一种改进的WOx忆阻器模型及其突触特性分析

忆阻器被定义为第四种基本电子元器件, 其模型的研究呈现多样性. 目前, 忆阻器模型与忆阻器实际特性的切合程度引起了研究者的广泛关注. 通过改变离子扩散项, 提出了一种新的WO_x忆阻器模型, 更好地匹配了忆阻器的实际行为特性. 首先, 新的模型不仅能够描述忆阻器的一般特性, 而且能够俘获记忆丢失行为. 另外, 将新的忆阻器作为神经突触, 分析了脉冲速率依赖可塑性、短期可塑性、长期可塑性, 并发现了与生物系统中极为相似的“经验学习”现象. 最后, 考虑到温度与离子扩散系数的关系, 探讨了温度对突触权值弛豫过程的影响. 实验表明, 新忆阻器模型比原来的模型更切合实际, 且更适合作为突触而应用到神经形态系统之中.     

忆阻突触耦合环形Hopfield神经网络动力学分析及其电路实现

提出一种新型忆阻器模型, 利用标准非线性理论分析三个忆阻特性, 并设计模拟电路。基于忆阻突触, 构建一个忆阻突触耦合环形Hopfield神经网络模型。采用分岔图、李雅普诺夫指数谱、时序图等方法, 揭示与忆阻突触密切相关的特殊动力学行为。数值仿真表明: 在忆阻突触权重的影响下, 它能够产生多种对称簇发放电模式和复杂的混沌行为。实现了该忆阻环形神经网络的模拟等效电路, 并由PSIM电路仿真验证MATLAB数值仿真的正确性。     

基于忆阻器的spiking神经网络在图像边缘提取中的应用

根据生物视觉系统的功能原理,用忆阻器模拟生物突触,结合忆阻器的记忆特性和spiking神经网络的高效处理能力,构造了一种可用于图像边缘提取的三层spiking神经网络模型,该网络用忆阻器电导的变化量来表征图像边缘信息,仿真结果表明,该方法的边缘提取结果具有连续性、光滑性、低误检漏检性和边缘定位准确性,该神经网络的处理过程符合生物信息处理机制,为视觉系统的仿生实现提供了新的思路。     

Reversible alternation between bipolar and unipolar resistive switching in La-SrTiO_3 thin films

The alternation from bipolar to unipolar resistive switching is observed in perovskite La0.01Sr0.99TiO3thin films.These two switching modes can be activated separately depending on the compliance current(Icomp)during the electroforming process:with a higher Icomp(5 mA)the unipolar resistance switching behavior is measured,while the bipolar resistance switching behavior is observed with a lower Icomp(1 mA).On the basis of I–V characteristics,the switching mechanisms for the URS and BRS modes are considered as being a change in the Schottky-like barrier height and/or width at the Pt/La-SrTiO3interface and the formation and disruption of conduction filaments,respectively.     

Pyrolysed pitch-polysilane blends for use as anode materials in lithium ion batteries II: the effect of oxygen

As previously reported, blends of pitch and polysilanes, (Me₂Si)_x(PhMeSi)_y, with various pitch/polysilane ratios were pyrolysed at 1000 °C [W. Xing, A.M. Wilson, G. Zank, J.R. Dahn, Solid State Ionics 93 (1997) 239]. Some of the pyrolysed mixtures demonstrated large reversible capacities for lithium insertion (600 mA h g⁻¹), small irreversible capacities (150–200 mA h g⁻¹) and small hysteresis between charge and discharge cycles. Here, we investigate the role of the oxygen in these materials. The magnitude of the irreversible capacity and hysteresis are correlated to the oxygen content. This suggests that these materials are disordered carbons containing nanodispersed silicon oxycarbide clusters and not nanodispersed silicon, as was previously suggested. This does not change our opinion that pyrolysed pitch-polysilane blends are good alternatives to carbons for anode materials in lithium ion batteries.     

Characterization of solid oxide fuel cells based on solid electrolytes or mixed ionic electronic conductors

The relation between cell voltage (V_cell), applied chemical potential difference (Δμ(O₂)) and cell current (I_t) for solid oxide fuel cells (SOFC) based on mixed ionic electronic conductors is derived by considering also the effect of electrode impedance. Four-probe measurements, combined with current interruption analysis, are considered to yield the relation between ionic current (I_i) and overpotential (η). The theoretical relations are used to analyze experiments on fuel cells with Ce_0.8Sm_0.2O_1.9 and Ce_0.8Gd_0.2O_1.9 electrolytes with La_0.8Sr_0.16CoO₃ or Pt as the cathode and Ni/Ce_0.9Ca_0.1O_1.9−xor Pt as the anode. The electrode overpotentials of these cells, determined by current interruption measurements, are discussed assuming different models including impeded mass transport in the gas phase for molecular and monoatomic oxygen and Butler-Volmer type charge transfer overpotential.     

氧分压对Ni/HfO_x/TiN阻变存储单元阻变特性的影响

采用射频磁控溅射的方法,基于不同氧分压制备的氧化铪构建了Ni/HfO_x/TiN结构阻变存储单元.研究发现,随着氧分压的增加,薄膜表面粗糙度略有降低;另一方面,阻变单元功耗降低,循环耐受性能可达10~3次,且转变电压分布的一致性得到改善.结合电流-电压曲线线性拟合结果及外加温度测试探究了器件的转变机理,得出在低阻态的传导机理为欧姆传导机理,在高阻态的传导机理为肖特基发射机理,并根据氧空位导电细丝理论,对高低阻态的阻变机理进行了详细的理论分析.     

Material design of new lithium ionic conductor, thio-LISICON, in the Li2S–P2S5 system

A new lithium ionic conductor of the thio-LISICON (LIthium SuperIonic CONductor) family was found in the binary Li₂S–P₂S₅ system; the new solid solution with the composition range 0.0≤x≤0.27 in Li_3+5xP_1−xS₄ was synthesized at 700 °C and characterized by X-ray diffraction measurements. Its electrical and electrochemical properties were studied by ac impedance and cyclic voltammetry measurements, respectively. The solid solution member at x=0.065 in Li_3+5xP_1−xS₄ showed the highest conductivity value of 1.5×10⁻⁴ S cm⁻¹ at 27 °C with negligible electronic conductivity and the activation energy of 22 kJ mol⁻¹ which is characteristic of high ionic conduction state. The extra lithium ions in Li₃PS₄ created by partial substitution of P⁵⁺ for Li⁺ led to the large increase in ionic conductivity. In the solid solution range examined, the minimum conductivity was obtained for the compositions, Li₃PS₄ (x=0.0 in Li_3+5xP_1−xS₄) and Li₄P_0.8S₄ (x=0.2 in Li_3+5xP_1−xS₄); this conductivity behavior is similar to other thio-LISICON family with the general formula, Li_xM_1−yM_y′S₄ (M=Si, Ge, and M′=P, Al, Zn, Ga, Sb). Conduction mechanism and the material design concepts are discussed based on the conduction behavior and the structure considerations.     

Artificial synaptic behavior of the SBT-memristor

The synapse of human brain neurons is not only the transmission channel of information, but also the basic unit of human brain learning and information storing. The artificial synapse is constructed based on the Sr_(0.97)Ba_(0.03) TiO_(3-x)(SBT) memristor, which realizes the short-term and long-term plasticity of the synapse. The experiential learning and non-associative learning behavior in accordance with human cognitive rules are realized by using the SBT-memristor-based synapse. The process of synaptic habituation and sensitization is analyzed. This study provides insightful guidance for realization of artificial synapse and the development of artificial neural network.     

Effect of cobalt substitution on cationic distribution in LiNi1 − y CoyO2 electrode materials

A crystal chemistry study of LiNi_{1 − y}Co_yO₂ phases, used as positive electrode in lithium batteries, is presented. These materials crystallize in the rhombohedral system (space group: R m) with a layered structure. Rietveld profile refinement of the X-ray data shows that for low substitution amounts ( ≤ 0.20) extra-nickel ions are always present leading to the Li_{1 − z}Ni_{1 + z − t}CotO₂ (t = y(1 + z)) formula (z * > 0), while for y ≥ 0.30, a pure 2D structure is obtained (z = 0). The stabilization of the 2D character of the structure by cobalt substitution in lithium nickelate leads to the improvement of the electrochemical properties.     

Positive electrode materials for lithium batteries based on VOPO4

A detailed electrochemical study of Li insertion in the -form of VOPO₄ and the optimization of the cycling performance are presented. The redox process occurs in one step close to 3.76 V, along with a phase transition. In order to improve the intercalation kinetics, the electronic conductivity was optimized by introducing a mixed valency, and the ionic conductivity was favored by introducing ‘pillaring’ molecules or ions in the interlayer space. In this way, the electrochemical behaviors of -VOPO₄·2H₂O, -Na_xVOPO₄, -K_xVOPO₄ and -Mg_xVOPO₄ (0≤x≤1) have been studied: the hydrate compound shows a good specific capacity (100 mA h/g at a C/5 regime), but a poor cyclability was mainly because of water oxidation. The inserted large alkaline ions improved the cyclability up to 80 cycles (Na⁺) or over 100 cycles (K⁺). Enhancements of the VOPO₄ specific capacity have been obtained as well by mechanical ball-millings.     

Improvement of electrochemical properties of the spinel LiMn2O4 using a Cr dopant effect

A series of LiCr_xMn_2−xO₄ spinels were synthesised by the Pechini method which enables dopant Cr ions to distribute at Mn sites homogeneously. Neutron diffraction and EDS analysis confirmed that Cr ions do occupy 16d sites (octahedral intestial) evenly in the spinel structure. The Cr dopant effect improves the cyclability of spinel LiMn₂O₄ electrodes and decreases the self-discharge rate substantially. Cyclic voltammetry and AC impedance spectroscopy were employed to characterise the reactions of lithium insertion into and extraction from LiCr_xMn_2−xO₄ electrodes. It was found that a thicker surface layer was formed on the surface of the pure LiMn₂O₄ electrode than on the LiMn₂O₄ electrode.     

Structural and lithium intercalation studies of Mn(0.5−x)CaxTi2(PO4)3 phases (0≤x≤0.50)

Materials from the Mn_(0.5−x)Ca_xTi₂(PO₄)₃ (0≤x≤0.50) solid solution were obtained by solid-state reaction in air at 1000 °C. Selected compositions were investigated by powder X-ray diffraction analysis, ³¹P nuclear magnetic resonance (NMR) spectroscopy and electrochemical lithium intercalation. The structure of all samples determined by Rietveld analysis is of the Nasicon type with the R space group. Mn²⁺/Ca²⁺ ions occupy only the M1 sites in the Ti₂(PO₄)₃ framework. The divalent cations are ordered in one of two M1 sites, except for the Mn_0.50Ti₂(PO₄)₃ phase, where a small departure from the ideal order is observed by XRD and ³¹P MAS NMR. The electrochemical behaviour of Mn_0.50Ti₂(PO₄)₃ and Mn_(0.5−x)Ca_xTi₂(PO₄)₃ phases was characterised in Li cells. Two Li ions can be inserted without altering the Ti₂(PO₄)₃ framework. In the 0≤y≤2 range, the OCV curves of Li//Li_yMn_0.50Ti₂(PO₄)₃ cells show two main potential plateaus at 2.90 and 2.50–2.30 V. Comparison between the OCV curves of Li//Li_(1+y)Ti₂(PO₄)₃ and Li//Li_yMn_0.50Ti₂(PO₄)₃ shows that the intercalation occurs first in the unoccupied M1 site of Mn_0.50Ti₂(PO₄)₃ at 2.90 V and then, for compositions y>0.50, at the M2 site (2.50–2.30 V voltage range). The effect of calcium substitution in Mn_0.50Ti₂(PO₄)₃ on the lithium intercalation is also discussed from a structural and kinetic viewpoint. In all systems, the lithium intercalation is associated with a redistribution of the divalent cation over all M1 sites. In the case of Mn_0.50Ti₂(PO₄)₃, the stability of Mn²⁺ either in an octahedral or tetrahedral environment facilitates cationic migration.     

Lithium insertion to ReO3-type metastable phase in the Nb2O5–WO3 system

Lithium insertion to distorted ReO₃-type metastable solid solution Nb_xW_1−xO_3−x/2 (0≤x<0.25) has been studied by chemical and electrochemical methods. In the course of lithium insertion into tetragonal compounds, transition to a cubic phase was found to occur in the region where values of y (in Li_yNb_xW_1−xO_3−x/2) fall between 0.2 and 0.3, and the phase transition was found to depend on the conditions of the reaction. Changes in OCV and lattice parameters in tetragonal region (y<0.2) were discussed from the viewpoint of the ordering of lithium ions. Also, the component diffusion coefficient of lithium in tetragonal compounds Li_0.1Nb_xW_1−xO_3−x/2 (0≤x≤0.23) was found to increase with niobium content when x≤0.10, and to saturate at 4×10⁻⁹ cm²/s.     

Evaluation of a gate-first process for AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors with low ohmic annealing temperature

In this paper, TiN/AlO_x gated Al Ga N/Ga N metal–oxide–semiconductor heterostructure field-effect transistors(MOSHFETs) were fabricated for gate-first process evaluation. By employing a low temperature ohmic process, ohmic contact can be obtained by annealing at 600℃ with the contact resistance approximately 1.6 ?·mm. The ohmic annealing process also acts as a post-deposition annealing on the oxide film, resulting in good device performance. Those results demonstrated that the TiN/AlO_x gated MOS-HFETs with low temperature ohmic process can be applied for self-aligned gate Al Ga N/Ga N MOS-HFETs.     

A phenomenological memristor model for short-term/long-term memory

Memristor is considered to be a natural electrical synapse because of its distinct memory property and nanoscale. In recent years, more and more similar behaviors are observed between memristors and biological synapse, e.g., short-term memory (STM) and long-term memory (LTM). The traditional mathematical models are unable to capture the new emerging behaviors. In this article, an updated phenomenological model based on the model of the Hewlett–Packard (HP) Labs has been proposed to capture such new behaviors. The new dynamical memristor model with an improved ion diffusion term can emulate the synapse behavior with forgetting effect, and exhibit the transformation between the STM and the LTM. Further, this model can be used in building new type of neural networks with forgetting ability like biological systems, and it is verified by our experiment with Hopfield neural network.     

忆阻Hopfield神经网络动力学分析及其电路实现

提出一种超多稳态忆阻Hopfield神经网络, 它仅包含3个神经元和一个多稳态忆阻突触。从理论上分析神经网络的耗散性和平衡点的稳定性, 并利用分岔图、李雅普诺夫指数谱和相位图等数值方法分析不同忆阻突触耦合强度对神经网络动力学的影响。网络参数固定时, 揭示与初始状态值密切相关的超多稳态性动力学行为。最后, 设计忆阻Hopfield神经网络的模拟等效电路, 并通过PSIM电路仿真验证MATLAB数值仿真结果。     

具有感觉记忆的忆阻器模型

人类记忆的形成包括感觉记忆、短期记忆、长期记忆三个阶段,类似的记忆形成过程在不同材料忆阻器的实验研究中有过多次报道.这类忆阻器的记忆形成过程存在有、无感觉记忆的两种情况,已报道的这类忆阻器的数学模型仅能够描述无感觉记忆的忆阻器.本文在已有模型的基础上,根据有感觉记忆的忆阻器的研究文献中所报道的实验现象,设计了具有感觉记忆的忆阻器模型.对所设计模型的仿真分析验证了该模型对于存在感觉记忆的这类忆阻器特性的描述能力:对忆阻器施加连续脉冲激励,在初始若干脉冲作用时忆阻器无明显的记忆形成,此时忆阻器处于感觉记忆阶段,后续的脉冲作用下忆阻器将逐渐形成短期、长期记忆,并且所施加脉冲的幅值越大、宽度越大、间隔越小,则感觉记忆阶段所经历的脉冲数量越少.模型状态变量的物理意义可用连通两电极的导电通道在外加电压作用下的形成与消失来给出解释.     

基于二维材料MXene的仿神经突触忆阻器的制备和长/短时程突触可塑性的实现

兼具长时程可塑性与短时程可塑性的电子突触被认为是类脑计算系统的重要基础.将一种新型二维材料MXene应用到忆阻器中,制备了基于Cu/MXene/SiO_2/W的仿神经突触忆阻器.结果表明, Cu/MXene/SiO_2/W忆阻器成功实现了稳定的双极性模拟阻态切换,同时成功模拟了生物突触短时程可塑性的双脉冲易化功能和长时程可塑性的长期增强/抑制行为,其中双脉冲易化的易化指数与脉冲间隔时间相关. Cu/MXene/SiO_2/W忆阻器的突触仿生特性,归功于MXene辅助的Cu离子电导丝形成与破灭的类突触响应机理.由于Cu/MXene/SiO_2/W忆阻器兼具长时程可塑性与短时程可塑性,其在突触仿生电子学和类脑智能领域将会具有巨大的应用前景.     

A mimic model of Pt---Rh catalyst prepared by electrochemical deposition of Rh on the Pt(100) surface

Electrochemical deposition of Rh ions on a (5 × 20) Pt(100) surface gave a (1 × 1) LEED pattern with high background intensity. By exposing the (1 × 1) Rh/Pt(100) surface to O₂ or NO, a characteristic p(3 × 1) Rh---O overlayer is built up at about 400 K, which is the same structure observed on the Pt_0.25Rh_0.75(100) surface exposed to NO or O₂. Once the p(3 × 1) Rh---O overlayer is formed, a reversible structural change, ,p(3 × 1) (1 × 1), can be caused at room temperature by adding H₂ and O₂. The p(3 × 1) Rh---O overlayer on the Pt(100) surface may represent a highly efficient catalyst for NO_x reduction.