High temperature ferrimagnetic semiconductors by spin-dependent doping in high temperature antiferromagnets

To realize room temperature ferromagnetic (FM) semiconductors is still a challenge in spintronics. Many antiferromagnetic (AFM) insulators and semiconductors with high Neel temperature $T_N$ are obtained in experiments, such as LaFeO$_3$, BiFeO$_3$, etc. High concentrations of magnetic impurities can be doped into these AFM materials, but AFM state with very tiny net magnetic moments was obtained in experiments, because the magnetic impurities were equally doped into the spin up and down sublattices of the AFM materials. Here, we propose that the effective magnetic field provided by a FM substrate could guarantee the spin-dependent doping in AFM materials, where the doped magnetic impurities prefer one sublattice of spins, and the ferrimagnetic (FIM) materials are obtained. To demonstrate this proposal, we study the Mn-doped AFM insulator LaFeO$_3$ with FM substrate of Fe metal by the density functional theory (DFT) calculations. It is shown that the doped magnetic Mn impurities prefer to occupy one sublattice of AFM insulator, and introduce large magnetic moments in La(Fe,Mn)O$_3$. For the AFM insulator LaFeO$_3$ with high $T_N$ = 740 K, several FIM semiconductors with high Curie temperature $T_C >$ 300 K and the band gap less than 2 eV are obtained by DFT calculations, when 1/8 or 1/4 Fe atoms in LaFeO$_3$ are replaced by the other 3d, 4d transition metal elements. The large magneto-optical Kerr effect (MOKE) is obtained in these LaFeO$_3$-based FIM semiconductors. In addition, the FIM semiconductors with high $T_C$ are also obtained by spin-dependent doping in some other AFM materials with high $T_N$, including BiFeO$_3$, SrTcO$_3$, CaTcO$_3$, etc. Our theoretical results propose a way to obtain high $T_C$ FIM semiconductors by spin-dependent doping in high $T_N$ AFM insulators and semiconductors

Experimental and Numerical Studies on Gas Production from Methane Hydrate in Porous Media by Depressurization in Pilot-Scale Hydrate Simulator

Dissociation processes of methane hydrate in porous media using the depressurization method are investigated by a combination of experimental observations and numerical simulations. In situ methane hydrate is synthesized in the Pilot-Scale Hydrate Simulator (PHS), a three-dimensional (3D) 117.8-L pressure vessel. During the experiment, constant-pressure depressurization method is used during the hydrate dissociation. A vertical well at the axis of the PHS is used as the production well. The initial hydrate and aqueous saturations before dissociation are S-H0 = 27% and S-A0 = 37% in volume, respectively. The hydrate dissociates continuously under depressurization and there is little hydrate remaining in the PHS. The hydrate dissociation is an analog of a moving boundary ablation process, and the hydrate dissociation interface separates the hydrate dissociated zone containing only gas and water from the undissociated zone containing the hydrate. The temperature increases in the hydrate dissociated zone near the boundaries, while that in the hydrate undissociated zone around the PHS center basically remains constant. The numerical results of the cumulative gas produced, the remaining hydrate in the deposit, and the temperature spatial distribution all agree well with the experiments, which completes the validation of the mathematical model and numerical codes employed in this study. The heat transfer from the surroundings is predominant in our experimental and numerical cases. The analysis of sensitivity to the intrinsic permeability and the initial hydrate saturation of the numerical simulation are investigated.</p

Gas Production from Methane Hydrate in a Pilot-Scale Hydrate Simulator Using the Huff and Puff Method by Experimental and Numerical Studies

A novel three-dimensional 117.8-L pressure vessel, which is called a Pilot-Scale Hydrate Simulator (PHS), is developed to investigate the gas production performance from hydrate-bearing porous media using the huff and puff method through both experimental and numerical simulations. The methane gas and deionized water are injected into the pressure vessel to synthesize methane hydrate. The grain sizes of the quartz sand in the vessel are between 300 and 450 mu m. The huff and puff stages, including the injection, the soaking, and the production, are employed for hydrate dissociation. A single vertical well at the axis of the PHS is used as the injection and production well. The whole experiment consists of 15 huff and puff cycles. The numerical simulation results agree well with the experiment. Both the experimental and numerical simulation results indicate that the injected water is mainly restricted around the well during the injection stage. The system pressure fluctuates regularly in each cycle, and the secondary hydrate is formed under the pressurization effect caused by the hot water injection in the injection stage. The gas production rate maintains approximately stable in a relatively long period. The sensitivity analysis indicates that the gas production can be enhanced with high intrinsic permeability of the deposit or by raising the temperature of the injected hot water. The mass of the water produced in each cycle has little difference and is manageable when using the huff and puff method.</p