High Density Phase Change Research Articles

Thickness dependence and crystallization properties of amorphous GeTe thin films on silicon dioxide

Radio frequency magnetron sputtering was used to prepare the amorphous GeTe thin films on silicon dioxide and the thickness effects on the crystallization behavior were investigated. With the film thickness reducing, the crystallization temperature, crystallization activation energy, amorphous and crystalline resistance increase remarkably, indicating the great improvement in thermal stability and power consumption. Ozawa’s model was used to estimate the crystallization kinetics of GeTe thin films, it shows that nucleation and grain growth occur simultaneously, and grain growth dominates ultimately. XRD analysis demonstrated that the grain size can be reduced and the crystallization process of GeTe thin film can be inhibited with the film thickness decreasing. Furthermore, the thinner film has smaller resistance drift index and surface roughness, which are beneficial to improve the reliability of storage device. T-type phase change memory devices based on 25 nm GeTe thin film were fabricated by 0.13 μm CMOS technology, and the current–voltage and resistance-voltage characteristics demonstrate the excellent electrical performance, including the fast resistance switching between SET and RESET processes, low threshold current and voltage. All the results proved the strong dependency relationships between the crystallization properties and film thickness of GeTe thin film, which paves the way for developing high-density phase change memory in the fields of big data and artificial intelligence.

Phase change materials integrated buildings: A short review

Buildings account for one-third of global energy consumption and 38% of greenhouse gas emissions. Improving a building’s energy efficiency is vital in minimizing climate change. As an alternative to active cooling systems, passive cooling methods are promising. Thermal energy storage employing latent heat is an effective passive cooling strategy for increasing a building’s thermal inertia and, in turn, reducing temperature fluctuations and improving thermal comfort for building occupants. To do this, high-density phase change materials (PCMs) for thermal energy storage (TES) can be put to good use. Recent developments in TES techniques using PCMs have gained much research focus, primarily to improve energy efficiency and promote clean energy sources. PCMs are regarded as the most promising materials due to their high energy storage density for developing high-performance and energy-efficient buildings. The primary disadvantage of PCM is its low thermal conductivity, limiting its practical usage, which could be resolved by loading nano or micro-sized conductive fillers. The investigated system’s initial findings show that they effectively lower indoor temperature changes and energy demand during winter seasons and can cause load reduction or shifting.

Flow pattern determination and rheological characteristics of high-density phase change material slurry in homogeneous flow

Erythritol slurry has shown great potential as a heat transport medium. We investigated the versatility of the flow pattern determination method and the effect of gravity on the rheological characteristics of erythritol slurry, with a focus on homogeneous flow. In vertical flow, the flow pattern was always homogeneous, whereas in horizontal flow, it depended on the flow velocity. Thus, the plots obtained from the horizontal flow were divided into two parts using linear regression analysis, and the discussion focused on the homogeneous part, which is the high-velocity region. Erythritol slurry behaved as a power law fluid in a homogeneous flow, regardless of the flow direction. However, the viscosity indices differed in the vertical and horizontal flows. The non-Newtonian behavior became more pronounced in the horizontal flow than in vertical flow. These findings may be valuable for the design and optimization of heat transport systems that use high-density PCM slurry.

Encoding multistate charge order and chirality in endotaxial heterostructures

High-density phase change memory (PCM) storage is proposed for materials with multiple intermediate resistance states, which have been observed in 1T-TaS2 due to charge density wave (CDW) phase transitions. However, the metastability responsible for this behavior makes the presence of multistate switching unpredictable in TaS2 devices. Here, we demonstrate the fabrication of nanothick verti-lateral H-TaS2/1T-TaS2 heterostructures in which the number of endotaxial metallic H-TaS2 monolayers dictates the number of resistance transitions in 1T-TaS2 lamellae near room temperature. Further, we also observe optically active heterochirality in the CDW superlattice structure, which is modulated in concert with the resistivity steps, and we show how strain engineering can be used to nucleate these polytype conversions. This work positions the principle of endotaxial heterostructures as a promising conceptual framework for reliable, non-volatile, and multi-level switching of structure, chirality, and resistance.

Temperature-sensitive and self-healing shape-stable phase change materials containing temperature adjustable ionic liquid for improving the recycling stability and the applicability in cold energy storage

Advanced shape-stable phase change materials (SSPCMs) with environment self-adaptation are becoming more meaningful in regulating temperature in diverse applications. In this manuscript, a series of self-healing SSPCMs containing ionic liquids were fabricated by temperature-responsive polymers (PNIPAM) through the free radical polymerization method. 1-ethyl-3-methylimidazolium dicyanamide ([Emim][DCA]) ionic liquid was selected as the cooling agent and PCM. 2-ureido-4[1H]-pyrimidinone (UPy) and chitosan oligosaccharide (COS) were chosen as modifiers. Nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy were used to determine the chemical structure. The surface morphology of SSPCMs was observed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and polarizing optical microscopy (POM). A rotational rheometer and electronic universal testing machine were used to assess the self-healing effectiveness and mechanical properties of SSPCMs. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to examine the thermal characteristics and thermal reliabilities of self-healing SSPCMs. The results indicate that the self-healing efficiency of SSPCM is 91.04 % within 5 min, and compressive strength is improved by 65.7 % by the ionic liquid. The SSPCMs perform thermo-sensitive properties, which allow SSPCMs to maintain a stable shape in response to temperature change and reduce PCMs leakage. Furthermore, the SSPCMs can quickly and effectively recover their mechanical properties after damage thanks to the excellent self-healing properties. Meanwhile, the SSPCMs have a longer lifespan for improving recycling efficiency and are more environmentally friendly. The developed SSPCMs with recycle stability have a high energy density (251.5 J/g) and adjustable phase change temperature properties. The developed SSPCMs can be made to fit various temperature applications by changing the concentration of ionic liquids in the phase change system. Overall, the SSPCMs show promising potential for cold energy storage and thermal management.

Experimental results of embedded phase change material capsules for increasing the performance of a wrapped heat pump water heater

To enhance the capacity of heat pump water heaters (HPWHs), high energy density phase change material (PCM) was used in two experimentally tested HPWH prototypes. Preliminary findings showed that in the PCM, stored thermal potential energy could be effectively converted into sensible heat in the water over a slight temperature difference, with the goal of increasing the capacity of hot water above a desired cut-off temperature. This exchange of energy results in an increased amount of hot water necessary for mixing at the location utilized by the consumer. It was also found that for a wrapped HPWH, there was an average increase of 27.7% in the capacity when 7.6 L of novel PCM filled spherical capsules were added to a 22.7 L tank downstream of the HPWH. In the second design (Beta), PCM capsules were immersed into the tank with a cylindrical geometry, and the UEF was increased by 5.5% over the baseline HPWH test result. As for the Beta design, heat loss was reduced significantly compared to a typical HPWH and the Alpha design. Food-grade PCMs were selected as appropriate. The novel encapsulation design increased the effective conductivity inside the capsule by adding metal fins.

Nanoscale Phase Change Material Array by Sub-Resolution Assist Feature for Storage Class Memory Application

High density phase change memory array requires both minimized critical dimension (CD) and maximized process window for the phase change material layer. High in-wafer uniformity of the nanoscale patterning of chalcogenides material is challenging given the optical proximity effect (OPE) in the lithography process and the micro-loading effect in the etching process. In this study, we demonstrate an approach to fabricate high density phase change material arrays with half-pitch down to around 70 nm by the co-optimization of lithography and plasma etching process. The focused-energy matrix was performed to improve the pattern process window of phase change material on a 12-inch wafer. A variety of patternings from an isolated line to a dense pitch line were investigated using immersion lithography system. The collapse of the edge line is observed due to the OPE induced shrinkage in linewidth, which is deteriorative as the patterning density increases. The sub-resolution assist feature (SRAF) was placed to increase the width of the lines at both edges of each patterning by taking advantage of the optical interference between the main features and the assistant features. The survival of the line at the edges is confirmed with around a 70 nm half-pitch feature in various arrays. A uniform etching profile across the pitch line pattern of phase change material was demonstrated in which the micro-loading effect and the plasma etching damage were significantly suppressed by co-optimizing the etching parameters. The results pave the way to achieve high density device arrays with improved uniformity and reliability for mass storage applications.

Modeling and simulations of high and hypervelocity impact of small ice particles

We present a computational model based on the Hot Optimal Transportation Meshfree (HOTM) method and a thermo-visco-elasto-plastic constitutive model for the high-fidelity simulation of high and hypervelocity impact of small ice particles. The competition and combination among various energy dissipation mechanisms in the high energy density event, including plasticity, fracture/fragmentation, and phase change, are predicted by minimizing the thermomechanical system’s potential energy within a variational structure. The variational Eigenerosion algorithm is incorporated in the HOTM method to simulate crack nucleation, propagation, and fragmentation. A multiphase thermo-visco-elasto-plastic model is developed to describe the dynamic response of ice under extreme loading conditions, such as strain and strain-rate hardening, temperature-dependent strength, pressure-dependent viscosity, and phase diagram. The proposed computational framework is validated by comparing to experimental observations and measurements from two ballistic tests at different strain rates. Numerical studies are performed for the impact tests of a microscopic spherical ice particle(⌀800nm) against a Ti-6Al-4V plate at velocities ranging from 250m/s to 5000m/s at an initial temperature of 200K. The evolution of failure modes in the ice projectile is well captured as a result of the energy partitioning explicitly into plasticity, fracture and phase change at different moments. The analysis demonstrates the transition of the dominant failure mechanism due to the increasing input energy density and the nature of stress wave propagation. The competition between fracture and phase change, in the presented configuration of numerical studies, starts when the impact velocity approaching 800m/s and thermal effects play a critical role in determining the local deformation and failure.

A techno-economic assessment on the adoption of latent heat thermal energy storage systems for district cooling optimal dispatch & operations

The adoption of high-density phase change materials as cold thermal energy storage media have been considered as potential alternatives to water and ice. In this paper, the implementation of these media has been investigated in the context of district cooling in mixed-used building micro-grids whereby the cold thermal energy storage is retrofitted into an existing eco-building cluster. The techno-economic feasibility of implementing phase change materials, which includes eutectic salt, polyethylene glycol and paraffin as cold thermal energy storage media for district cooling, have been evaluated by computing the economic dispatch of a cooling bus network using mixed-integer quadratic programming. Furthermore, the impact of high-density storage media with a reduced footprint on the integration and design of cold thermal energy storage systems in district cooling has been included in the analysis. The capital expenditure of cold thermal energy storage systems, the land rental price for required occupancy and operating expenditure are used to determine the net present value after 20 years. Through peak shaving, optimally sized cold thermal energy storage has shown a better alternative substituting one of the two chillers. The smaller chiller (1200 kWc) is no longer required in the plant, giving significant savings in capital expenditure. The cost-benefit analysis also showed that only eutectic salt phase change materials are competitive with ice, with a net present value approaching 200,000 $ after 20 years. When phase change materials are adopted, especially if compared with water, the building operations and related costs are significantly less affected by large fluctuations in land rental price given its smaller footprint, thus posing significantly lower financial risks.

Thickness effect of Sn<sub>15</sub>Sb<sub>85</sub> phase change film

Sn<sub>15</sub>Sb<sub>85</sub> thin films with different thickness are prepared by magnetron sputtering. The evolution of Sn<sub>15</sub>Sb<sub>85</sub> thin film from the amorphous state to the crystalline state is studied by an <i>in-situ</i> resistance temperature measurement system. The crystallization temperature, electrical resistance, crystallization activation energy, and data retention capacity of Sn<sub>15</sub>Sb<sub>85</sub> thin film increase significantly with film thickness decreasing. The near infrared spectrophotometer is used to record the diffuse reflectance spectra of amorphous Sn<sub>15</sub>Sb<sub>85</sub> film. The results show that the band gap energy increases with film thickness decreasing. The surface morphology of Sn<sub>15</sub>Sb<sub>85</sub> film after being crystalized is observed by atomic force microscope, which shows that the thinner film has lower roughness. The analysis of X-ray diffraction indicates that the grain size becomes smaller and the crystallization may be inhibited by reducing the film thickness. T-type phase change memory cells based on Sn<sub>15</sub>Sb<sub>85</sub> thin films with different thickness are fabricated by the CMOS technology. The electrical performances of phase change memory show that the thinner Sn<sub>15</sub>Sb<sub>85</sub> film has a larger threshold switching voltage and smaller RESET operation voltage, which means the better thermal stability and lower power consumption. The outcomes of this work provide the guidance for designing the high-density phase change memory by reducing the size of Sn<sub>15</sub>Sb<sub>85</sub> thin film.

Investigation of V2O5/Ge8Sb92 multilayer thin film for high-data-retention and high-speed phase change memory applications

In this paper, the V2O5/Ge8Sb92 multilayer thin films were prepared and investigated. Compared with Ge8Sb92, V2O5/Ge8Sb92 film had a higher crystallization temperature and a larger conductivity activation energy. The surface roughness for V2O5/Ge8Sb92 film was small before and after crystallization. The crystallization of V2O5/Ge8Sb92 was inhibited and the grain size was only 5.0 nm. A higher stability and smaller grain made V2O5/Ge8Sb92 multilayer film a potential application in high-density phase change memory.

Zn-doped Sb70Se30 thin films with multiple phase transition for high storage density and low power consumption phase change memory applications

The single layer Zn44Sb39Se17 thin film, being fabricated by Sb70Se30 and Zn using co-sputtering method, exhibits double phase change transition. The double phase change processes are mainly attributed to the crystallization of the Sb and ZnSb phases. The potential operating temperature for ten years of two phase change processes are 64 °C and 179 °C, of which the good stability of the second phase change can be sufficient for the autoelectronic applications. Meanwhile we also discovered the Zn44Sb39Se17 thin film presents apparent low power consumption in comparison with Ge2Sb2Te5 and other reported Sb-Se based phase change materials.

Sb-rich CuSbTe material: A candidate for high-speed and high-density phase change memory application

Sb-rich CuSbTe material is proposed to meet the requirements of low-power consumption and high-speed of phase change memory application. Sb-rich CuSbTe material is based on Sb-rich SbTe (Sb3Te) material which has a fast crystallization speed. The poor amorphous thermal stability of Sb3Te material can be solved by the addition of Cu. Cu0.25Sb3Te-based device is expected to achieve fast access speed (6 ns) and maintain a high data retention (10-year: 85 °C) in comparison with that of Ge2Sb2Te5 (10-year: 78 °C). With the doping of Cu, the power consumption and the grain size of Sb3Te-based device are greatly reduced. Under the pulse width of 6 ns, the RESET voltage is only 4.1 V. Moreover, the operation cycles is close to 1 × 105, and the resistance ratio is nearly two orders of magnitude. Finally, the XRD, TEM and XRR measurements were carried out to obtain the microscopic analysis for Cu0.25Sb3Te alloy.

SbSe/ZnSb stacked thin films with multi-level phase transition for high density phase change memory applications

As a highly integrated non-volatile memory device, phase change memory (PCM) has attracted considerable attention. In this paper, SbSe/ZnSb (SS/ZS) stacked thin films were developed for high density phase change memory applications. SS/ZS stacked thin films show two pronounced resistance steps with the increase in temperature and possess excellent thermal stability. The X-ray diffraction reveals that Sb, Sb2Se3 phases crystallize first, hexagonal ZnSb phase forms then at higher temperatures. The stacked thin films exhibit subtle variations in thickness and roughness after crystallization. Ultrafast crystallization speed (9.68 ns) in the SS(25 nm)/ZS(25 nm) thin film was validated by picosecond laser pump-probe system. Phase-change memory cells based on SS(25 nm)/ZS(25 nm) stacked thin film can realize multi-level data storage and the whole SET and RESET operations can be implemented with a 20 ns electrical pulse. Thus, SS/ZS stacked thin films have advantages for multi-level data storage capability, better thermal stability, and fast phase change speed, and are therefore good candidates for high density PCM device.

Size effect on the thermal conductivity of octadecanoic acid: A molecular dynamics study

Octadecanoic acid (OA) is well-known as an excellent phase change material (PCM) for heat storage due to its high energy density and narrow phase change temperature. In the practical application of PCMs, OA is often filled into the porous or microencapsulated matrix in micro/nanoscale forms. Generally, the thermal transport properties of OA are greatly subject to its size effect, and unfortunately only its macroscopic thermal transport properties are well studied in the composite field. To meet the demands of efficient OA-filled composite PCMs, OA’s nanoscale thermal transport properties are undoubtedly significant. In this study, an atomic simulation was conducted through a top-down approach based on equilibrium molecular dynamics to calculate the size-dependent thermal conductivity (κ) of OA in bulk, nanowire, and nanochain forms. The vibrational density of states and phonon overlap energy for the three forms of OA were compared to extract the size effect of κ. The largest κ value occurred for the bulk form while the smallest value occurred for the nanochain form. The limited sizes of nanowire forms reduce the atom vibration and weaken the phonon matching level, thus resulting in a lower κ than the bulk form. The curled structures of the nanochain forms exhibit higher molecular distances and more phonon scattering probabilities under weak van der Waals forces, leading to the lowest thermal transport. This study’s findings provide a clear understanding of the size dependence of thermal transport in OA for constructing OA-filled PCMs.

Numerical calibration and experimental validation of a PCM-Air heat exchanger model

Ambitious goals have been set at international, European and French level for energy consumption and greenhouse gas emissions decrease of the building sector. Achieving them requires renewable energy integration, a technology that presents however an important drawback: intermittent energy production. In response, thermal energy storage (TES) technology applications have been developed in order to correlate energy production and consumption of the building. Phase Change Materials (PCMs) have been widely used in TES applications as they offer a high storage density and adequate phase change temperature range. It is important to accurately know the thermophysical properties of the PCM, both for experimental (system design) and numerical (correct prediction) purposes.In this paper, the fabrication of a PCM – Air experimental prototype is presented at first, along with the development of a numerical model simulating the downstream temperature evolution of the heat exchanger. Particular focus is given to the calibration method and the validation of the model using experimental characterization results. Differential scanning calorimetry (DSC) is used to define the thermal properties of the PCM. Initial numerical results are underestimated compared to experimental ones. Various factors were investigated, pointing to the ineptitude of the heat capacity parameter, as DSC results depend on heating/cooling rates. Adequate heat capacity curves were empirically determined, depending on heat transfer rates and based on DSC results and experimental observations.The results of the proposed model are confronted with experimental characterization data at different points of the unit and for various airflow rates. A good agreement is observed, showing an average difference ranging from 0.53°C to 0.75°C for the surface and PCM values and from 0.87°C to 1.2°C for the outlet air temperature values.

Improvement of phase change properties of stacked Ge2Sb2Te5/ZnSb thin films for phase change memory application

Ge2Sb2Te5/ZnSb (GST/ZS) stacked thin films were proposed for high density phase change memories (PCM). Electrical and structural properties were studied by in-situ resistance measurements and X-ray diffraction (XRD), respectively. The films exhibited good thermal stability and two resistance steps during heating process. A picosecond laser pump-probe system was used to measure phase change speed. Phase change memory cells based on [GST(35nm)/ZS(15nm)]1 thin film were fabricated to test and verify multi-level switch between set and reset states.

Crystallization behaviors of Ga30Sb70/GeTe nanocomposite multilayer thin films

The multi-step phase change behaviors in Ga30Sb70/GeTe (GS/GT) nanocomposite multilayer films are investigated through in situ film resistance and x-ray diffraction (XRD) measurements as well as transmission electron microscopy (TEM). Analyses of XRD and TEM indicate that the multi-step phase change in GS/GT films results from its unique crystallization mechanism (amorphous-mix crystalline–crystalline). What is more, for single period samples, the thickness of each layer has an obvious influence on multi-step transition performance. In terms of [GS(a nm)/GT(a nm)]1 film configuration, the optimized thickness of each layer should be in the range 50–100 nm. Above all, GS/GT nanocomposite thin films with one period harbor great potential for high-density phase change random access memory applications.

Wear Relief for High-Density Phase Change Memory Through Cell Morphing Considering Process Variation

Due to the scalability and large leakage power, dynamic random-access memory (DRAM) has a lot of challenges in scaling. As an alternative, phase change memory (PCM) has demonstrated promising potential to serve as the main memory in deep submicrometer regime. The broad resistance range of PCM cells enables several cell modes with various densities, pertaining to multiple level cell (MLC), triple state cell (TSC), and single level cell (SLC). High-density mode outperforms low-density ones in terms of capacity and cost-per-bit, but suffers from a weaker cell endurance. Wear leveling strategies are proposed to enhance the memory endurance but encounter more challenges with the aggravating process variation. Due to endurance variations, physical domains are fabricated with irregular tenacity. As a result, balanced write traffic, which is the objective of traditional wear leveling, cannot fully exploit the PCM endurance since the weak parts will be worn out sooner than others. In this paper, considering process variation, we propose a cell morphing based wear leveling scheme. Cell morphing refers to the cell mode transformation between high density (e.g., MLC) and low densities (e.g., TSC and SLC). Instead of redistributing write operations, the proposed wear leveling scheme dynamically transforms weak and frequently written portions into low-density mode for endurance benefits. Multitier cell morphing schemes are proposed to support mode transformation among multiple density levels. The experimental results show 236% endurance improvement for single-tier cell morphing and 209% for two-tier cell morphing with 2% low-density page percentage, when compared with the most related work.

Modeling of Solidification of CCHH (CaCl2, 6H2O) in a Shell-and-Tube PCM based Heat Storage Unit

Phase Change Material (PCM) based thermal energy storage plays a great role in the era of energy storage due to its high energy storage density and constant phase change temperature. In the present work, solidification behavior of CCHH has been studied numerically in a shell-and-tube PCM based heat storage unit. The enthalpy based 2-D numerical model for shell and tube PCM heat exchanger unit has been considered to study the solidification characteristics. The numerical model has been validated using experiments measuring the temperature variation with time at different locations in the numerical domain of PCM. The predicted isotherm plots clearly show the temperature distribution in the domain during solidification. The melt fraction plots with time are plotted to find out the solid and liquid fraction of the PCM in the numerical domain. The stream function plots with time reflect the solid and liquid zones in the numerical domain during solidification. A close agreement of numerical and experimental study of the solidification behavior of CCHH has been observed in the present study which improves in design of such devices.