneous sublimation of ammonium azide at ambient and elevated temperatures by means of two microencapsulation techniques involving solvent/non-solvent and solvent evaporation methods in which stearic acid, Viton and nitrocellulose (NC) have been tested as coating agents. Scanning electron microscopy (SEM) was employed to examine the coating morphology. The thermal behavior of pure and coated ammonium azide samples have been studied by using simultaneous thermogravimetery–differential thermal analysis (TG–DTA) and differential scanning calorimetry (DSC). The results showed that ammonium azide microparticle could be effectively coated with nitrocellulose through a solvent/non-solvent experiment in which the coating quality depends on some experimental factors such as coating agent to NH 4N 3 weight ratio and volume and addition time of non-solvent. The effect of these factors on coating quality and thermal properties of NH 4N 3 has been revealed by results of SEM and thermal experiments. The most stabilized coated ammonium azide was achieved by using 4.5% (w/w) of NC as stabilizer, and by addition of 30 ml n-hexane as non-solvent within 75 min, where the maximum sublimation temperature increases about 30 °C with respect to uncoated sample and reaches to 145.5 °C. The influence of the heating rate (5, 10, 15 and 20 °C/min) on the DSC behavior of the pure and coated ammonium azide particles at the optimum condition was verified, whereas sublimation temperature of the NH 4N 3 was increased as the heating rate was increased. Also, the kinetic parameters such as the activation energy and frequency factor of the sublimation processes for pure and coated ammonium azide were obtained from the DSC data by non-isothermal methods proposed by Kissinger and Ozawa. The results showed that, activation energy for sublimation of completely coated NH 4N 3 particles is considerably higher than (nearly 1.5 times) that of the pure one. Also, the first order rate constant of sublimation of completely coated sample is noticeably lower than that of pure sample ( k coat k pure = 3 × 10 − 4 ). The kinetic results led us to conclude that the applied microencapsulation technique caused efficient stabilization of volatile NH 4N 3.

Molecular and ionic sublimation of neodymium tribromide polycrystals and single crystals

The molecular and ionic sublimation of polycrystals and single crystals under Knudsen effusion and Langmuir evaporation conditions is reported. In both sublimation regimes, the sublimation product at 780–1050 K contains neodymium tribromide monomer and dimer molecules, as well as the negative ions NdBr 4 − , Nd2Br 7 − , and Br−. The dimer-to-monomer flux ratio j(Nd2Br6)/j(NdBr3)is larger in the molecular beam coming out of the effusion hole, while the ratio of the sublimation fluxes of the negative ions, j(Nd2Br 7 − )/j(NdBr 4 − ), is independent of the sublimation conditions. The partial pressures of the neutral components of the vapor have been determined, and the enthalpies and activation energies of sublimation of neodymium tribromide as monomer and dimer molecules and NdBr 4 − and Nd2Br 7 − ions have been calculated. The equilibrium constants of ion-molecule reactions have been measured, and the enthalpies of these reactions have been determined. Based on these data, values of the thermodynamic properties Δ s H 0(298.15) and Δ f H 0(298.15) are recommended for the monomer and dimer molecules and the NdBr 4 − and Nd2Br 7 − ions.

Vapor pressure, kinetics and enthalpy of sublimation of bis(2,2,6,6-tetramethyl-3,5-heptanedionato)copper(II)

The kinetics of sublimation of bis(2,2,6,6-tetramethyl-3,5-heptanedionato)copper(II), [Cu(tmhd) 2] was studied by non-isothermal and isothermal thermogravimetric (TG) methods. The non-isothermal sublimation activation energy values determined following the procedures of Friedman, Kissinger, and Flynn–Wall methods yielded 93 ± 5, 67 ± 2, and 73 ± 4 kJ mol −1, respectively and the isothermal sublimation activation energy was found to be 97 ± 3 kJ mol −1 over the temperature range of 375–435 K. The dynamic TG run proved the complex to be completely volatile and the equilibrium vapor pressure ( p e) T of the complex over the temperature range of 375–435 K determined by a TG-based transpiration technique, yielded a value of 96 ± 2 kJ mol −1 for its standard enthalpy of sublimation (Δ sub H°).

The molecular and ionic sublimation of holmium tribromide

The molecular and ionic composition of vapor over holmium tribromide was studied by high-temperature mass spectrometry under the conditions of sublimation from a Knudsen effusion cell and from the open single crystal surface. The partial pressures of HoBr3 and Ho2Br6 in saturated vapor and the ratio between their sublimation coefficients under free vaporization conditions were determined. The intensities of lines in the electron impact mass spectra and their temperature dependences were different under Knudsen and Langmuir conditions, which was evidence of superthermal vibrational-rotational excitation of molecules sublimed from the open surface. The enthalpies and activation energies of sublimation of HoBr3 crystals as monomers and dimers were calculated. The emission of HoBr4− and Ho2Br7− negative ions was observed under both sublimation conditions. Ion-molecular equilibria in the LaBr3-HoBr3 system were studied. The enthalpies of formation of the HoBr3 and Ho2Br6 molecules and HoBr4− and Ho2Br7− negative ions in the gas phase were calculated.

Molecular and ionic sublimation of erbium tribromide

The molecular and ionic composition of vapor over erbium tribromide sublimed from the Knudsen effusion cell and the open surface of a single crystal was studied by high-temperature mass spectrometry. The partial pressures of ErBr3 and Er2Br6 molecules in saturated vapor and the ratio between their sublimation coefficients under free vaporization conditions were determined. The enthalpies and activation energies of sublimation of ErBr3 crystals in the form of monomers and dimers were calculated. The emission of and Er2 was recorded in studies of ionic sublimation in both modes. The enthalpies of formation of gas molecules and ions were determined.

A mass spectrometric study of the vaporization of cerium tribromide under Knudsen and Langmuir conditions

The molecular and ionic vaporization of cerium tribromide from the effusion cell (Knudsen conditions) and the open surface of a CeBr 3 single crystal (Langmuir conditions) were studied by high-temperature mass spectrometry. The CeBr 3 and Ce 2Br 6 molecules and the CeBr 4 − and Ce 2Br 7 − negative ions were observed as vapor constituents. The partial pressures of the molecules in saturated vapor and the ratio between the vaporization coefficients of monomers and dimers under free vaporization conditions were determined. The enthalpies and the activation energies of sublimation in the form of monomers and dimers were measured. Ion–molecular equilibria with the participation of negative ions were studied. The enthalpies of formation of molecules and ions in the gas phase were obtained.

A Mass Spectrometric Study of the Sublimation of Lutetium Tribromide under Knudsen and Langmuir Conditions

The molecular and ionic sublimation of lutetium tribromide under thermodynamic equilibrium (Knudsen effusion) conditions and from the open surface of a LuBr3 single crystal (Langmuir conditions) was studied by high-temperature mass spectrometry. Vapor contained the LuBr3, Lu2Br6, Lu3Br9, and Lu4Br12 molecules and the Br−, LuBr4 −, Lu2Br 7 − , and Lu3Br 10 − negative ions. The partial pressures of the molecules in saturated vapor and the ratio between the sublimation coefficients of monomers and dimers under free vaporization conditions were determined. The degree of the electron impact-induced fragmentation of LuBr3 molecules under Knudsen and Langmuir sublimation conditions was analyzed. The second and third laws of thermodynamics were used to calculate the enthalpies of sublimation in the form of monomers and oligomers (Knudsen vaporization) and the corresponding activation energies of sublimation (Langmuir vaporization). Ion—molecular equilibria with the participation of negative ions were studied. The enthalpies of formation of molecules and ions in the gas phase were obtained.

Sublimation of Ammonium Salts: A Mechanism Revealed by a First-Principles Study of the NH4Cl System

The mechanism for the sublimation of ammonium salts in general is not well-elucidated; the relationship between sublimation energies and activation energies of sublimation has not been understood. We have studied the kinetics and mechanism for the sublimation of NH4Cl, a prototype ammonium salt, by first-principles calculations using generalized gradient approximation in the plane-wave density functional theory. Supercells containing 8 to 64 NH4Cl units were used, and the predicted sublimation energy, 41.0 kcal/mol for NH4Cl(c) → NH3(g) + HCl(g), is in excellent agreement with the experimental value, 42.2 kcal/mol. The result of statistical-theory calculations indicates that the desorption of the H3N···HCl molecular complex with a 15.5 ± 1.0 kcal/mol variational barrier, instead of the individual NH3 and HCl molecules, from the relaxed crystal surface is the rate-controlling step. The desorption rate is predicted to be 25.0 exp(−13.2 kcal/mol/RT) cm/s, which is in close agreement with the experimental dat...

Thermal desorption and IR spectrometric investigation of polyamorphic and polymorphic transformations in cryovacuum condensates of water

Simultaneous thermal desorption and IR spectrometric studies of thermally stimulated transformations in thin films of cryovacuum condensates of water are carried out. It is shown that the temperature interval 12–36K is the existence region of a high-density form of amorphous solid water. The transition of amorphous high-density ice to a low-density amorphous state starts at T≈39K and is completed in the temperature region 60–70K. This process is accompanied by pore formation and reaches its maximum intensity at T≈42K. The transition temperature from the glassy amorphous ice to a super-viscous liquid state is determined: Tg=137±2K. Further increase in temperature leads to transformations of a stepped character in the layer. This may be due to competing processes of crystallization via the growth of cubic and hexagonal nuclei and also to direct crystallization of the super-viscous liquid water formed at Tg and existing together with the crystalline phase up to temperatures ∼200K. Anomalous behavior of the sample at temperatures preceding sublimation is revealed. This confirms that a multicomponent system consisting of amorphous and crystalline samples has different values of the equilibrium pressures of the gas phase, corresponding to partial activation energies of sublimation, i.e., at high temperatures amorphous water evaporates at an earlier stage, and then recondenses on the crystalline components.

Kinetics of dihydro-dibenz[ b, f]azepine derivatives sublimation

Sublimation of dihydro-dibenz[ b, f]azepine derivatives upon heating was studied and confirmed by thermogravimetry (TGA) and differential scanning calorimetry (DSC). DSC was used to analyze thermodynamics of melting. The kinetics of dihydro-dibenz[ b, f]azepine derivatives sublimation were monitored by TGA as a function of temperature. Activation energies of sublimation were deduced and correlated with melting enthalpies and molecular structure. The results indicated that the sublimation was controlled by intermolecular forces in the crystalline lattice of dihydro-dibenz[ b, f]azepine derivatives.

A computational study on the decomposition of NH 4ClO 4: Comparison of the gas-phase and condensed-phase results

The decomposition of NH 4ClO 4 (AP) in both gaseous and condensed phases has been studied at the CCSD(T)/6-311 + G(3df,2p)//B3LYP/6-311 + G(3df,2p) level. Formation of NH 3 + HClO 3 is the main channel in the gas phase with 14.2 kcal/mol enthalpy change. In solution, the enthalpy change for this channel is 29.5 kcal/mol, which is close to the sublimation activation energy, ∼30 kcal/mol in crystalline AP. Formation of H 3N–HO +ClO 3 has higher enthalpy changes, 56.3 kcal/mol and 77.1 kcal/mol in the gas phase and in solution, respectively. Strong solvent effects may exist in the AP decomposition reaction.

Effects of interfacial interaction potential on the sublimation rates of TNT films on a silica surface examined by QCM and AFM techniques

The study of 2,4,6-trinitrotoluene (TNT) sublimation rates from the bulk surface and a substrate surface have been evaluated quantitatively with both atomic force microscopy and quartz crystal microbalance (QCM) techniques. A first principle theoretical model is proposed, which allows obtaining three critical parameters, bulk sublimation rate, surface interaction potential, and the effective decay length, with no arbitrary parameters. The bulk sublimation rate predicted by the model is quantitatively confirmed by QCM experiments. The isothermal measurements with QCM showed that the sublimation activation energy of bulk TNT is 131 kJ/mol. More importantly, all results were obtained at one atmosphere and near room temperature. Thus, it should have direct impacts on explosive trace detection device applications.