Developing Next Generation Phase Change Materials for Heat Storage Applications

A large proportion of energy consumption in the UK is in the form of heat, associated with domestic and commercial heating of buildings, and the heating requirements for a wide range of industrial processes. Since many heating requirements rely ultimately on the combustion of fossil fuels, inevitably this has a major impact on the release of CO2. Furthermore, with the ever-increasing price of fuel and electricity, there are significant economic impacts for both domestic and industrial consumers. Hence there is a very strong driver towards the exploitation of renewable heat, and a key challenge for renewable heat must be effective heat storage. Latent heat storage systems have the potential to be more economical and reduce CO2 emissions compared to heating systems currently used in homes and industry. Phase-change materials (PCMs) are the key materials used in the technology, and can include organic compounds such as waxes and sugar alcohols, and inorganic compounds such as salt hydrates. On melting these materials absorb heat and on freezing they release heat. This poster describes the development of new PCMs based on hydrates of magnesium , calcium, and strontium compounds which have tailored properties such as specific melting temperature ranges, improved long-term stabilities (over many heating and cooling cycles), and high energy densities. Variable temperature crystallographic studies (single crystal and powder X-ray diffraction) provide valuable insight into phenomena such as incongruent melting, supercooling, the appearance of intermediate hydrates, and the effects of additives that promote crystal nucleation. Such information leads to a better understanding of the behaviour of these PCMs, ultimately leading to more effective methods for heat storage.

Pressure-induced phase transitions in energetic materials

Explosives and propellants, known generically as energetic materials, are widely used in applications that include mining, munitions, and automotive safety. Key properties of these materials include: reliable performance under a range of environmental conditions; long-term stability; environmental impact; processability; sensitivity to accidental initiation through stimuli such as impact, shock, friction, and electrostatic discharge. Many of these properties are affected by the crystal structure of the energetic material. Explosives experience elevated pressures and temperatures under detonation conditions - such conditions often induce phase transitions in the energetic material. Hence detailed studies of pressure-induced structural changes in these materials are essential in order to understand and model fully their behaviour. This presentation will describe some recent high-pressure studies (using a combination of X-ray and neutron diffraction techniques) on 2,4-dinitroanisole (DNAN), an insensitive explosive that is replacing TNT in some applications [1,2]. DNAN shows rich pressure-induced polymorphism, with at least four high-pressure forms having been identified to date. One of the structures provides insight into as to why DNAN is particularly insensitive to initiation by shock. The presentation will also describe the interplay between experiment and theory, which will be illustrated by experimental and computational high-pressure studies of 1,1-diamino-2,2-dinitroethene (DADNE or FOX-7). A very subtle phase transition has been identified at a pressure of ~2.0 GPa and the implications of this will be discussed in relation to the observed structural changes and properties of this material.

The effects of pressure on structural strain in crystals with acetamide groups

Polymorphism is an ever growing area of interest in chemistry. Many active pharmaceutical ingredients (APIs) have the potential to have polymorphic forms, which can subsequently be used to the advantage of the pharmaceutical industry. There are a variety of conditions in which polymorphism can be examined; one way which has sparked interest in recent years is the influence of pressure and its effect on the behaviour of intermolecular bonds. The polymorphs of paracetamol were the first solid drugs for which the properties were compared at different pressures [1-2]. Another interesting research direction involves the comparison of the structural response to pressure of a series of chemically different compounds with similar molecular fragments, but possessing different molecular packing and intermolecular interactions. This is important for crystal engineering and for understanding structure-properties relationships. In the present study, we compare the response to pressure of a series of organic compounds with a common acetamide fragment: two polymorphs of paracetamol, two polymorphs of acetotoluidine, polymorphs and a hydrate of metacetamol, methacetin, and phenacetin. Both single-crystal Raman spectroscopy and X-ray diffraction were used. The effects of various pressure media have also undergone examination. The study was supported by the Year Abroad Programme of the University of Edinburgh (LM, CB), Russian Ministry of Science and Education and Russian Academy of Sciences (BAZ, SVG, EVB).

Explosives at Extreme Conditions: Polymorphism of 2,4-Dinitroanisole

2,4-dinitroanisole (DNAN) is an energetic material, developed as an insensitive replacement for TNT in melt-cast explosive formulations. While DNAN-based formulations demonstrate greatly reduced sensitivity to accidental initiation compared to those using TNT, issues remain with the replacement of TNT with DNAN. For instance, DNAN based formulations have demonstrated catastrophic levels of irreversible growth during heat-cycling, with volume increases of up to 15% reported. [1] In order to investigate the role of polymorphism in the irreversible growth of DNAN, high-pressure and variable-temperature neutron and x-ray diffraction studies have been performed. Two polymorphs of DNAN have been found to exist at ambient temperature and pressure, the thermodynamic form, DNAN-I, and the kinetic form, DNAN-II.[2,3] The phase diagrams of both form-I and -II of DNAN have been explored for the first time. In the case of DNAN-II, two high-pressure phase transitions were found. DNAN-II initially transformed to DNAN-III, which at higher pressures transformed to DNAN-IV. In addition, variable temperature studies demonstrated that the DNAN-II to DNAN-III transition also occurs when DNAN-II is cooled below room temperature. The thermal expansion of the DNAN-II/III lattice was investigated from 150K to 363K, demonstrating that an abrupt change in the thermal behaviour of lattice parameters occurs at the DNAN-II/III transition. From these combined crystallographic studies, the structure of DNAN-III has been solved, showing it is closely related to DNAN-II. In the case of DNAN-I, high-pressure neutron powder diffraction studies demonstrated that it transforms to a new form (DNAN-V) that is distinct from DNAN-II,-III or -IV. Rietveld refinement of the high-pressure DNAN-I data also determined that the material exhibits negative linear compressibility, which is of interest given the use of DNAN as a shock-insensitive energetic material. Comparison of the behaviour of DNAN-I and -II under variable temperature and high-pressure conditions indicates that the kinetic form, DNAN-II, is the denser phase under all conditions studied. This work highlights the importance of crystallographic techniques in order to understand the polymorphism of energetic materials.

Serotonin in different compounds: crystallographic and computational insight

Biologically active substances are in the focus of pharmaceutical and chemical research. Serotonin, one of the most common neurotransmitters, is widely studied in relation to its effect on humans from cellular to neurological levels. Although serotonin plays a key role in some biological processes, its chemistry and crystallography are not sufficiently understood. The aim of the present study was to crystallize serotonin adipate and creatinine sulfate monohydrate, determine their crystal structures, and analyze them in a comparison with other previously known serotonin crystal structures. Special attention was paid to the interrelation between the molecular conformation and crystalline environment. This issue was addressed using crystallographic and computational chemistry (DFT-D, MD) approaches. In our research was shown that the crystal structure of the creatinine sulfate complex significantly differs from what was previously determined. The conformation of serotonin in the new structure differs from serotonin conformations in all other known complexes, as well as from the most stable conformation, predicted by the adiabatic conformational analysis using quantum chemical calculations (DFT, MP) in different phases. This work has explicitly shown the influence of different interactions on serotonin molecular conformation in the crystalline state, described from a crystallographic and theoretical point of view. It has been previously demonstrated that salt formation in the presence of different anions produces variation in pharmacological, therapeutic and physic-chemical properties. This study has shown that alterations of the anion affects the molecular geometry of the bioactive substance and invite further investigation to rationalize the geometry changes. The work was supported by the RFBR Grants No.14-03-31866, 13-03-92704, Russian Ministry of Science and Education and RAS, Siberian Supercomputer Center SB RAS Integration Grant No.130, Edinburgh Compute and Data Facility

Energetic Cocrystals - Structural Studies of Nitrotriazolone Salts & Cocrystals

Developments in energetic materials are currently focused on the requirements for safer, yet still powerful materials for uses within mining, munitions and rocket propulsion systems One strategy that can be used to achieve these desirable properties is to synthesise new molecules, but this is both time-consuming and resource-intensive. Instead, another strategy is to crystallise energetic molecules with other molecules to form salts or cocrystals. This approach has been used extensively within the pharmaceutical industry in order to enhance desirable properties, e.g. solubility and bioavailability. To date, however, there has been very little research on the cocrystallisation of energetic materials. Examples include trinitrotoluene (TNT) with pyrene, naphthalene, and CL-20. To start this design process, the relationships between the types and strengths of interactions within a crystal structure and materials properties need to be established. Once these structure-property relationships have been established, the engineering of new and improved energetic materials can be achieved. The main focus of this work is on the energetic material 3-nitro-1,2,4-triazol-5-one (NTO) and the characterisation of a selection of new salts and cocrystals. NTO is an insensitive high explosive that has a similar performance to the more widely used explosive, RDX, yet is more stable, less prone to accidental detonation, and more soluble in water. Its high solubility in water is a major issue, as NTO is biologically active and represents a potential risk to the environment. There are only a few known salts of NTO and no published cocrystals, so the design and preparation of the first NTO cocrystals is a key objective. A selection of crystal structures of salts and cocrystals of NTO with nitrogen-rich aromatic systems has been obtained and the results are presented here. Interesting trends between pKa, functional groups, and intermolecular interactions have been observed.