Gerhard Wildermuth

Bio: Gerhard Wildermuth is an academic researcher from University of Konstanz. The author has contributed to research in topics: Thermal analysis & Particle size. The author has an hindex of 4, co-authored 5 publications receiving 123 citations.

How stable are inorganic fullerene-like particles? Thermal analysis (STA) of inorganic fullerene-like NbS2, MoS2, and WS2 in oxidizing and inert atmospheres in comparison with the bulk material

Abstract: Two-dimensional compounds (like graphite or MoS2) are capable of bending their layers and forming a closed cage structure, which is denoted the inorganic-fullerene-like (IF) phase in the case of inorganic materials. Simultaneous thermal analysis (TG-DTA) of a number of nanomaterials (IF-MoS2, IF-NbS2, and IF-WS2) and their bulk counterparts was carried out in inert and oxidizing environments at various heating rates. Oxidation occurred at 295 °C for the smallest particle size (100 nm) but the oxidation temperature increased to 440 °C for the largest particle size (3 μm). At low heating rates, the oxidation occurred through sulfates and lower oxides, while at high heating rates mostly one-step oxidation to the highest oxide was observed. The stability in N2 atmosphere depended on the criterion of stability chosen: Choosing the onset temperatures of the decomposition step (between 1200 °C and 1390 °C), the stability decreased in the order WS2 ≈ MoS2 > NbS2; and the bulk material reacted mostly at higher temperatures compared to the IF-phase. Choosing the extent of decomposition as a measure of stability, no specific trend could be found. Surprisingly, a single IF-WS2 sample was found to be most stable by both methods.This study shows that IF nanoparticles are comparably (though somewhat less) stable against oxidation as well as decomposition than the bulk material exhibiting the usual plate-like structure.

How Stable Are Inorganic Fullerene-Like Particles? Thermal Analysis (STA) of Inorganic Fullerene-Like NbS2, MoS2, and WS2 in Oxidizing and Inert Atmospheres in Comparison with the Bulk Material.

Abstract: Two-dimensional compounds (like graphite or MoS2) are capable of bending their layers and forming a closed cage structure, which is denoted the inorganic-fullerene-like (IF) phase in the case of inorganic materials. Simultaneous thermal analysis (TG-DTA) of a number of nanomaterials (IF-MoS2, IF-NbS2, and IF-WS2) and their bulk counterparts was carried out in inert and oxidizing environments at various heating rates. Oxidation occurred at 295 °C for the smallest particle size (100 nm) but the oxidation temperature increased to 440 °C for the largest particle size (3 μm). At low heating rates, the oxidation occurred through sulfates and lower oxides, while at high heating rates mostly one-step oxidation to the highest oxide was observed. The stability in N2 atmosphere depended on the criterion of stability chosen: Choosing the onset temperatures of the decomposition step (between 1200 °C and 1390 °C), the stability decreased in the order WS2 ≈ MoS2 > NbS2; and the bulk material reacted mostly at higher temperatures compared to the IF-phase. Choosing the extent of decomposition as a measure of stability, no specific trend could be found. Surprisingly, a single IF-WS2 sample was found to be most stable by both methods.This study shows that IF nanoparticles are comparably (though somewhat less) stable against oxidation as well as decomposition than the bulk material exhibiting the usual plate-like structure.

Synthesis, characterization and phase transitions in the inorganic–organic layered perovskite‐type hybrids [(CnH2n + 1NH3)2PbI4], n = 4, 5 and 6

Abstract: Three inorganic–organic layered perovskite-type hybrids of the general formula [(CnH2n + 1NH3)2PbI4], n = 4, 5 and 6, display a number of reversible first-order phase transitions in the temperature range from 256 to 393 K. [(C4H9NH3)2PbI4] has a single phase transition, [(C5H11NH3)2PbI4] has two phase transitions and [(C6H13NH3)2PbI4] has three phase transitions. In all three cases, the lowest-temperature phase transition is thermochromic and the crystals change colour from yellow in their lowest-temperature phase to orange in their higher-temperature phase for [(C4H9NH3)2PbI4] and [(C6H13NH3)2PbI4], and from orange to red for [(C5H11NH3)2PbI4]. The structural details associated with this phase transition have been investigated via single-crystal X-ray diffraction, SC-XRD, for all three compounds.

Fullerene-like MoS 2 Nanoparticles and Their Tribological Behavior

Abstract: Using a new quartz-made reactor, large amounts of fullerene-like (IF) MoS2 nanoparticles were synthesized by reacting MoO3 vapor with H2S in a reducing atmosphere. The nanoparticles were found to be of high crystalline order; with an average size of 70 nm and consist of more than 30 closed shells. Extensive tribological testing of the nanoparticles in two types of synthetic oils- poly-alpha olefins (PAO)- was carried out and compared to that of bulk (2H platelets) MoS2 and IF-WS2. These tests indicated that under high pressure and relatively low humidity, the IF-MoS2 exhibited a friction coefficient as low as 0.03 and the smallest wear rate of the measured systems. However, its performance was found to be lower in comparison to IF-WS2 after 2500 cycles, due probably to its inferior chemical stability. This study indicates that the tribological performance of the IF nanoparticles depends strongly on their crystalline order and size.

Dynamics of the n‐decylammonium chains in the perovskite‐type layer structure compound (C10H21NH3)2CdCl4

Abstract: The two structural phase transitions in the perovskite layer structure compound (C10H21NH3)2CdCl4 at Tc1=35 °C and Tc2=39 °C have been studied by x‐ray diffractional, calorimetric and dielectric measurements, proton NMR spin–lattice relaxation and second moment investigations, 35Cl and 14N quadrupole resonance spectroscopy as well as group theoretical considerations. The results show that the transition (Tc1) from the low temperature phase (P21/n) to the intermediate temperature phase (Pmnn) is basically a dynamic order–disorder transition of rigid alkyl chains, whereas the transition (Tc2) from the intermediate to the high temperature phase (Amaa) is connected with a melting of the alkyl chains. In the intermediate phase the rigid alkyl chains are flipping around their long axes between two equivalent orientations separated by 90°. The two transitions are somewhat analogous to the ones found in the lipid bilayer membranes and can be described by order parameters used for smectic liquid crystals.