Phase-change Elements Research Articles

Effect of heating/cooling dynamics in the hysteresis loop and tunable IR emissivity of VO2 thin films

We experimentally investigate the semiconductor-to-metal transition (SMT) in vanadium dioxide thin films using an infrared thermographic technique. During the semiconductor to metal phase change process, VO2 optical properties dynamically change and infrared emission undergoes a hysteresis loop due to differences between heating and cooling stages. The shape of the hysteresis loop was accurately monitored under different dynamic heating/cooling rates. In order to quantify and understand the effects of different rates, we used a numerical modelling approach in which a VO2 thin layer was modeled as metamaterial. The main experimental findings are interpreted assuming that both the rate of formation and shape of metallic inclusions are tuned with the heating/cooling rate. The structural transition from monoclinic to tetragonal phases is the main mechanism for controlling the global properties of the phase transition. However, our experimental results reveal that the dynamics of the heating/cooling process can become a useful parameter for further tuning options and lays out a macroscopic optical sensing scheme for the microscopic phase change dynamics of VO2. Our study sheds light on phase-transition dynamics and their effect on the infrared emission spectra of VO2 thin films, therefore enabling the heating/cooling rate to be an additional parameter to control infrared emission characteristics of thermal emitters. The hysteresis loop represents the phase coexistence region, thus being of fundamental importance for several applications, such as the operation of radiative thermal logic elements based on phase transition materials. For such applications, the phase transition region is shifted for heating and cooling processes. We also show that, depending on the way the phase change elements are heated, the temperature operation range will be slightly modified.

Electrothermal Modeling and Design Strategies for Multibit Phase-Change Memory

Electrothermal transport and crystallization dynamics govern the speed and bit stability of multibit phase-change memory (PCM). This paper develops a transient simulation methodology incorporating electrical, thermal, and phase transition models to investigate multibit PCM cell structures and programming strategies. The simulations evaluate two standard PCM structures, namely, the mushroom cell and the confined pillar cell, with feature sizes smaller than 40 nm. The transient simulation captures the phase distribution and cell resistance profile, which are corroborated by transmission electron microscope imaging and the corresponding measured resistance values. This paper also explores a more compact architecture, i.e., the stacked vertical cell, with precise control of Joule heating and potentially more stable intermediate resistance levels. For an electrode area of 10 nm × 20 nm, a low programming current of 60-90 μA generates sufficient heating power to amorphize the phase-change elements sequentially, resulting in four distinct resistance levels distributed over a two-order-of-magnitude resistance range with promise for multibit storage.

Studies on the synthesis, characterization and application of a novel copolymer macromonomer for polymeric solidsolid phase change materials

AbstractA novel copolymer as a macromonomer with a polyethylene glycol (PEG) unit as the phase change element and a vinyl group was synthesized for the preparation of polymeric solid-solid phase change materials. The IR, 1HNMR, DSC and WAXD and POM were employed to determine the structure and properties of the novel monomer. The results show that the novel monomer possess steady molecular structure, excellent crystal properties, higher phase change enthalpy and reactivity. Furthermore, the novel polymeric solid-solid phase change material was also prepared via the copolymerization of the novel copolymer macromonomer and styrene. The phase change properties of the polymeric solid-solid phase change material were characterized.

Thermodynamics of Phase-Change Storage in Series With a Heat Engine

This paper addresses thermodynamics of phase-change storage elements in series with heat engines. It is assumed that the duration of the heat storage and the discharge are equal. It is also assumed that the same heat transfer fluid (HTF) with a constant flow rate is used for the whole cycle. The major constraint imposed on these systems is the stability of the temperature of the HTF supplied to the engine during the storage-discharge cycle. It is shown, for this setup, that the freezing point of the phase-change material (PCM) is defined by the First Law. Maximal stability corresponds to the freezing point equal to the arithmetic mean of the inlet temperatures of the hot and the cold streams. An analytic expression is developed for the Second Law efficiency of the heat storage-removal cycle for the phase-change element in series with an engine. It yields maximal entropy production in the absolute stability limit. Two analytically tractable models of a phase-change storage in series with a heat engine are studied in detail. One involves a PCM slab, and the second involves a PCM tube-and-shell heat exchanger.

Microcomputer analysis of solid/liquid phase change heat transfer

The development of a microcomputer based finite element program featuring phase change (melting and freezing) simulation and facilities is outlined. A closed form Galerkin finite element method derived from a delta function formulation of the latent heat discontinuity in the heat capacity versus temperature function is used within phase change elements of the solution domain. Storage reduction data structures are implemented and compared on the basis of overall program execution time. Analytical solutions for melting and freezing are used to vertify program accuracy and to explore other simulation parameters such as time step size, mesh density and start-up technique. Several “life like” phase change simulations are compared to the results obtained from other numerical models.