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Phase Changes [Explained]

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Research reveals how order first appears in liquid crystals

Liquid crystals undergo a peculiar type of phase change. At a certain temperature, their cigar-shaped molecules go from a disordered jumble to a more orderly arrangement in which they all point more or less in the same direction. LCD televisions take advantage of that phase change to project different colors in moving images.

For years, however, experiments have hinted at another liquid crystal state—an intermediate state between the disordered and ordered states in which order begins to emerge in discrete patches as a system approaches its transition temperature. Now, chemists at Brown University have demonstrated a theoretical framework for detecting that intermediate state and for better understanding how it works.

"People understand the ordered and disordered behaviors very well, but the state where this transition is just about to happen isn't well understood," said Richard Stratt, a professor of chemistry at Brown and coauthor of a paper describing the research. "What we've come up with is a sort of yardstick to measure whether a system is in this state. It gives us an idea of what to look for in molecular terms to see if the state is present."

The research, published in the Journal of Chemical Physics, could shed new light not only on liquid crystals, but also molecular motion elsewhere in nature—phenomena such as the protein tangles involved in Alzheimer's disease, for example. The work was led by Yan Zhao, a Ph.D. student in Stratt's lab who expects to graduate from Brown this spring.

Missing link between new topological phases of matter discovered

HZB-Physicists at BESSY II have investigated a class of materials that exhibit characteristics of topological insulators. During these studies they discovered a transition between two different topological phases, one of which is ferroelectric, meaning a phase in the material that exhibits spontaneous electric polarisation and can be reversed by an external electric field.

This could also lead to new applications such as switching between differing conductivities.

The HZB researchers studied crystalline semiconductor films made of a lead, tin, and selenium alloy (PbSnSe) that were doped additionally with tiny amounts of the element bismuth. These semiconductors belong to the new class of materials called topological insulators, materials that conduct very well at their surfaces while behaving as insulators internally. Doping with 1-2 per cent bismuth has enabled them to observe a new topological phase transition now. The sample changes to a particular topological phase that also possesses the property of ferroelectricity. This means that an external electric field distorts the crystal lattice, whereas conversely, mechanical forces on the lattice can create electric fields.

The effect can be used to develop new functionality, which is also of interest for potential applications. Ferroelectric phase-change materials are employed in DVDs and flash memories, for example. An electrical voltage displaces atoms in the crystal, transforming the insulating material into a metallic one.

Theoretical technique to reconcile inconsistencies between crystallographic and chemical experimental results

An ANSTO researcher has co-authored a novel theoretical approach to explain inconsistencies between crystallographic and chemical experimental data in the apparent transformation of a pyrochlore to defect fluorite in La2Zr2O7.

The model can be extended to understand the ageing of a large class of complex oxides, such as spinels, with practical applications ranging from solid oxide fuel cells to the design and management of nuclear waste forms.

The research was published in Scientific Reports.

Prof. Gordon Thorogood, a nuclear fuel cycle researcher, who collaborated with colleagues including David Simeone from CEA and Da Huo a PhD student and others said what was thought to be a phase change to defect fluorite structure, may, in fact, not be occurring.

"Matrix mathematics suggested that a summation of peaks in the crystallographic data will make it look like the peaks have disappeared," said Thorogood.

Alien ice on Earth: Scientists discover how dense, extraterrestrial ice can form in just billionths of a second

Stanford researchers have for the first time captured the freezing of water, molecule-by-molecule, into a strange, dense form called ice VII ("ice seven"), found naturally in otherworldly environments, such as when icy planetary bodies collide.

In addition to helping scientists better understand those remote worlds, the findings - published online July 11 in Physical Review Letters - could reveal how water and other substances undergo transitions from liquids to solids. Learning to manipulate those transitions might open the way someday to engineering materials with exotic new properties.

"These experiments with water are the first of their kind, allowing us to witness a fundamental disorder-to-order transition in one of the most abundant molecules in the universe," said study lead author Arianna Gleason, a postdoctoral fellow at Los Alamos National Laboratory and a visiting scientist in the Extreme Environments Laboratory of Stanford's School of Earth, Energy & Environmental Sciences.

The shape of melting in two dimensions

Snow falls in winter and melts in spring, but what drives the phase change in between?

Although melting is a familiar phenomenon encountered in everyday life, playing a part in many industrial and commercial processes, much remains to be discovered about this transformation at a fundamental level.

In 2015, a team led by the University of Michigan's Sharon Glotzer used high-performance computing at the Department of Energy's (DOE's) Oak Ridge National Laboratory to study melting in two-dimensional (2-D) systems, a problem that could yield insights into surface interactions in materials important to technologies like solar panels, as well as into the mechanism behind three-dimensional melting. The team explored how particle shape affects the physics of a solid-to-fluid melting transition in two dimensions.

Using the Cray XK7 Titan supercomputer at the Oak Ridge Leadership Computing Facility (OLCF), a DOE Office of Science User Facility, the team's work revealed that the shape and symmetry of particles can dramatically affect the melting process. This fundamental finding could help guide researchers in search of nanoparticles with desirable properties for energy applications.

Scientists Observe Self-Healing of a Confined Phase Change Memory Device

A collaboration between the lab of Judy Cha, the Carol and Douglas Melamed Assistant Professor of Mechanical Engineering & Materials Science, and IBM’s Watson Research Center could help make a potentially revolutionary technology more viable for manufacturing.

Phase-change memory (PCM) devices have in recent years emerged as a game-changing alternative to computer random-access memory. Using heat to transform the states of material from amorphous to crystalline, PCM chips are fast, use much less power and have the potential to scale down to smaller chips – allowing the trajectory for smaller, more powerful computing to continue. However, manufacturing PCM devices on a large scale with consistent quality and long endurance has been a challenge.

“Everybody’s trying to figure that out, and we want to understand the phase change behavior precisely,” said Yujun Xie, a PhD candidate in Cha’s lab and lead author of the study. “That’s one of the biggest challenges for phase-change memory.”