Reversible Phase Change Research Articles

Single-crystal X-ray and neutron powder diffraction investigation of the phase transition in tetrachlorobenzene

The polymorphic phase transition of 1,2,4,5-tetrachlorobenzene (TCB) has been investigated using neutron powder diffraction and single-crystal X-ray diffraction. The diffraction experiments show a reversible phase change that occurs as a function of temperature with no apparent loss of sample quality on transition between the two phases. Neutron powder diffraction gives detailed information on the molecular structural changes and lattice parameters from 2 K to room temperature. The structure of the low-temperature form has been elucidated for the first time using single-crystal X-ray diffraction. Comparison of the alpha and beta structures show that they are both based on the same sheet motif, with the differences between the two being very subtle, except in terms of crystal symmetry. Detailed analysis of the structures revealed the changes required for inter-conversion. A computational polymorph search showed that these two sheet structures are more thermodynamically stable than alternative herringbone-type structures.

Structure of Phase Change Materials for Data Storage

Phase change materials based on chalcogenide alloys play an important role in optical and electrical memory devices. Both applications rely on the reversible phase transition of these alloys between amorphous and metastable cubic states. However, their atomic arrangements are not yet clear, which results in the unknown phase change mechanism of the utilization. Here using ab initio calculations we have determined the atomic arrangements. The results show that the metastable structure consists of special repeated units possessing rocksalt symmetry, whereas the so-called vacancy positions are highly ordered and layered and just result from the cubic symmetry. Finally, the fast and reversible phase change comes from the intrinsic similarity in the structures of the amorphous and metastable states.

Observation of Ge2Sb2Te5 thin film phase transition behavior according to the number of cycles using Transmission Electron Microscope and Scanning Probe Microscope

ABSTRACTRecently, the development of information technology (IT) increases the demands of memory devices. Phase change random access memory (PRAM), based on the reversible phase change of the chalcogenide alloy, Ge2Sb2Te5, is widely regarded as a favourite candidate for the next generation memory. Because of PRAM has a simple cell structure with high scalability; it is non-volatile, has a relatively high read/write operation speed (Â50ns). The PRAM operation relies on the fact that chalcogenide-based materials can be reversible switched from an amorphous phase to a crystalline state by an external electrical current. It is important to study the electrical property with set/reset cycles, since film thickness shrinkage occurs with the phase transition.In this work, we fabricated the 100nm amorphous Ge2Sb2Te5 thin film on TiN/Ti/Si substrate using dc-magnetron sputtering. The 50X50§2 isolated Ge2Sb2Te5 cell was lithographed by the lift-off pattern and wet etching. And TiN top electrode was deposited using pattern align process at room temperature after the SiO2 insulator CMP. Phase transition behavior with the set/reset cycle was observed using I-V measurement and transmission electron microscope (TEM) on isolated Ge2Sb2Te5 cell. The set/reset programming was operated using tungsten SPM tip which was fabricated using focused ion beam (FIB) lithography. I-V curve which was observed by the I-V probe clearly showed that the phase transition was occurred by applying the electric field through the I-V probe. The resistivity difference between amorphous and crystal state was more than 102. After the phase transition, it was also demonstrated with transmission electron microscope (TEM) analysis. For the preparation of TEM specimen of the amorphous and crystalline cell, focused ion beam (FIB) lithography was adopted.

A Reversible Polymorphic Phase Change Which Affects the Luminescence and Aurophilic Interactions in the Gold(I) Cluster Complex, [μ3-S(AuCNC7H13)3](SbF6) [J. Am. Chem. Soc. 2005, 127, 10838−10839

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionA Reversible Polymorphic Phase Change Which Affects the Luminescence and Aurophilic Interactions in the Gold(I) Cluster Complex, [μ3-S(AuCNC7H13)3](SbF6) [J. Am. Chem. Soc. 2005, 127, 10838−10839].Emily M. Gussenhoven, James C. Fettinger, David M. Pham, , and Alan L. BalchCite this: J. Am. Chem. Soc. 2005, 127, 46, 16338Publication Date (Web):November 1, 2005Publication History Published online1 November 2005Published inissue 23 November 2005https://doi.org/10.1021/ja056393pCopyright © 2005 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views392Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (26 KB) Get e-Alerts Get e-Alerts

A Reversible Polymorphic Phase Change Which Affects the Luminescence and Aurophilic Interactions in the Gold(I) Cluster Complex, [μ3-S(AuCNC7H13)3](SbF6)

Crystallographic examination of [mu3-S(AuCNC7H13)3](SbF6) shows that it undergoes a reversible phase change from orthorhombic to monoclinic upon cooling. At 190 K, the structure shows that two cations self-associate to form a pseudo-octahedral array of six gold atoms connected by both intra- and interionic aurophilic interactions. On cooling, the clusters become less symmetric, and in one, the interionic Au...Au separations increase, while they decrease in the second cluster. The luminescence of crystalline [mu3-S(AuCNC7H13)3](SbF6) shows corresponding changes in emission, with two emissions of similar lifetimes but with different excitations at 77 K, but only a single emission at 298 K. In contrast, [mu3-S(AuCNC6H11)3](PF6), which has a similar structure to that of the high-temperature form of [mu3-S(AuCNC7H13)3](SbF6), does not undergo a phase change or change in its luminescence upon cooling.

Reaction of hydrogen with sodium oxide: A reversible hydrogenation/dehydrogenation system

Sodium oxide, Na 2O, reversibly absorbs hydrogen, H 2, to form sodium hydride, NaH, and sodium hydroxide, NaOH, which possesses a potential to find its application to reversible hydrogen storage. X-ray diffraction measurements illustrate reversible phase and composition changes during the hydrogen absorption and desorption in the Na–O–H system. Pressure–composition (P–C) isotherm and thermogravimetric (TG) measurements exhibit a hydrogen capacity up to 3.0 wt.%. Temperature-programmed desorption (TPD) measurements demonstrate that mixing NaOH to NaH significantly shifts the hydrogen desorption of NaH towards lower temperature, to which TiCl 3 and SiO 2 show catalytic activity. A reaction intermediate, Na H δ − ⋯ H δ ′ + ONa , involving dihydrogen bonding between the negatively charged hydrogen atom bonded to sodium and the positively charged hydrogen atom bonded to oxygen is proposed for the reaction mechanism.

A nonvolatile memory based on reversible phase changes between fcc and hcp

A nonvolatile memory technology utilizing reversible changes between fcc and hcp crystalline phases is proposed. In this new type of phase-change memory, data are stored in different forms of crystalline phases of (Ge/sub 1/Sb/sub 2/Te/sub 4/)/sub 0.8/(Sn/sub 1/Bi/sub 2/Te/sub 4/)/sub 0.2/ chalcogenide alloy. RESET operation produces the less conductive metastable fcc phase via melt-quenching from the more conductive stable hcp phase and SET operation involves a phase change from fcc directly to hcp. Both RESET and SET operations can be completed as fast as 70 ns with large changes in cell resistance.

Reversible Phase Change for C-RAM Nano-Cell-Element Fabricated by Focused Ion Beam Method

A single nano-cell-element of chalcogenide-random access memory wasfabricated by using the focused ion beam method. The minimum contactsize between the Ge2Sb2Te5 phase change film andthe top electrode film for the cell element is with diameter of 60 nm.The reversible phase transition between the RESET state and the SETstate was successively realized. The minimum SET current is obtained tobe about 0.28 mA.

Direct Evidence for Reversible Diffusional Phase Change in Nanometer-Sized Alloy Particles

Solid solubility in isolated nanometer-sized particles has been studied by in situ transmission electron microscopy using alloy particles in the Pb-Sn binary system. In approximately 17-nm-sized particles of a lead-56 at. % Sn solid solution, a phase change from a single phase of the lead solid solution to two phases, a lead solid solution and a tin-solid solution, takes place when the temperature is reduced from 110 degrees C to room temperature (RT). Furthermore, it is confirmed that this phase change could occur rather reversibly when the temperature is cyclically changed between 110 degrees C and RT. This observation provides direct evidence for reversible diffusional phase change in nanometer-sized alloy particles. It seems safe to conclude that the solubility limit of tin in lead is higher than 56 at. % at 110 degrees C, which is almost 5 times higher than the solubility limit of tin in bulk lead (i.e., 10 at. %).

Reversible phase changes of Ni-rich Al?Co?Ni decagonal quasicrystals studied by means of transmission electron microscopy and thermal analysis

In the stability region of the Ni-rich Al–Co–Ni decagonal quasicrystals, phase relationships among three different modulations, i.e. the so-called Ni-rich basic, S1-state and Type-I phases, and their phase changes have been thoroughly investigated by various transmission electron diffraction and imaging techniques in combination with thermal analysis. Electron diffraction experiments have clearly indicated that the S1-state phase formed in the Al 70 Co 12 Ni 18 alloy, which is stable at 900 °C, transforms into the Type-I phase by annealing at 650 °C and that the Ni-rich basic phase formed in the Al 71.5 Co 9 Ni 19.5 alloy, which is stable at 900 °C, transforms into the S1-state phase by annealing at 650 °C. In addition, differential scanning calorimetry measurements have evidenced for the first time that both phase changes occur reversibly at certain temperatures ranged between 700 and 800 °C. The structural features of the modulations mentioned above have been distinguished and uniquely characterized by making the best use of high-angle annular detector dark-field scanning transmission electron microscopy as well as high-resolution transmission electron microscopy.

Reversible phase changes of Ni-rich Al–Co–Ni decagonal quasicrystals studied by means of transmission electron microscopy and thermal analysis

In the stability region of the Ni-rich Al–Co–Ni decagonal quasicrystals, phase relationships among three different modulations, i.e. the so-called Ni-rich basic, S1-state and Type-I phases, and their phase changes have been thoroughly investigated by various transmission electron diffraction and imaging techniques in combination with thermal analysis. Electron diffraction experiments have clearly indicated that the S1-state phase formed in the Al 70Co 12Ni 18 alloy, which is stable at 900 °C, transforms into the Type-I phase by annealing at 650 °C and that the Ni-rich basic phase formed in the Al 71.5Co 9Ni 19.5 alloy, which is stable at 900 °C, transforms into the S1-state phase by annealing at 650 °C. In addition, differential scanning calorimetry measurements have evidenced for the first time that both phase changes occur reversibly at certain temperatures ranged between 700 and 800 °C. The structural features of the modulations mentioned above have been distinguished and uniquely characterized by making the best use of high-angle annular detector dark-field scanning transmission electron microscopy as well as high-resolution transmission electron microscopy.

Sodium percarbonate between 293 and 100 K.

Sodium carbonate perhydrate, Na(2)CO(3).1.5H(2)O(2), commonly known as sodium percarbonate, has been structurally characterized at temperatures ranging from 293 down to 100 K. As the temperature drops, there is a reversible phase change between 250 and 240 K, in which the crystallographic mirror plane of the room-temperature space group, Cmca, is lost, transforming it to Pbca. This transformation is accompanied by a reversal in the expansion coefficient of cell side b, which, like the other two sides, initially contracts on cooling. The two crystallographically independent hydrogen peroxide sites undergo the largest change, where the disorder seen at room temperature gradually diminishes and is almost completely absent at 100 K, thus indicating dynamic rather than static disorder. It is noteworthy that, despite Cmca being the preferred room-temperature space group in the current work, two previous single-crystal studies rejected this space group in favour of non-centrosymmetric Aba2 [Carrondo et al. (1977). J. Chem. Soc. Dalton Trans. pp. 2323-2331; Adams & Pritchard (1977). Acta Cryst. B33, 3650-3653]. A re-evaluation of these determinations indicates that they are identical to the current structure and should be re-assigned to Cmca.

Lithium intercalation in electrodeposited vanadium oxide bronzes

By using an electrodeposition method to form hexavanadates, followed by a thermal process, a range of bronze stoichiometries, M x V 6O 13+ y , where M: K, Ni, or Mo, 0< x<0.5, 0.2< y<3.3 in the ternary system of V 2O 4–V 2O 5–M 2O were prepared. These bronzes adhered well to a conducting substrate and could be directly fabricated into a coin cell without the need for binders or electronic conductors. The phase changes on discharge and the lithium insertion ability were influenced by the amount of the guest metal in the bronze structure. When the value of x was small (<0.5) reversible phase changes resembled those of V 2O 5 or V 6O 13 depending on the initial O 2 content of the bronze after heating. The cycle life was adversely affected by the depth of discharge below 2.2 V for the V 2O 5 type bronzes. The V 6O 13 bronze cathodes had better cycle lives at average operating voltages of about 2.5 V and over 350 Ah/kg for their initial cycles.

Low temperature hydrogen transport using a palladium/copper membrane

Results are presented from low temperature hydrogen permeation experiments using a palladium/copper membrane. Inlet pressure was varied from 5 psig to 180 psig, while temperature was varied from 25°C to 275°C. The palladium/copper membranes exhibited flow stability problems at low temperatures and pressures when using ultra high purity hydrogen. A preconditioning step of high temperatures and inlet pressures of pure hydrogen was necessary to stimulate any substantial permeate flows. After pre-conditioning, results showed zero hydrogen flow when using 3–4% hydrogen mixed with helium or argon. It is thought that the inert gas atoms were adsorbed into the membrane surface and thus blocked the hydrogen atom dissolution. When using pure hydrogen at low to moderate temperatures and low pressures, no measurable permeate flow was observed. Also, zero permeate flow was observed at relatively high temperatures (e.g., 150°C) and a low inlet pressure (5 psig). The cause of the zero permeate flow, when using pure hydrogen, was attributed to interface control of the permeation process. Interface control could be due to: (a) insufficient energy to split the hydrogen molecule into hydrogen atoms, or (b) a reversible phase change from beta to alpha of crystals at the near surface.

Characteristics of OUM Phase Change Materials and Devices for High Density Nonvolatile Commodity and Embedded Memory Applications

ABSTRACTPhase change memory devices were originally reported by S. R.Ovshinsky [1] in 1968. A 256-bit phase-change memory array based on chalcogenide materials was reported in 1970 [2] Recent advances in phase change materials, memory device designs, and process technology have resulted in significant advances in phase change device performance, and a new memory device, called Ovonic Unified Memory (OUM), has been developed. This paper will discuss various device and materials characteristics of OUM phase change memory materials of interest in applications for nonvolatile high-density memories. These materials are generally Te chalcogenide based, exploiting the congruent crystallization of the FCC phase and the associated reduction in resistivity that results from crystallization from the quenched amorphous state. Data storage is a thermally initiated, rapid, reversible structural phase change in the film. While rewriteable DVD disks employ laser heat to induce the phase change and modulate reflectivity, OUM technology uses a short electrical current pulse to modulate resistivity. The device geometry and thermal environment dictate the power and energy required for memory state programming.

A Benchmark Simulation for Moist Nonhydrostatic Numerical Models

A benchmark solution that facilitates testing the accuracy, efficiency, and efficacy of moist nonhydrostatic numerical model formulations and assumptions is presented. The solution is created from a special configuration of moist model processes and a specific set of initial conditions. The configuration and initial conditions include: reversible phase changes, no hydrometeor fallout, a neutrally stable base-state environment, and an initial buoyancy perturbation that is identical to the one used to test nonlinearly evolving dry thermals. The results of the moist simulation exhibit many of the properties found in its dry counterpart. Given the similar results, and acceptably small total mass and total energy errors, it is argued that this new moist simulation design can be used as a benchmark to evaluate moist numerical model formulations. The utility of the benchmark simulation is highlighted by running the case with approximate forms of the governing equations found in the literature. Results of these tests have implications for the formulation of numerical models. For example, it is shown that an equation set that conserves both mass and energy is crucial for obtaining the benchmark solution. Results also suggest that the extra effort required to conserve mass in a numerical model may not lead to significant improvements in results unless energy is also conserved.

Variable Adhesion of Micropatterned Thermoresponsive Polymer Brushes: AFM Investigations of Poly(N-isopropylacrylamide) Brushes Prepared by Surface-Initiated Polymerizations

Thermoresponsive polymer brushes have been grown via surface-initiated atom-transfer radical polymerization from micropatterned substrates (see Figure). Atomic force microscopy both in topography and adhesion force mode shows that the brush domains undergo reversible phase changes in water around the polymer’s lower critical solution temperature. These switchable patterned surfaces may form a new class of microdevices.

Active vibration control of a pneumatic vibration isolator embedded with the electrorheological fluid

Pneumatic vibration isolator is achieved through either passive or active control system, which offers the simple and reliable means to reduce the vibration of a platform system due to a vibration environment. In this paper, a pneumatic passive vibration isolation system is designed and fabricated. The maximum amplitude in Bode plot of the pneumatic passive vibration isolation system is about 6Hz and the value of the transmissibility is 4. Using adaptive control, the improvement of the peak frequency is not effective but the transmissibility can be reduced to be the value 2. The ER fl uid exhibits dramatic and reversible phase change in the presence of the electric field. After applying the ER fluid, the characteristics of the system proposed in the paper are improved and investigated via computer simulation.

Three-coordinate copper(II)-phenolate complexes.

The reactions of LCuCl (L = 2,4-bis((2,6-diisopropylphenyl)imido)pentane (L(iPr)), 2,4-bis((2,6-diisopropylphenyl)imido)-3-chloropentane (L(CliPr))) with the phenolates TlOAr (Ar = C(6)H(3)Me(2), C(6)H(4)OMe, C(6)H(4)tBu) and NaOC(6)H(3)(tBu)(2) were explored. Novel three-coordinate Cu(II)-phenolates, LCuOAr, were isolated from the reactions with the thallium phenolates and were characterized by X-ray crystallography and spectroscopy (UV-vis, EPR). The complexes feature short Cu-O(phenolate) distances (average Cu-O = 1.81 A) and, with one exception, irregular N-Cu-O(phenolate) angles that differ within each compound (15 degrees < Delta < 28 degrees, where Delta = angleN(1)-Cu-O - angleN(2)-Cu-O). The exception is L(iPr)Cu(OC(6)H(4)tBu), for which X-ray structures at -100 and 25 degrees C differed due to an unusual reversible phase change with nonmerohedral twinning (2:1 ratio) in the low-temperature form. The high-temperature form has local C(2)(v) symmetry (Delta = 0 degrees ), and upon cooling below the phase transition temperature (-8 +/- 5 degrees C) lateral movement of the phenolate ligand (Delta = 17.6 degrees ) and rotation of the phenolate plane by 10.7 degrees occurs. Resonance Raman spectroscopic data acquired for L(iPr)Cu(OC(6)H(4)tBu) corroborated assignment of phenolate --> Cu(II) LMCT character in the UV-vis spectra. Cyclic voltammetry experiments (THF, 0.5 M NBu(4)PF(6)) revealed negative E(1/2) values for the Cu(II)/Cu(I) couples relative to NHE, consistent with enhanced stabilization of the Cu(II) state by both the strongly electron donating beta-diketiminate ligand and the phenolates. Although thermally stable, the Cu(II)-phenolates are unusually reactive with dioxygen, albeit to give product(s) that have yet to be identified. In the reaction of L(iPr)CuCl with NaOC(6)H(3)(tBu)(2) no Cu(II)-phenolate was observed. Instead, a Cu(I) complex was generated quantitatively by trapping with added isocyanide, [L(iPr)CuNC(C(6)H(3)Me(2))], along with 3,3',5,5'-tetra-tert-butyl-4,4'-dibenzoquinone and 2,6-di-tert-butylphenol in 27 +/- 3% and 46 +/- 6% yields, respectively, corresponding to the overall reaction 4L(iPr)Cu(II)Cl + 4NaOAr --> 4L(iPr)Cu(I) + 4NaCl + dibenzoquinone + 2(phenol).

Reversible phase change in Bi xSe 100− x chalcogenide thin films for using as optical recording medium

The microstructure of electron beam deposited Bi x Se 100− x thin films (where x varies from 0 to 16 at.%) was investigated. The morphology of crystallization for in situ thermally annealed and electron beam heated Bi 16Se 84 films was investigated using transmission electron microscopy. Selected area electron diffraction was used to characterize different phases observed during the crystallization process where the crystalline Bi 2Se 3 phase was separated. Optical absorption measurements were carried out on as-deposited Bi x Se 100− x films of different compositions. It was found that the mechanism of the optical absorption follows the rule of non-direct transition. The optical energy gap decreases with increasing Bi content. The effect of thermal annealing on the optical properties of Bi 16Se 84 films was investigated. The study indicated that Bi 16Se 84 films have low threshold energy for amorphization, high optical absorption coefficient and optimum contrast between the amorphous and the crystalline states. The decrease in the optical gap is discussed on the basis of amorphous-crystalline transformations.