Nanoscale phase change memory materials

Phase change memory materials store information through their reversible transitions between crystalline and amorphous states. For typical metal chalcogenide compounds, their phase transition properties directly impact critical memory characteristics and the manipulation of these is a major focus in the field. Here, we discuss recent work that explores the tuning of such properties by scaling the materials to nanoscale dimensions, including fabrication and synthetic strategies used to produce nanoscale phase change memory materials. The trends that emerge are relevant to understanding how such memory technologies will function as they scale to ever smaller dimensions and also suggest new approaches to designing materials for phase change applications. Finally, the challenges and opportunities raised by integrating nanoscale phase change materials into switching devices are discussed.     

Reverse selectivity of zeolites and metal-organic frameworks in the ethane/ethylene separation by adsorption

Major advances in the ethane/ethylene separation will be related with the discovery of adsorbents that present preferential adsorption of ethane over ethylene. Metal-organic frameworks (MOFs) are crystalline materials consisting of metal ions, or ion clusters, and organic ligands. Gas chromatography (GC) is most informative for materials screening for the ethane/ethylene separation since it needs only few mg of sample and gives selectivity results at various temperatures. This is illustrated by the methodology presented in this work where materials from two types of families (zeolites and MOFs) were used to show the reversed selectivity that can be found towards the ethane/ethylene separation.     

长波红外硫系玻璃制备技术的研究进展

硫系玻璃具有折射率温度系数低、透过谱段范围宽、光学均匀性好、性能可调和易于加工等优点,被视作新一代温度自适应红外热成像系统用核心元件材料,在红外追踪、红外制导、安防监控、辅助驾驶等多个领域具有广阔的应用前景。为了解决极端服役环境对红外材料的需求,硫系玻璃制备技术研究主要涉及以下三个方面:(1)设计开发大尺寸高纯硫系玻璃的制备方法;(2)进行气氛熔制技术探索研究以解决大尺寸硫系玻璃的工程应用需求;(3)将高能球磨、热压等方式引入到硫系玻璃陶瓷制备上,拓展红外光学材料可选范围,提升硫系玻璃极端环境适应能力。本文基于上述三个方面综述了红外硫系玻璃制备技术的研究进展。     

Mining tailings as a raw material for glass-bonded thermally sprayed ceramic coatings: Microstructure and properties

Magnesium aluminate, MgAl₂O₄, spinel powders for thermal spraying, were synthesized from secondary raw materials by spray drying and subsequent reaction sintering. Talc ore mining tailings and aluminium hydroxide precipitate from aluminium anodizing process were studied. A stoichiometric MgAl₂O₄ spinel coating was prepared as a reference using pure raw materials. Atmospheric plasma spraying resulted in the formation of ceramic coatings. Microstructural investigations revealed that the reference coatings exhibited crystalline lamellar microstructure of MgAl₂O₄ but secondary coatings contained amorphous areas between the crystalline MgAl₂O₄ clusters. Abrasive wear test results revealed considerably lower wear rate for secondary coatings. It is suggested that the different structure of coatings, particularly the high degree of amorphous phase between the isolated crystalline MgAl₂O₄ clusters caused the higher abrasive wear resistance by changing the wear mechanism. The dielectric breakdown strength of the secondary coatings were at the same level, 24 V/μm, as compared to reference coating, 23 V/μm.     

Physical and structural properties of Ge-rich chalcogenide glass sandwiched by GeS crystalline layers

Ge-rich glass-ceramics sandwiched by GeS crystalline layers were fabricated through 10?h thermal treatments at different temperatures. Surface crystallization is evidenced by XRD investigation of glass-ceramic samples polished by different times. SEM observation shows that the thickness of the crystalline layer is about 100?µm for the sample thermally treated at 395?°C for 10?h. The physical properties, including transmission spectra, density, Vickers hardness, and thermal expansion coefficient, were characterized and discussed with the evolution of GeS crystalline layers. This work not only establishes the correlation between microstructure and physical properties of chalcogenide glass-ceramics sandwiched by GeS, but also provides important evidence of structural similarity for understanding the network structure of Ge-S chalcogenide glasses.     

基于脉冲喷射技术的硫系玻璃光纤制备方法探索

硫系玻璃光纤因具有独特的红外光学特性,在红外成像、激光传输和传感等诸多领域具有广阔的应用前景。目前,硫系玻璃光纤的拉制方法主要包括双坩埚法和预制棒拉制法。其中,双坩埚法装置复杂,预制棒拉制法需要提前制备高质量的预制棒。此外,这两种方法均要求玻璃具有较高的抗析晶能力,限制了硫系玻璃光纤新材料的开发。本工作创新性地将脉冲喷射技术引入到硫系玻璃光纤制备领域,通过硫系玻璃光纤纤芯的拉制,探索该方法在玻璃光纤制备上的可行性。通过对玻璃熔体施加持续性的脉冲扰动,坩埚底部小孔处能产生连续的射流,并且在下落过程中发生凝固,从而获得玻璃纤芯。采用该方法,成功制备了一种组成为Ge₂₈Sb₁₂Se₆₀的玻璃光纤纤芯。脉冲喷射法具有装置简单、操作容易等优点,通过连续且规律的脉冲和坩埚内外压力差实现硫系玻璃光纤的拉制,与传统依靠重力拉制的方法相比,脉冲喷射法具有更为丰富的调控手段,从而为新型硫系玻璃光纤的制备提供参考。     

Sol-gel methods for the assembly of metal chalcogenide quantum dots

Sol-gel chemistry represents a powerful method for assembling metal chalcogenide quantum dots into 3D connected architectures without the presence of intervening ligands to moderate particle-particle interactions. Wet gels prepared by the oxidative loss of thiolate surface groups from chalcogenide nanoparticles can be converted to xerogels (low porosity) or aerogels (high porosity), and the quantum-confinement effects in these low-dimensional networks decrease with increasing density of the network. In this Account, we describe the application of sol-gel chemistry to the formation of CdSe architectures and discuss how surface modification can lead to highly luminous monoliths, concluding with the prospects of these unique materials for applications in sensing and photovoltaics.     

The morphology and crystallography of diesel particulate emissions

The morphology and crystallography of particulate emissions from indirect injection diesel engines has been studied using transmission electron microscopy. Samples were collected in a diluted exhaust stream directly onto amorphous carbon films supported on electron microscope grids. Diesel particles closely resemble carbon blacks and consist of chains or clusters of quasispherical subunits. Diffraction patterns were quantified using a microdensitometer and a graphical background correction. Peak intensity was then normalized to background to eliminate exposure variations. Small clusters were found to be less crystalline than large clusters and the centers of two chain-like particulates were more crystalline than the ends. By analogy with the crystallinity studies of heat treated carbon blacks, these differences were interpreted as differences in the time/temperature histories of the particles.     

Nanocrystallization of lead-free Cs3Sb2Br9 perovskites in chalcogenide glass

Lead-free halide perovskite nanocrystals (HPNCs) are an emergent alternative of lead-based halide perovskites owing to their environmentally benign nature. In this work, lead-free Cs₃Sb₂Br₉ HPNCs were precipitated firstly in chalcogenide glass through an elaborated composition design and appropriate crystallization process. The microstructural evolution and nanocrystallization behavior of the Cs₃Sb₂Br₉ crystallized glass-ceramics were analyzed by employing advanced characterization techniques. It is revealed that the crystallization process is temperature-dependent and can be described as two stages. At the first crystallization stage, the nucleation and crystal growth of Cs₃Sb₂Br₉ HPNCs occur due to the presence of structural similarity of [Sb₂Br₉]³⁻ dioctahedral clusters in the precursor glass. And then the second crystalline phase of GeS₂ is successively separated from the residual glassy matrix under high-temperature treatment. This work should be of great guiding significance for developing novel glass-ceramic materials embedded with lead-free HPNCs.     

基于不同材料特性的水辅注塑模拟

基于广义非牛顿流体本构方程,采用有限体积法,对不同类型材料(结晶型材料PE和非结晶型材料HIPS、ABS)的短射法水辅注塑成型进行了数值模拟研究。研究发现,结晶型材料PE因结晶导致的体积收缩性,使其水辅注塑制件的水穿透长度比其它两种非结晶材料更长。当冷却时间过长时,结晶型材料PE因结晶导致的内应力增加,使其水辅注塑制件的水穿透长度比其他两种非结晶材料更短。这3种材料在注水延迟时间或注射量增加的情况下,均不利于水穿透长度的增加。另外,结晶型材料PE比非结晶型材料HIPS和ABS更容易发生二次穿透现象,且当熔体经过一定时间冷却,黏度提高时,更容易发生该现象。     

Investigation of Nitramine‐Based Amorphous Energetics

To date, the use of high explosives (HEs) has been largely limited to their crystalline forms. The glassy forms of these materials have not received much attention. While the highest density and stability of HEs is only attainable in the crystalline state, the unique properties exhibited by glasses may offer significant practical advantages. In this work, the production and properties of glassy molecular dispersions based on CL‐20 with HMX and polyvinyl acetate additives were investigated. The glassy form was achieved by rapid precipitation of the composite ingredients using spray drying. The role of the additives on glass formation was examined. Characterization revealed multiple novel properties in comparison to the traditional crystalline HE‐based compositions. Differential scanning calorimetry (DSC) analysis showed that in the glassy state the materials have a higher configurational enthalpy, by as much as 42 J g⁻¹. Thermomechanical analysis (TMA) results indicate that the glassy materials undergo a glass transition at around 50 °C, during which a transformation from a rigid to a liquid‐like state occurs. The glasses showed a tendency to crystallize, as was observed with TMA and DSC analysis. It was observed that the stability of the amorphous phase with respect to crystallization improved with increasing pellet density. These findings serve to highlight the unique opportunity presented by glassy energetics to achieve improvements beyond those attainable with traditional crystalline materials.     

Effect of heat treatment on AgI-rich chalcogenide glasses with enhanced ionic conductivity

The glass-ceramics of AgI-based electroconductive chalcogenide system was realized using an appropriate heat treatment at a fairly high temperature (T_g + 40°C) and different times ranging from 4 to 20 hours. The crystallization behavior and electroconductive properties of the heat-treated samples were studied in detail. Transmission study was performed, and the results show that the cut-off edge of the short wavelength is red-shifted at prolonged annealing time but remains an excellent transmittance in the mid-infrared (IR) region. XRD and scanning electron microscopy results indicated that the precipitated crystalline phases are mainly β-/γ-AgI. Moreover, a small amount of α-AgI, which rarely existed at room temperature, is precipitated in the AgI-rich chalcogenide glass-ceramics. The ionic conductivity of all glass-ceramics was enhanced by heat treatment in contrast to that of base glass. Raman analysis exhibited the structure variation in the glass sample after heat treatments. This study provided an observation of crystallization in chalcogenide glass containing large amounts of AgI and be of good guidance to fabricate novel AgI-based chalcogenide glass-ceramics that can be candidates in infrared optics and solid electrolyte applications.     

Electrochemical kinetics and X-ray absorption spectroscopy investigations of select chalcogenide electrocatalysts for oxygen reduction reaction applications

Transition metal-based chalcogenide electrocatalysts exhibit a promising level of performance for oxygen reduction reaction applications while offering significant economic benefits over the state of the art Pt/C systems. The most active materials are based on Ru_xSe_y clusters, but the toxicity of selenium will most likely limit their embrace by the marketplace. Sulfur-based analogues do not suffer from toxicity issues, but suffer from substantially less activity and stability than their selenium brethren. The structure/property relationships that result in these properties are not understood due to ambiguities regarding the specific morphologies of Ru_xS_y-based chalcogenides. To clarify these properties, an electrochemical kinetics study was interpreted in light of extensive X-ray diffraction, scanning electron microscopy, and in situ X-ray absorption spectroscopy evaluations. The performance characteristics of ternary M_xRu_yS_z/C (M = Mo, Rh, or Re) chalcogenide electrocatalysts synthesized by the now-standard low-temperature nonaqueous (NA) route are compared to commercially available (De Nora) Rh- and Ru-based systems. Interpretation of performance differences is made in regards to bulk and surface properties of these systems. In particular, the overall trends of the measured activation energies in respect to increasing overpotential and the gross energy values can be explained in regards to these differences.     

Studies on comb-like polymers: 5 A d.s.c. investigation of samples of poly(octadecylethylene) with different history

The crude product of the stereospecific polymerization of octadecylethylene shows, on d.s.c. analysis, two enthalpy changes associated with the melting or the crystallization of two immiscible crystalline phases. It is shown that the crystallization of the crude polymer from dilute heptane solutions leads to a satisfactory separation of supposedly isotactic from atactic material. Contrary to previous statements, it is demonstrated that both these materials display single melting and crystallization phenomena. It is also shown that appropriate annealing treatments lead to a narrow distribution of the crystallite sizes for all the crystalline structured formed by poly(octadecylethylene).     

In situ synchrotron radiation grazing incidence X-ray diffraction—A powerful technique for the characterization of solid-state ion-selective electrode surfaces

An in situ surface study of the iron chalcogenide glass membrane ion-selective electrode (ISE) in aqueous media has been undertaken using a tandem technique of mixed potential/synchrotron radiation grazing incidence X-ray diffraction (SR-GIXRD) and atomic force microscopy (AFM). This work has simultaneously monitored the mixed potential and in situ surface diffraction patterns of this crystalline glassy material, showing that the observed gradual shift of the electrode potential in the anodic direction is linked to the preferential dissolution of the GeSe (1 1 1), GeSe (1 0 1) and GeSe (1 4 1) and/or Sb₂Se₃ (0 1 3), Sb₂Se₃ (2 2 1) and Sb₂Se₃ (0 2 0) surfaces. Expectedly, these observations are internally consistent with preferential oxidative attack of the crystalline regions of the membrane comprising GeSe and/or Sb₂Se₃, as evidenced by AFM imaging of the electrode surface. Clearly, this work corroborates the results of previous ex situ surface studies on the iron chalcogenide glass ISE, whereby it was shown that alkaline saline solutions have a tendency to alter the surface chemistry and concomitant response characteristics of the ISE.     

可加工氟云母玻璃陶瓷研究进展

可加工氟云母玻璃陶瓷是一种新型的结构材料,可用普通的金属刀具进行机械加工,自问世以来一直是陶瓷材料领域的一个研究热点.传统的可加工氟云母玻璃陶瓷中的主晶相是氟金云母和四硅氟云母,近年来又开发出钡云母、钙云母、锂云母和复相氟云母等新型的氟云母玻璃陶瓷,本文综述了近年来国内外在这方面的研究进展.     

The two-step mechanism of nucleation of crystals in solution

The formation of crystalline nanoparticles starts with nucleation and control of nucleation is crucial for the control of the number, size, perfection, polymorph modification and other characteristics of particles. Recently, there have been significant advances in the understanding of the mechanism of nucleation of crystals in solution. The most significant of these is the two-step mechanism of nucleation, according to which the crystalline nucleus appears inside pre-existing metastable clusters of size several hundred nanometers, which consist of dense liquid and are suspended in the solution. While initially proposed for protein crystals, the applicability of this mechanism has been demonstrated for small-molecule organic and inorganic materials, colloids, and biominerals. This mechanism helps to explain several long-standing puzzles of crystal nucleation in solution: nucleation rates which are many orders of magnitude lower than theoretical predictions, nucleation kinetic dependencies with steady or receding parts at increasing supersaturation, the role of heterogeneous substrates for polymorph selection, the significance of the dense protein liquid, and others. More importantly, this mechanism provides powerful tools for control of the nucleation process by varying the solution thermodynamic parameters so that the volume occupied by the dense liquid shrinks or expands.     

高纯硫系玻璃和光纤的制备技术

高纯度硫系玻璃如As-A,As-Se和Ge-As-Se系统都是很有希望作为中红外低损耗的光纤材料，本文主要回顾了制备高纯度的硫系玻璃和光纤材料的一些问题，并对这些问题进行了讨论，最后举例说明了这种光纤的一些实际应用。     

Nanoparticles in Liquid Crystals: Synthesis,Self-Assembly,Defect Formation and Potential Applications

Revolutionary developments in the fabrication of nanosized particles have created enormous expectations in the last few years for the use of such materials in areas such as medical diagnostics and drug-delivery, and in high-tech devices. By its very nature, nanotechnology is of immense academic and industrial interest as it involves the creation and exploitation of materials with structural features in between those of atoms and bulk materials, with at least one dimension limited to between 1 and 100 nm. Most importantly, the properties of materials with nanometric dimensions are, in most instances, significantly different from those of atoms or bulk materials. Research efforts geared towards new synthetic procedures for shape and size-uniform nanoscale building blocks as well as efficient self-assembly protocols for manipulation of these building blocks into functional materials has created enormous excitement in the field of liquid crystal research. Liquid crystals (LCs) by their very nature are suitable candidates for matrix-guided synthesis and self-assembly of nanoscale materials, since the liquid crystalline state combines order and mobility at the molecular (nanoscale) level. Based on selected relevant examples, this review attempts to give a short overview of current research efforts in LC-nanoscience. The areas addressed in this review include the synthesis of nanomaterials using LCs as templates, the design of LC nanomaterials, self-assembly of nanomaterials using LC phases, defect formation in LC-nanoparticle suspensions, and potential applications. Despite the seeming diversity of these research topics, this review will make an effort to establish logical links between these different research areas.     

The effectiveness of published continuum constitutive laws to predict stress-assisted densification of powder compacts

Commonly used constitutive laws for crystalline and viscous materials have been compared to predict the densification behavior under hot-pressing and sinter-forging. Experimental results, from literature for one loading condition, have been used to extract the constitutive laws for amorphous and crystalline materials and, these in-turn, have been used to predict behavior under a different set of loading conditions. Ideally, the constitutive parameters obtained from one set of loading conditions and thermal history should apply to a different set of conditions. However, there is a lack of systematic experimental studies in which this can be checked. In this paper, we use constitutive parameters obtained from one set of conditions to predict the densification response under a different set of loading conditions. For both sintering of amorphous and crystalline materials, we use two different constitutive parameters and compare the predictions of these for the case where experimental results are not available. In addition, the effect of temperature on densification behavior for stress-assisted sintering has been investigated. It is shown that the two commonly used constitutive models for viscous sintering (Scherer and Skorohod–Olevsky) predict similar behavior for amorphous materials. However, for crystalline materials, the predictions of the Riedel–Svoboda and the Kuhn–Sofronis–McMeeking (KSM) models are different. Finally, it is shown that the dependence of the normalized densification on temperature, under constant heating rate conditions, with parameters obtained from isothermal experiments, is a good test for the models.