Structure and properties of confined sodium nitrite

The temperature evolution of the structure of NaNO(2) nanocomposite ferroelectric material in a porous glass with 7 nm pores was studied by neutron diffraction in temperature region from room temperature up to the melting, i.e. in the ferro- and paraelectric phases. It is demonstrated that in the ferroelectric phase the structure is consistent with the structure of the bulk, but above the ferroelectric phase transition (and up to approximately 513 K) a volume premelted state is formed, manifesting itself in a growth of amplitudes of ion thermal vibrations, a steep increase of elementary cell volume and "softening" of lattice. For the first time the temperature dependence of order parameter eta for confined sodium nitrite is determined. eta (T) follows a power law with T(C)=425.6+/- 2.1 K and beta= 0.31+/- 0.04, which is essentially different from that for bulk NaNO(2). Our obtained data are in a good agreement with the results of earlier dielectric and neutron diffraction measurements.     

Acoustic studies of melting and crystallization of sodium nitrite nanocrystals in the pores of mesoporous silicate matrices

A conventional phase-pulse acoustic method was used to study melting and crystallization of sodium nitrite embedded in the pores of mesoporous silicate matrices. The pore diameter was 20, 37, and 52 Å. The measurements were performed at a frequency of 3–8 MHz in the temperature interval 290–560 K. The temperature dependence of ultrasonic velocity was found to exhibit anomalies corresponding to phase transitions of sodium nitrite. The transitions were smeared in temperature and shifted to lower temperatures from the melting point T _b of bulk sodium nitrite; the shift in crystallization temperature was greater than that of the melting temperature. The irreversible character of melting was revealed. The size dependence of the melting temperature of sodium nitrite was obtained. Phenomena observed in the experiments were discussed with the use of different size effect models.     

用相图原理研究玻璃性质与结构(Ⅰ)核磁共振谱研究Na_2O-B_20_3-SiO_2玻璃结构

本文用电子计算机计算Na_2O-B_2O_3-SiO_2玻璃系统的NMR数据,并根据实验结果,应用相图原理,提出了一种新的玻璃结构模型.本文认为组份简单的玻璃与相同成分化合物的结构相似,而多组份玻璃则是由相图中最邻近的同成份熔融化合物组成的混合物.     

固体和熔融亚硝酸钠的Raman光谱研究

用高温共焦显微激光Raman光谱仪测定了不同温度下固体和熔融状态的亚硝酸钠的拉曼光谱图,获得了有关固体和熔态NaNO2的结构信息,分析了NaNO2熔化过程中的结构变化。     

Dielectric and NMR studies of nanoporous matrices loaded with sodium nitrite

NMR and dielectric studies have been performed on NaNO₂ loaded in mesoporous matrices of MCM-41 and SBA-15 with pore sizes of 20, 37, and 52 Å. The spin-lattice relaxation rate and ²³Na NMR line shape, as well as the complex impedance, were measured within a broad temperature interval including the ferroelectric phase transition in bulk NaNO₂. Two different phases of sodium nitrite, the crystalline and melt phases, are shown to coexist under conditions of a restricted geometry. The crystalline phase undergoes a ferroelectric phase transition. The melt fraction increases with temperature. The existence of two phases accounts for all experimental data on NaNO₂ under conditions of a restricted geometry.     

Evolution of NaNO<Subscript>2</Subscript> in porous matrices

NMR and dielectric studies of NaNO₂ loaded into an SBA-15 mesoporous matrix are reported. The spin-lattice relaxation rate and the ²³Na NMR line shift, as well as the permittivity, were measured within a broad temperature interval including the ferroelectric phase transition in NaNO₂. The phase transition temperature of sodium nitrite in as-prepared samples was shown to differ substantially from that characteristic of a bulk crystal. The permittivity grows strongly in the vicinity of the phase transition. Heating a sample causes the properties of NaNO₂ embedded in pores to gradually approach those of bulk crystals.     

The corrosion properties of phosphate coating on AZ31 magnesium alloy: The effect of sodium dodecyl sulfate (SDS) as an eco-friendly accelerating agent

Sodium nitrite has been used as an accelerating agent in phosphating bath to improve its properties. However, it is well known that sodium nitrite is a carcinogenic component in phosphating sludge. In this study, it has been aimed to replace sodium nitrite by an environmentally friendly accelerating agent. To this end, sodium dodecyl sulfate (SDS) was used in phosphating bath to improve the phosphate coating formation on an AZ31 magnesium alloy. The effect of SDS/sodium nitrite ratio on the phosphated samples properties was also studied. Using field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), direct current (DC) polarization and electrochemical impedance spectroscopy (EIS) the properties of phosphated magnesium samples were studied.Results showed uniform phosphate coating formation on the magnesium sample mostly in hopeite phase composition. In addition, a denser and less permeable coating can be obtained at these conditions. The corrosion resistance of the phosphated samples was superiorly improved using higher SDS concentration in the phosphating bath.     

Nuclear magnetic resonance investigation of metallic sodium nanoparticles in porous glass

Sodium nanoparticles embedded in porous glass have been studied by NMR. The measurements have been carried out on pulse spectrometers in magnetic fields of 9.4 and 17.6 T in a wide temperature range. Changes in the magnitude and temperature dependence of the ²³Na Knight shift with respect to those in bulk sodium have been discovered. An additional component of the NMR line shifted to high frequencies has been observed in the temperature range from 240 to 100 K. Investigation of the specific heat has revealed a considerable decrease in the melting and crystallization temperatures of sodium under nanoconfinement, which were not accompanied by abrupt changes in the Knight shift.     

A 125Te and 23Na NMR investigation of the structure and crystallisation of sodium tellurite glasses

125Te static nuclear magnetic resonance (NMR) and 23Na and 125Te magic angle spinning (MAS) NMR have been used, in conjunction with X-ray diffraction, to examine the structure and crystallisation behaviour of glasses of composition xNa2O.(1-x)TeO2 (0.075 x 0.4). The MAS NMR 23Na spectra from the glasses are broad and featureless but shift by approximately +5 ppm with increased x, i.e. as the system becomes more ionic. The static 125Te NMR spectra show an increase in axial symmetry with increasing x, indicating a shift from predominantly [TeO4] to [TeO3] structural units. The 23Na and 125Te spectra from the crystallised samples have been fitted to obtain information on the sites in the metastable crystal phases, which are the first to form on heating and which are therefore more closely related to the glass structure than thermodynamically stable crystal phases. New sodium tellurite phases are reported, including a sodium stabilised, face centred cubic phase related to delta-TeO2; a metastable form of Na2Te4O9 containing 3 sodium and 4 tellurium sites; and a metastable form of Na2Te2O5 containing 2 sodium sites. There is evidence of oxidation of TeIV to TeVI occurring in glasses with high values of x and, at x=0.40 and 0.50 (outside the glass forming range), some sodium metatellurate (Na2TeO4) is formed at the same time as sodium metatellurite (Na2TeO3). The 125Te shift is very sensitive to environment within the sodium tellurite system, covering more than 320 ppm, with anisotropies varying from 640 to 1540 ppm. The lack of features in the 125Te spectra of the glass phases, combined with the large shift range and high but variable anisotropy, means than it is not possible to obtain a unique fit to any presumed species present. Furthermore, the chemical shift anisotropy parameters for three of the four Te sites in the Na2Te4O9 phase are found to lie outside the range used for previous simulations of glass spectra.     

Temperature dependences of the order parameter for sodium nitrite embedded into porous glasses and opals

The temperature dependences of the order parameter η(T) for sodium nitrite NaNO₂ embedded in porous glasses with average pore diameters of 320 and 20 nm, as well as in artificial opals, have been investigated. It has been demonstrated that the dependence η(T) for sodium nitrite in the porous glass almost coincides with that for the bulk material, whereas this dependence for NaNO₂ in opals differs substantially from that observed in the bulk material and from those previously determined for sodium nitrite in porous glasses with average pore diameters of 3 and 7 nm. It has been revealed that the dependence of the order parameter for sodium nitrite in opals exhibits a temperature hysteresis (approximately equal to 8 K). The temperature dependence η(T) has been described using a simple model, which takes into account the nanopore diameter distribution existing in artificial opals.     

Ferroelectricity in nanocomposites based on porous glass with inclusions of NaNO<Subscript>2</Subscript>

The dielectric nonlinearity and pyroeffect in matrix composite based on porous glass with inclusions of sodium nitrite are investigated. It is found that the composite polarization turns to zero at a temperature below that of the phase transition in the bulk NaNO₂. The obtained dependence of the pyrocoefficient on temperature also corroborates the existence of the phase transition in sodium nitrite nanoparticles included in the porous matrix.     

Influence of energy and entropy on the melting of sodium clusters

Energetic and entropic influences on the melting temperatures of size selected sodium clusters are experimentally separated. It is shown that the energetic difference between solid and liquid is the leading influence for the still puzzling features in the size dependence of sodium melting points. Additionally, this energy difference decreases towards smaller cluster sizes and causes steplike melting phase transitions to vanish. The entropy difference between solid and liquid has been found to be strongly correlated with the energy and causes a pronounced damping of the energetic influences.     

Dielectric parameters of mesoporous sieves filled with NaNO<Subscript>2</Subscript>

This paper reports on a dielectric study of MCM-41 molecular sieves with cellular channels of different sizes filled with the NaNO₂ ferroelectric. The temperature dependences of the permittivity and electrical conductivity of sodium nitrite in cellular channels are calculated from experimental data on the permittivity and electrical conductivity of the composite. The calculations are performed using the relationships obtained for the hexagonal matrix with parallel cylindrical inclusions within pores. The observed increase in the conductivity of sodium nitrite in confined geometry at high temperatures is attributed to partial melting. It is shown that the increase in the permittivity of the composite is caused by Maxwell-Wagner relaxation processes.     

First-principles simulations on the nature of the melting line of sodium

We report a first-principles molecular dynamics study of the reentering behavior that has been recently observed experimentally in the melting line of bcc sodium [Gregoryanz et al., Phys. Rev. Lett. 94, 185502 (2005)10.1103/PhysRevLett.94.185502]. Our results show the liquid phase to be more compressible than the solid phase, and to remain so at high pressures, eventually becoming denser than the solid phase and hence causing the change of slope in the melting line from positive to negative. The maximum of the melting line thus occurs without any accompanying first-order liquid-liquid phase transition.     

Laser implantation of sodium nitrite ferroelectric into pores of synthetic opal

We present the experimental results on implantation of NaNO₂ ferroelectric into pores of synthetic opal by illuminating with focused ultraviolet excimer-laser radiation of a sodium nitrite film located on the synthetic-opal surface. The reflection spectra of broadband radiation from the surface of implanted specimens are compared with the reflection spectra of initial (without any treatment) opals, synthetic opals filled with a saturated aqueous solution of sodium nitrite with subsequently evaporated solvent, and specimens with NaNO₂ embedded as a melt. The possible mechanisms of shifts of reflected- and transmitted-radiation peaks are analyzed. Methods of implanting materials with high melting points into pores of 3D photonic crystals are proposed.     

Order-Parameter Temperature Dependences in Nanocomposites of Porous Glass–Sodium Nitrite

Temperature dependences of the order parameter in nanocomposites based on porous borosilicate glasses with mean pore diameters of 20 and 46 nm and filled with sodium nitrite are studied via the diffraction of synchrotron radiation. The mean diffraction sizes of NaNO₂ nanoparticles in the pores of these matrices and the temperatures of the transition of sodium nitrite to the ferroelectric state are determined. It is shown that the phase transition for these nanoparticles remains a first-order phase transition.     

14N Pulsed nuclear quadrupole resonance. 4. Two-pulse sequences for the determination of T1 and T2 relaxation times

A general theory, based on density matrix calculations, has been developed for the special case of a two-pulse sequence applied to spin 1 (¹⁴N) nuclear quadrupole resonance (NQR) of a powder sample. It is shown that the homolog of the NMR inversion-recovery experiment leads easily to the spin-lattice relaxation time T ₁ (associated with the diagonal elements of the density matrix) provided that an appropriate phase cycling is used. Conversely, in spite of two-step phase cycling schemes adapted to spin-spin relaxation measurements, the homolog of the NMR Hahn spin-echo sequence may pose some problems if the results are displayed in the magnitude mode. First, at short decay times, the echo may be corrupted by unwanted signals. Secondly, in that case, the amplitude of the resulting signal can evolve unexpectedly and differently as a function of the phase of the second pulse. Thirdly, at long decay times, the echo maximum occurs earlier than expected. All these problems in principle disappear with a complete four-step phase cycling scheme and the echo decay curve yields reliably the spin-spin relaxation time T ₂ (associated with off-diagonal elements). This theory allowed the exploitation of many test experiments performed at different frequencies on hexamethylenetetramine (HMT) and sodium nitrite.     

Electrical conductivity of polycrystals of nitrites of metals from the 1A group of the Periodic Table

In the process of research [1], a study was made of electrical conductivity of polycrystals of the nitrites of lithium, sodium, potassium, rubidium, and cesium, as a function of temperature from –150 °C to their melting points: 222, 438, 283, 422, and 400 ° C, respectively. The graphs of specific conductance as a function of inverse absolute temperature of the nitrites of the 1A group of the Periodic Table are broken lines whose sections characterize internal and impurity conductivities. At the points of first order phase transformations, the electrical conductivity of polycrystals of nitrites changes in jumps, as for nitrates [1]; it increases in transition from the noncentrosymmetrical to centrosymmetrical structure. Changes in ionic composition in several nitrites of the 1A group generate regular change of the parameters and of the system of crystalline lattice. The interconnection between ionic migrations in polycrystals of nitrites, energy of crystalline lattice, specific charge of ions, their radii, and energy of activation of ionic migration are shown. Polymorphic transformations in phase transitions in rubidium nitrite at –12 °C, and cesium nitrite at –98 and 80 °C, not described in the literature, were established. Polymorphic modifications of lithium, sodium, and potassium nitrites were confirmed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 7, pp. 105–108, July, 1971.     

典型高分子材料的固体核磁共振研究

本论文通过固体核磁共振（NMR）谱及动力学参量的测量,并结合X-射线衍射技术和DSC测量等研究了两种典型高分子材料的相结构、链的运动以及相与相之间的关系.  乙烯-醋酸乙烯共聚物( EVA) 是最主要的乙烯共聚物之一. 研究发现,EVA的相组成非常复杂,共有5个不同的组分. 除了PE中所观察到的常规单斜晶相和刚性的正交晶相外,我们发现还存在第三个晶相分量－运动性较强的晶相（SOCP,可能是转动相）. 它不仅拥有自己的熔点,而且它的化学位移和分子运动性不同于刚性正交晶相（LOCP）. 另一方面,非晶相也由两种不同的分量组成：运动受限的各相异性的非晶界面相和高度可动的橡胶型的非晶相. 我们进一步详细研究了EVA中的晶区链动力学和非晶区的低温冻结行为. 实验发现,在正交晶相中,高分子链以180° flip-flop方式运动,同时伴随沿链方向的平移型跳跃运动,并引起正交晶相和非晶相之间的长程链扩散,通过NOE的测量证实了这种相间链扩散的存在,并进一步通过实验证实这种相间链扩散是一种受限扩散而不是自由扩散. 同时非晶相的两个组分具有不同的低温冻结行为：当温度低于-弛豫转变温度时,橡胶型的非晶相中的长程分子运动被冻结,但仍存在分子的局域运动;而界面非晶在低温时冻结成一种有序取向结构,并用质子自旋扩散实验证实该有序结构与正交晶相相邻近.  少量纳米级片层状粘土分散在聚合物中就可赋予材料许多优异的性能,我们用固体NMR技术对EVA/REC复合材料的结构和其中粘土的分散性质进行研究,发现上述复合材料中所形成的晶体类型不仅依赖于各组分的性质还依赖于所形成的复合材料的类型.  偏氟乙烯/三氟乙烯共聚物（P（VDF-TrFE））是最主要的铁电高聚物之一. 我们利用变温固体¹⁹F MAS NMR 谱及弛豫数据的测量详细研究了电子辐照对P(VDF-TrFE)共聚物的分子结构、构型、运动性以及相变等的影响. 发现,电子辐照不仅改变了分子链段的构型和运动性,同时也改变了局部分子化学结构. 电子辐照促使铁电相向顺电相(或者非晶相)转变,与此同时诱发了富含VDF和含-TrFE链段从全反式的构型到混合的反式-旁式构型的转变. 电子辐照加剧顺电区域中的分子运动而在高温熔融态中(>100 ℃）,分子的运动反而受限.     

Synthesis, crystal structure, spectral and thermal properties of 4-dimethylaminopyridinium salicylate monohydrate

4-dimethylaminopyridinium salicylate monohydrate (DMAPSA) was synthesized and its crystal structure was determined using single crystal X-ray diffraction analysis. From the crystal structure analysis it can be inferred that the crystal belongs to monoclinic system with space group of P2₁/n. Investigation has been carried out to assign the vibrational frequencies of the grown crystals by FTIR spectral studies. ¹H and ¹³C FT–NMR has been recorded to elucidate the molecular structure. The molecular mass of DMAPSA has been measured using mass spectroscopic analysis. The thermal stability and thermal decomposition of DMAPSA have been investigated by means of thermogravimetric analysis and differential thermal analysis. The melting point of crystal was observed as 172 °C by melting point apparatus. Fluorescence spectra were taken for the excitation wavelength of 240 nm.