基于多弛豫信号补偿的快速磁共振T1ρ散布成像

定量磁共振成像（MRI）可量化组织特性,是科学研究和临床研究的重要工具.旋转坐标系下的自旋-晶格弛豫时间（T_1ρ）能反映水与大分子之间的低频交互作用,在3 T及以上的高场环境下,T_1ρ受水和不稳定质子之间化学交换的影响较大,通过测量弛豫率随自旋锁定场强度的变化而得到其分布情况（T_1ρ散布）,可用于分析和量化质子的交换过程,因此T_1ρ散布是一种重要的定量MRI技术.然而,获得不同自旋锁定场强下T_1ρ加权图像的时间过长,限制了其应用范围.针对这一问题,本研究提出一种基于多弛豫信号补偿策略的快速T_1ρ散布成像方法.该方法将不同锁定频率下的T_1ρ加权图像补偿到同一信号强度水平,并结合低秩与稀疏建立重建模型.实验结果表明,该方法在加速倍数高达7倍时仍获得了较好的重建结果.     

基于T2截止值的幂函数构建毛管压力曲线

岩心核磁共振（NMR）T₂谱和毛管压力曲线都在一定程度上反映了岩石孔隙结构，理论分析表明可利用T₂谱构建毛管压力曲线，由此快速获取储层孔隙结构的信息．本文对18块岩样T₂分布和毛管压力曲线进行了分析，提出将T₂截止值作为幂函数的分段点，采用分段幂函数方法构建岩样的NMR毛管压力曲线，并与采用不分段幂函数方法获得的毛管压力曲线进行了对比．研究结果表明，与不分段幂函数方法相比，分段幂函数方法得到的结果和压汞实验测定的毛管压力曲线吻合度更高，平均拟合度（R²）达到0.943 1，并且论证了T₂截止值作为分段点进行分段幂函数法构建NMR毛管压力曲线的合理性和可靠性．T₂截止值的引入提高了幂函数法构建NMR毛管压力曲线的精度，是利用NMR T₂谱构建岩样毛管压力曲线的有价值探索．     

未掺杂聚乙炔中质子的自旋晶格弛豫——局域化的孤子

测量了未掺杂的顺式聚乙炔(cis-polyacetylene)、反式聚乙炔(trans-polyacetylene)和部分氘代的反式聚乙炔样品的质子自旋晶格驰豫时间T₁,测量了反式聚乙炔的旋转座标系中的质子自旋晶格弛豫时间T_1ρ。结果表明,反式聚乙炔中确实存在大量被称为孤子(Soliton)的自旋中心,它们是质子弛豫的支配因素。孤子是可动的,但又是局域化的。用局域化的孤子模型,不仅说明了T_1ρ过程的多指数型衰减,也解释了它的衰减速率不随温度和自旋锁定场强度而变化的特点。     

13C及29Si核磁共振研究苯乙烯-二甲基硅氧烷嵌段共聚物的弛豫及分子运动

用¹³C及²⁹Si核磁共振研究了苯乙烯(S)及二甲基硅氧烷(Si)嵌段共聚物中硅氧烷软段的固体及溶液谱的自旋-晶格弛豫时间T₁。固态嵌段共聚物主链²⁹Si及侧甲基¹³C的T₁都与均聚物的T₁相近,但在CdCl₃溶液中各种嵌段共聚物的T₁与均聚硅氧烷相差颇大。用偶极-偶极相互作用来解释高聚物的自旋-晶格弛豫。苯乙烯-二甲基硅氧烷嵌段共聚物具两相结构,所以嵌段共聚物中软段及硬段微区中链段的运动与在均聚物分子中链段的运动模式基本相同。而CdCl₃对聚苯乙烯或聚硅氧烷都是良溶剂,软段硬段之间有相互影响。所以其链段运动与均聚物不同,从而导致链段运动的相关时间τ_c变短和T₁的增长。     

基于蒙特卡罗算法的低场核磁共振测量效率提高方法

核磁共振（NMR）的纵向弛豫时间（T₁）、横向弛豫时间（T₂）、自扩散系数（D₀）,以及T₂-T₁、T₂-D₀测量目前广泛应用于石油测井行业.在测量D₀的SGSE序列中,通过逐渐增大90°和180°脉冲之间的时间间隔（T_d）,可以对液体扩散行为产生的影响进行调节.然而T_d的"起点"、"步进数"和"终点"等参数必须设置得当才能准确测量T₁和D₀.目前参数的设置依赖多次的人工调整和测量人员的经验,耗时且使用门槛较高.本文用蒙特卡罗方法进行大量随机模拟,根据前面若干点的测量结果筛选出满足要求的随机值,预测下一个测量点的位置.该算法可以实时更新参数设置,实现自动化测量,达到降低测量门槛、缩短测量时间的目的.经验证,该算法可以适用于T₁、D₀的测量.     

交叉关联噪声对光学双稳系统平均首通时间的影响

研究了乘性色噪声和加性白噪声驱动的光学双稳系统中噪声对系统两个方向平均首通时间T₊（x_s1→x_s2）和T_-（x_s2→x_s1）的影响（平均首通时间是指从一稳态出发越过势垒到另一稳态所用时间的平均值）.通过Novikov定理和Fox近似方法得到相应的Fokker-Planck方程,利用最速下降法得到T₊（x_s1→x_s2）和T_-（x_s2→x_s1）的表达式.研究发现：乘性噪声强度Q和加性噪声强度D对T₊（x_s1→x_s2）的影响相同,对T_-（x_s2→x_s1）的影响不同;T₊（x_s1→x_s2）随乘性色噪声自关联时间τ的增大而增大,但随噪声间的交叉关联强度λ的增大而减小;T_-（x_s2→x_s1）随τ的增大而减小,随λ的增大而增大. 关键词： 平均首通时间 光学双稳系统 乘性色噪声 加性白噪声     

回波间隔对核磁共振表观孔隙度的影响及矫正方法

本文对具有特定横向弛豫时间（T₂）的硫酸铜溶液进行了多回波间隔（T_E）的核磁共振（NMR）实验，并利用数值模拟对32组具有不同弛豫分量的模型进行了变T_E模拟实验，定量研究了T_E对致密油气、页岩气等低孔低渗储层NMR孔隙度的影响规律.实验结果表明，随着T_E的增大，各T₂弛豫组分NMR孔隙度先维持在100%左右，然后迅速衰减，当T_E增加到一定数值时，趋近于0；不同T₂弛豫组分NMR孔隙度开始迅速衰减及最后变为0的T_E值存在显著差异.根据不同T₂弛豫组分NMR孔隙度与T_E的关系，将整个NMR测量分为无损测量区、快速衰减区、无效参数区和仪器盲区4个区域.对特定弛豫组分而言，在快速衰减区弛豫组分损失量与T_E呈对数关系，本文还给出了该区域NMR孔隙度的校正公式及方法.     

结晶PMMA的NMR研究

用NMR方法和DSC技术研究了结晶聚甲基丙烯酸甲酯(PMMA)性质和玻璃化转变温度,并与无定形PMMA作了比较.通过¹³C CPMAS谱,观察到了结晶PMMA与无定形PM MA的谱的线形,线宽,化学位移都十分类似,其中α-CH₃分裂为两个峰,我们将它归属为γ旁式效应所对应的反式和旁式构象.而反门控碳谱显示PMMA中有四种状态,即等规PMMA结晶态和无定形态,间规PMMA结晶态和无定形态.通过测定T₂(H)和旋转坐标系中质子的自旋晶格弛豫时间T_1ρ(H),我们分别得到了两T₂(H)值和两个T_1ρ(H)值,对应于晶区与非晶区,而且其含量分别与反门控碳谱积分所得含量一致.     

固体核磁共振研究半晶聚-3-羟基丁酸酯和聚羟基丁酸戊酸酯的分子动力学

本文在150~370 K温度范围内,采用固体核磁共振（NMR）测定了半晶聚-3-羟基丁酸酯（PHB）,以及3-羟基戊酸酯单体质量分数分别为5%（PHBV5）和12%（PHBV12）的聚羟基丁酸戊酸酯共聚物在实验室坐标系和旋转坐标系条件下质子的自旋-晶格弛豫时间T₁和T_1ρ.通过弛豫时间随温度变化的理论拟合,分别获得上述半晶聚合物晶区和结晶区的分子动力学参数（包括E_a和τ₀）.这些结果从分子水平上阐述了PHB结构修饰和增强的原因.     

块体非晶合金的黏度与玻璃形成能力的关系

基于经典结晶理论讨论了非晶合金的晶化动力学因素和晶化热力学因素对玻璃形成能力（GFA）的影响.分析表明,合金的等温转变（TTT）曲线“鼻尖”温度T_n对应的黏度与晶化阻力因子成正比;重新加热时晶化开始温度T_x对应的黏度与晶化驱动力因子成反比.由此得到了新的GFA参数ω₀=（T_g-T₀）/（T_x-T₀）-（T_g-T₀）/（T_n-T₀）,其中T_g为玻璃转变温度,T₀为理想玻璃转变温度.统计结果显示,ω₀与临界冷却速率具有较高的相关性,R²高达09626.进一步分析表明：新提出的ω₀参数可以合理地解释过冷熔体的黏度、脆性、液相稳定性、热稳定性以及T_rg、ΔT_x、γ、γ_m、ΔT_rg、α、β、δ和φ等参数与GFA的关系. 关键词： 块体非晶合金 黏度 脆性 玻璃形成能力     

用发射光谱法测量氮气直流辉光放电的转动温度

本文报道了氮气气压分别为10和20Pa时，对直流辉光放电的发射光谱进行测量和分析的结果。选择的研究对象为N2放电中形成的N2^ B^2∑u^ →X^2∑g^ 跃迁的Δv=v′-v″=0谱带系中v′=0→v″=0谱带的R支。在阴极背面辉光区、阴极鞘层区、正柱区以及阳极辉光区中分别选择一点进行了转动分辨的发射光谱的测量。利用自己编写的光谱拟合程序，获得了相应的实验条件下N2^ 的转动温度，给出了转动温度随放电电压的变化趋势，其结果可以用直流放电的帕邢定律得到很好的解释。在10和20Pa气压下，放电的阴极鞘层区、正柱区、阳极辉光区中的转动温度都随放电电压呈现出了不同的变化趋势，甚至是完全相反的变化趋势。我们认为这是由于气压不同时，放电状态不同所致：气压为10Pa时的放电是正常辉光放电，而气压为时20Pa的放电为反常辉光放电。     

Incommensurate-commensurate magnetic phase transition in double tungstate Li2Co(WO4)2

Magnetic susceptibility, specific heat, and neutron powder diffraction measurements have been performed on polycrystalline Li$_{2}$Co(WO$_{4}$)$_{2}$ samples. Under zero magnetic field, two successive magnetic transitions at $T_{\rm N1}\sim 9.4$ K and $T_{\rm N2}\sim 7.4$ K are observed. The magnetic ordering temperatures gradually decrease as the magnetic field increases. Neutron diffraction reveals that Li$_{2}$Co(WO$_{4}$)$_{2}$ enters an incommensurate magnetic state with a temperature dependent $\bm k$ between $T_{\rm N1}$ and $T_{\rm N2}$. The magnetic propagation vector locks-in to a commensurate value $\bm k = (1/2, 1/4, 1/4)$ below $T_{\rm N2}$. The antiferromagnetic structure is refined at 1.7 K with Co$^{2+}$ magnetic moment 2.8(1) $\mu_{\rm B}$, consistent with our first-principles calculations.     

High‐pressure μSR studies of ferro‐ and antiferromagnetic metals

High‐pressure μSR experiments on ferromagnetic nickel and \alpha‐iron and antiferromagnetic chromium are reported. In Ni above 260 K B_Fermi was found to be proportional to the saturation magnetization, whereas at lower temperatures it is temperature independent apart from a small anomaly below 30 K which is presumably caused by a magnetoelastic interaction. There was no evidence for an occupation of metastable sites by the μ⁺ below the Curie temperature. By contrast, in \alpha‐Fe the temperature dependence of \curpartialB_μ/\curpartialp shows a structure which might be attributed to the occupation of excited muon states at elevated temperatures. High‐pressure zero‐field experiments on Cr performed in the temperature regime between 4.5 K and 8 K revealed a pressure dependence of B_μ as large as \curpartialB_μ/\curpartialp=-(89.15\pm 0.06)\times 10^-12 T/Pa. In terms of volume dependence a very large negative Grüneisen parameter \gamma =-27 was obtained. This revised version was published online in July 2006 with corrections to the Cover Date.     

Magnetism and giant magnetocaloric effect in rare-earth-based compounds R_3BWO_9(R = Gd,Dy, Ho)

The magnetism and magnetocaloric effect(MCE) of rare-earth-based tungstate compounds R_3 BWO_9(R=Gd,Dy,Ho) have been studied by magnetic susceptibility,isothermal magnetization,and specific heat measurements.No obvious long-range magnetic ordering can be found down to 2 K.The Curie-Weiss fitting and magnetic susceptibilities under different applied fields reveal the existence of weak short-range antiferromagnetic couplings at low temperature in these systems.The calculations of isothermal magnetization exhibit a giant MCE with the maximum changes of magnetic entropy being 54.80 J/kg-K at 2 K for Gd_3 BWO_9,28.5 J/kg-K at 6 K for Dy_3 BWO_9,and 29.76 J/kg-K at 4 K for Ho_3 BWO_9,respectively,under a field change of 0-7 T.Especially for Gd_3 BWO_9,the maximum value of magnetic entropy change(-ΔS_M~(max)) and adiabatic temperature change(-ΔT_(ad)~(max)) are 36.75 J/kg·K and 5.56 K for a low field change of 0-3 T,indicating a promising application for low temperature magnetic refrigeration.     

Observation of multiferroic properties in pyroxene NaFeGe2O6

We report the observation of multiferroicity in a clinopyroxene NaFeGe(2)O(6) polycrystal from the investigation of its electrical and magnetic properties. Following the previously known first magnetic transition at T(N1) = 13 K, a second magnetic transition appears at T(N2) = 11.8 K in the temperature dependence of the magnetization. A ferroelectric polarization starts to develop clearly at T(N2) rather than T(N1) and its magnitude increases up to ~13 μC m(-2) at 5 K, supporting the idea that the ferroelectric state in NaFeGe(2)O(6) stems from a helical spin order stabilized below T(N2). When a magnetic field of 90 kOe is applied, the electric polarization decreases to 9 μC m(-2) and T(N2) slightly increases by 0.5 K. At intermediate magnetic fields, around 28 and 78 kOe, anomalies in the magnetoelectric current, magnetoelectric susceptibility, and field derivative of magnetization curves are found, indicating field-induced spin-state transitions. Based on these electrical and magnetic properties, we provide a detailed low temperature phase diagram up to 90 kOe, and discuss the nature of each phase of NaFeGe(2)O(6).     

激子转移对GaP:N发光热猝灭过程的影响

本文研究了20K-120K范围内GaP:N束缚激子发光与温度的关系,从实验结果发现,各发光中心之间存在着激子转移.初步分析表明,采用带带激发方式,激子转移会导致深束缚中心的热激活能拟合值偏高,其发光热猝灭是由于空穴的热离化所造成的.     

CRITICAL CURRENT DENSITY OF Y-Ba-Cu-Sn-O SUPERCONDUCTORS

The magnetic curves of bulk Y-Ba-Cu-Sn-O have been studied in the temperature range 60-77K and in fields up to 2.4T. The results indicate that J_c varies with the magnetic field as a power law and decreases exponentially with temperature. The critical current density J_c can be improved by the substitution of Sn for Cu.     

Pressure and field induced magnetic order in the spin liquid Tb2Ti2O7 as studied by single crystal neutron diffraction

We have studied the spin liquid Tb2Ti2O7 by single crystal neutron diffraction under high pressure up to 2.8 GPa, together with uniaxial stress, down to 0.1 K, in zero and high magnetic fields up to 7 T. In zero magnetic field, a long-range ordered antiferromagnetic structure is induced by pressure. The Néel temperature and ordered magnetic moment can be tuned by the anisotropic pressure component. Under magnetic field, the antiferromagnetic structure transforms into a canted ferromagnetic one at 0.6 T. Spin canting persists even at 7 T. The magnetic phase diagram under pressure shows a strong increase of the Néel temperature with the field.     

Spin fluctuations and magnetic order in the heavy fermion compound CePt2Sn2

We present zero and longitudinal field μ SR measurements of single crystal and polycrystalline specimens of the heavy fermion compound CePt₂Sn₂. Above 1 K the behaviour of the two samples is indistinguishable; the muon 1/T_1 increases with decreasing temperature until 25 K when it plateaus. The 1/T_1 relaxation rate differs strongly for the two cases below \sim\,0.8\ K. At 0.1 K a rate of about 20 μ s^-1 is seen in the polycrystal while in the single crystal it is only about 5 μ s^-1. Even more revealing is the fact that longitudinal field decoupling spectra at very low temperatures demonstrate an essentially static spin system to be present in the polycrystalline material while the single crystal shows definite dynamic spin properties. We conclude that, in the presence of the distortion, long range magnetic order occurs below 0.9 K while in tetragonal symmetry long range order is suppressed (probably due to frustration) and spin fluctuations remain for T\rightarrow0. This revised version was published online in July 2006 with corrections to the Cover Date.     

Magnetic properties and magnetocaloric effects in Tb6Co1.67Si3 compound

Magnetic properties and magnetocaloric effects of Tb₆Co_1.67Si₃ have been investigated by magnetization measurement. This compound is of a hexagonal Ce$_{6}$Ni$_{2}$Si$_{3}$-type structure with a saturation magnetization of 187\,emu/g at 5\,K and a reversible second-order magnetic transition at Curie temperature $T_{\rm C} = 186$\,K. A magnetic entropy change $\Delta S = 7$\,J\,$\cdot$\,kg$^{-1}$\,$\cdot$\,K$^{-1}$ is observed for a magnetic field change from 0 to 5\,T. A large value of refrigerant capacity (RC) is found to be 330\,J/kg for fields ranging from 0 to 5\,T. The large RC, the reversible magnetization around $T_{\rm C}$ and the easy fabrication make the Tb₆Co_1.67Si₃ compound a suitable candidate for magnetic refrigerants in a corresponding temperature range.