RESUMO
While tunnel boring machines (TBMs) tunneling in clayey strata, the adhered excavated soil on the cutterhead and cutting tools tends to form mudcake after compaction and consolidation. Mudcake can obstruct the cutterhead openings and rendering the cutting tools ineffective, leads to a substantial reduction in advance rate. Dispersants are recognized as an effective method for the disintegration of mudcakes. A novel set of equipment, comprising a mudcake compression device and a mudcake disintegration apparatus, is developed for assessing mudcake disintegration properties. The results showed that mudcakes underwent a tripartite disintegration process in water, including an initial stage, a rapid disintegration stage, and a stable stage. In the initial stage, the mudcakes absorbed water before disintegration, resulting in marginal changes in the weight of the disintegrated mudcakes. In the rapid disintegration stage, the weight of the disintegrated mudcakes increased quickly. During the stable stage, the weight of the disintegrated mudcakes remained relatively constant. The submersion of mudcakes in a dispersant solution substantially increased the rate of disintegration. Greater dispersant concentration corresponded to an increase in the disintegration rate. No weight gain was observed in mudcakes during the initial disintegration stage. When mudcakes disintegrated in a bentonite slurry, the weight of the disintegrated mudcakes initially decreased and then stabilized. The weight of the disintegrated mudcakes turned negative, indicating an increase in the weight of mudcakes. This suggested that bentonite significantly hindered mudcake disintegration.
RESUMO
Polycrystalline La0.8(Ca0.12Sr0.08)MnO3:mol%Ag x (LCSMO:Ag x , x = 0, 0.1, 0.2, 0.3 and 0.4) ceramics were synthesized by the sol-gel technique. Structural, electrical and magnetic properties of the LCSMO:Ag x ceramics were investigated in detail. X-ray diffraction (XRD) data analyses revealed that all the samples were crystalized in the orthorhombic structure with space group of Pnma. With the increase in Ag doping (x), the grain sizes of the LCSMO:Ag x samples increased and the amount of grain boundaries (GBs) decreased accordingly. At the same time, the Mn-O bond distance and the Mn-O-Mn bond angles changed correspondingly, leading to the slight increase in the lattice constants (a, b and c) and slight expansion of cell volume (V). For the LCSMO:Ag x sample with x = 0.3, the optimal values of temperature coefficient of resistivity (TCR) and magnetoresistance (MR) reached 16.22% K-1 (265.1 K) and 42.07% K-1 (270.48 K), respectively. In addition, the fitting analysis of ρ-T curves showed that the experimental data were consistent with the theoretical calculation data. In the T < T MI (metal-insulator transition temperature) region, the electrical conduction mechanism of LCSMO:Ag x was clarified by electron-magnon, electron-electron and electron-phonon scattering. In the T > T MI region, the resistivity data were interpreted by using the adiabatic small-polaron hopping model. Furthermore, in the entire temperature range, the phenomenological equation called the percolation model was used to explain the resistivity data and the phase-separation mechanism of ferromagnetic metallic (FM) and paramagnetic insulating (PI) phases. All the obtained results indicated that the improvement in the electrical properties of the LCSMO:Ag x samples was attributed to the doping of Ag, which changed the A-site (La, Ca and Sr ions) average ion radius, the Mn-O-Mn bond angles and the Mn-O bond distance. In addition, the grain size increased, which led to improvement in the Mn4+ ion concentration and the GBs connectivity in the LCSMO:Ag x polycrystalline ceramics.
