RESUMEN
Previous experimental and theoretical studies revealed that carbon insertion into the RCr2Si2 compounds drastically affects the magnetic behavior, since chromium does not carry any magnetic moment in RCr2Si2C (R = Y, La-Sm, Gd-Er) compounds in contrast to RCr2Si2 (R = Y, Sm, Gd-Lu, Th) compounds. In this study, we report on the unexpected magnetic ordering of chromium atoms in the isotype quaternary UCr2Si2C compound. While specific heat and magnetic measurements suggest a Pauli paramagnetic behavior, neutron powder diffraction reveals an antiferromagnetic ordering of the chromium substructure at high temperature ( TN > 300 K), while that of uranium remains nonmagnetically ordered down to 2 K. Its magnetic behavior, inverse in comparison to the RCr2Si2C carbides involving a magnetic lanthanide, is discussed in relation with the singularity of its crystal structure among the series. Moreover, the crystallographic structures and the structural stability of UCr2Si2C and of two other quaternary U-Cr-Si-C compounds (i.e., UCr3Si2C and U2Cr3Si2C3), based on the full occupancy of interstitial sites by carbon atoms, are discussed and compared to those of the related ternary intermetallics. Finally, the low-temperature form of UCr2Si2, corresponding to a displacive transformation around 210 K of the ThCr2Si2-type structure, is reinvestigated by considering a higher symmetry monoclinic unit cell ( C2/ m) instead of the previously reported triclinic cell ( P1Ì ). The antiferromagnetic ordering at low temperature ( TN = 30(2) K) of the uranium substructure is confirmed, and its magnetic structure is reanalyzed and discussed considering the monoclinic crystal structure.
RESUMEN
OBJECTIVE: To reproduce and compare the intraoral torsional behavior of 10 commonly used preformed upper NiTi 0.017 x 0.025 archwires in 0.018-slot brackets at 20 degrees C, 35 degrees C, and 55 degrees C. MATERIALS AND METHODS: Ten upper preformed NiTi archwires were compared to a multibraided stainless steel wire. An original testing bench was used to reproduce palatal root torque applied onto an upper central incisor with a maximum value of 1540 g x mm. Ten samples of each wire type were tested at 20 degrees C, 35 degrees C, and 55 degrees C each. RESULTS: Loading and unloading at 20 degrees C revealed three categories of wires: a group of four NiTi wires of relative stiffness bereft of any superelasticity, a group of six NiTi wires displaying some horizontal plateau, and finally the stainless steel wire of lesser stiffness. Testing at the average oral temperature of 35 degrees C produced the same three categories of wires, with only 2 of 10 NiTi wires displaying a superelastic effect (Copper NiTi 35 degrees C and 40 degrees C). None of the NiTi wires was superelastic at 55 degrees C. Moments increased with temperature as the martensite was replaced by the more rigid austenite. CONCLUSION: This study showed that most NiTi wires did not exhibit in torsion the superelastic effect traditionally described in bending. The combination of straight-wire prescriptions and rectangular superelastic NiTi archwires did not provide optimal constant moments necessary to gain third-order control of tooth movement early in treatment. A braided stainless steel rectangular archwire displayed better torsional behavior at 35 degrees C than most NiTi archwires of the same dimensions.