Fast Scanning Calorimetry Research Articles

Experimental Investigation of In-Plane Shear Behaviour of Thermoplastic Fibre-Reinforced Composites under Thermoforming Process Conditions

In order to meet environmental regulations and achieve resource efficiency in the series production of vehicles, recyclable polymer composites with a high strength-to-weight ratio are increasingly being used as materials for structural components. Particularly with thermoplastic fibre-reinforced polymers or organo-sheets, the advantage lies in the tailored mechanical properties of the final component by adapting the orientation of fibres based on the direction of loads. These components produced by thermoforming organo-sheets also offer a cost benefit and short cycle times. During the thermoforming process, the shear behaviour of the organo-sheet is the most dominant and determines the mechanical properties and quality of the resulting component. However, the current standard for characterising the shear behaviour of organo-sheets does not consider the strain and cooling rates inherent in the thermoforming process. This research investigates the influence of thermoforming process parameters on the shear behaviour of organo-sheets with a new methodology combining DSC and DMA experiments. During the thermoforming process, the transition of the matrix material from a molten state to a solid state is dictated by the crystallisation kinetics and their dependence on heating and cooling rates. Thus, non-isothermal DSC scans, which correspond to a temperature cycle in a thermoforming process, are used in the DSC experiments to establish the relationship between the recrystallisation temperature of the organo-sheet material and the cooling/heating rates in the thermoforming process. In order to achieve thermoforming-process-relevant cooling rates, fast scanning calorimetry (Flash DSC) is used in addition to conventional DSC measurements. DMA experiments carried out with 45° fibre orientation show that the recrystallisation temperature consequently influences the shear storage modulus of the organo-sheet. The results from DSC measurements show a shift of recrystallisation temperatures to lower temperatures as the cooling rate increases. The combined analysis of results from the DSC and DMA experiments supports the findings and shows the influence of the process temperature, cooling rate and strain rate on the recrystallisation temperature and, in turn, the shear behaviour of organo-sheets. Thus, a recommendation for establishing a new standard for characterising the shear behaviour of organo-sheets is made.

Melting Kinetics of Superheated Polymer Crystals Examined by Isothermal and Nonisothermal Fast Scanning Calorimetry

Melting Kinetics of Superheated Polymer Crystals Examined by Isothermal and Nonisothermal Fast Scanning Calorimetry

Thermal rejuvenation of an aged Au-based metallic glass by fast scanning calorimetry

A metallic glass (MG) annealed above its glass-transition temperature T g , and cooled, may show an enthalpy increase Δ H , and other property changes. The extent of this thermal rejuvenation depends on the state of the MG (represented by effective cooling rate Φ i ) and the post-anneal cooling rate Φ c . Previous studies examined effects of (Φ c /Φ i ) up to 10 2 . With a Au-based MG aged for up to 10 years at room temperature, and using fast calorimetry to anneal and then cool at up to 5000 K s −1 , we extend (Φ c /Φ i ) to 10 7 . The rejuvenation is limited by anneal temperature or by Φ c , when, for all MGs, Δ H / T g shows a universal approximate scaling with log(Φ c /Φ i ). We detect decoupling of vitrification from α relaxation, and highlight limitations in the use of fictive temperature to characterize glassy states. Rejuvenation of the Au-based MG decreases its elastic modulus and hardness, extending trends reported for other MGs. • Fast-scanning calorimetry permits extended thermal rejuvenation of a metallic glass. • The cooling-rate ratio giving rejuvenation, 10 2 in earlier work, is extended to 10 7 . • Rejuvenation is in two regimes, limited by anneal temperature or cooling rate. • Thermal rejuvenation gives extended decoupling of vitrification from α relaxation. • Greater cooling-rate ratio extends reductions in elastic modulus and hardness.

Crystallization kinetics, polymorphism fine tuning, and rigid amorphous fraction of poly(vinylidene fluoride) blends

Fast scanning calorimetry (FSC) is used to study crystallization of poly(vinylidene fluoride) (PVDF), and its blends with a random fluorinated copolymer (FCP) of poly(methyl methacrylate) and 1H,1H,2H,2H-perfluorodecyl methacrylate (PMMA-r-PFDMA). By varying the residence time at isothermal melt crystallization temperatures TMC = 80–120°C, the amount of α- and β-phase can be controlled, yielding partial or complete suppression of β-phase for all blends studied. Nonisothermal crystallization kinetics are also studied by FSC and analyzed using the Mo model. Crystallization rates increase when FCP is present. PVDF crystallizes into β-phase when cooled from the melt at rates faster than 3000 K/s giving PVDF crystal fractions of about 0.05–0.16. Only α-phase occurs at cooling rates slower than 2000 K/s yielding larger PVDF crystal fractions of about 0.30–0.43. Cooling rates between those limits result in mixed α- and β-phase. The rigid amorphous fraction (RAF) of PVDF varies with α and β crystal fraction in PVDF/FCP blends. RAF of α-phase PVDF ranges from 0.21 to 0.28 whereas RAF of β-phase PVDF spans a wider range, reaching values from 0.42 to 0.46.

Probing Three Distinct Crystal Polymorphs of Melt-Crystallized Polyamide 6 by an Integrated Fast Scanning Calorimetry Chip System

Probing Three Distinct Crystal Polymorphs of Melt-Crystallized Polyamide 6 by an Integrated Fast Scanning Calorimetry Chip System

Bio‐Sourced Poly l‐Lactide‐Flax Composites with Close to Maximum Stiffness at Low Fiber Content through Two‐Stage Annealing

AbstractPoly l‐Lactide (PLLA) composites with short flax fibers (from 0 to 10 wt/wt%) with close to maximum theoretical stiffness are prepared by melt‐compounding and injection‐molding followed by a two‐step isothermal crystallization protocol that fully separates the nucleation and growth stages (Tammann). The use of fast chip scanning calorimetry for thermal characterization avoids the complicating issues of crystal reorganization during the cooling and heating steps between the isothermal stages. Flax fibers are very efficient and selective nucleating agents of PLLA favoring the ordered α form. The resulting morphology exhibits trans‐crystallization on the fibers surface, predominantly at fiber defects, with a clear reduction of crystal size and a very strong fiber matrix cohesion. Efficient nucleation further leads to a large reduction of the overall crystallization time. Avrami analysis evidences a reduction of crystal growth dimensionality, consistent with both optical and scanning electron microscopy. The high modulus of the composites is unambiguously related to the strong orientation of the fibers in the tensile direction, to their high aspect ratio and to the excellent matrix‐fibers cohesion. On the other hand, the tensile strength and hardness appear isotropic within experimental uncertainty and are unfavorably influenced by the presence of the fibers and by the two‐stage annealing.

On the crystal stabilization during two-step isothermal crystallization of poly(butylene terephthalate) examined by fast scanning calorimetry

Crystallization behaviors on two-step isothermal crystallization at Tc1 and Tc2(<Tc1) were examined by fast scanning calorimetry for poly(butylene terephthalate) in order to identify the dominant process during the secondary stage of crystallization at Tc1 after the completion of spherulitic growth under isothermal condition. From the difference between the melting peaks obtained before and after the low-temperature crystallization step at Tc2, the distinction of the following two processes became realizable: the growth of crystals formed at Tc2 and the stabilization of crystals formed at Tc1. The dependences on Tc2 and the duration of times Δtc1 and Δtc2 revealed the following: The formation and growth of new crystals are controlled by thermodynamic driving force determined by supercooling, and hence are suppressed at Tc1 higher than Tc2. The crystal stabilization is controlled by the mobility of polymer chains, and became more pronounced at Tc1. Therefore, the dominant process of the secondary crystallization under isothermal condition at Tc1 should be the stabilization of crystals formed in the primary stage and the formation of new crystals will only have a secondary effect.

Physical aging of selenium glass: Assessing the double mechanism of equilibration and the crystallization process

This work focuses on the study of the physical aging of a selenium glass by differential scanning calorimetry (DSC) and fast scanning calorimetry (FSC). Aged samples of 30 and 40 years old at room temperature were analysed by classical DSC, and their enthalpies of recovery were calculated, showing that the glass has reached its thermodynamic equilibrium and is stable over time. The study of accelerated physical aging by using FSC on a rejuvenated sample allows reproducing the way by which this glass has reached its thermodynamic equilibrium at the laboratory time scale. We evidence that the glass reaches its equilibrium state in one or two steps, depending on the gap between the aging temperature and the glass transition temperature, resulting from the difference in the mechanisms governing the relaxation process. Furthermore, a second phenomenon is evidenced, consisting in the crystallization of the glass once the equilibrium has been reached for aging temperatures close to the glass transition temperature.

Temperature-modulated fast scanning calorimetry of isothermal crystallization of Poly(butylene terephthalate)

Crystallization kinetics of poly(butylene terephthalate) under high supercooling was examined by Temperature-Modulated Fast Scanning Calorimetry (TM-FSC). By using temperature modulation for isothermal crystallization, we can determine the temperature dependence of crystal growth rate. The positive/negative dependence brings the negative/positive sign of the phase angle of dynamic heat capacity, respectively. The obtained dependence was compared with that of peak time of heat flow on isothermal crystallization. The isothermal peak time is inversely proportional to the crystallization rate and showed a double peak, corresponding to the crystal-growth- and nucleation-dominant temperature ranges. The result of TM-FSC over the entire temperature range including the nucleation-dominant lower temperature range was in agreement with the temperature derivative of the fitting curve of the crystal-growth-dominant higher-temperature peak. Hence, the present analysis confirmed that TM-FSC detected the change in crystal growth rate but not in nucleation rate. The crystal growth rate was practically determined by a mobility factor near the glass transition.

Additive manufacturing of a precious bulk metallic glass

For the first time, a high-density amorphous and crack-free bulk metallic glass (BMG) based on a precious metal (PdCuNiP) was produced via additive manufacturing (AM). Laser powder-bed fusion (LPBF) was used for the fabrication of the samples, and led to a density of 99.6%. Excellent mechanical properties such as high hardness and compressive strength were achieved, overcoming the limitations usually found for precious metals in jewelry and watchmaking. Furthermore, without any post-processing, a mirror-like smooth and brilliant surface was directly obtained, which is highly beneficial for applications where surface finish or aesthetics matters. The effect of the main processing parameters, such as laser power and laser-scanning speed, on the shape of single tracks was investigated by laser confocal microscopy (LCM). Following the single-track experiments, highly amorphous LPBF samples were produced. The samples were characterized by optical microscopy (OM), scanning electron microscopy (SEM), conventional and synchrotron X-ray diffraction (XRD), micro-computed tomography (μ-CT), compression tests, and microhardness. The crystallization kinetics of the powder alloy was investigated via fast differential scanning calorimetry (FDSC). A small quantity of the powder (< 70 g) was used for the fabrication of samples, alleviating the cost of the process. Efficient production of precious metal parts with enhanced mechanical properties is demonstrated.

Nucleation Behavior of a Single Al-20Si Particle Rapidly Solidified in a Fast Scanning Calorimeter.

Understanding the rapid solidification behavior characteristics, nucleation undercooling, and nucleation mechanism is important for modifying the microstructures and properties of metal alloys. In order to investigate the rapid solidification behavior in-situ, accurate measurements of nucleation undercooling and cooling rate are required in most rapid solidification processes, e.g., in additive manufacturing (AM). In this study, differential fast scanning calorimetry (DFSC) was applied to investigate the nucleation kinetics in a single micro-sized Al-20Si (mass%) particle under a controlled cooling rate of 5000 K/s. The nucleation rates of primary Si and secondary α-Al phases were calculated by a statistical analysis of 300 identical melting/solidification experiments. Applying a model based on the classical nucleation theory (CNT) together with available thermodynamic data, two different heterogeneous nucleation mechanisms of primary Si and secondary α-Al were proposed, i.e., surface heterogeneous nucleation for primary Si and interface heterogenous nucleation for secondary α-Al. The present study introduces a practical method for a detailed investigation of rapid solidification behavior of metal particles to distinguish surface and interface nucleation.

Fragility of short‐chain poly(lactic acid)s derivatives by combining dielectric spectroscopy and fast scanning calorimetry

AbstractLow‐molecular‐mass bio‐polyesters, such as lactic acid oligomers, constitute a growing category of plasticizers for poly (lactic acid), since they show good compatibility and are respectful of the environment. This study focuses on the glass transition of a series of oligomers with different molecular weights, with the aim of investigating how their associated dynamics occur. Dielectric and calorimetric results are combined to study the molecular mobility in a large temperature range. In comparison with poly (lactic acid), the oligomers exhibit a lower fragility index and smaller cooperative rearranging regions. Among oligomers, the fragility is clearly dependent on the molecular weight, whereas the variations of the cooperativity are subtler.

Viscosity of Metallic Glass-Forming Liquids Based on Zr by Fast-Scanning Calorimetry

Fast-scanning calorimetry was applied to retrieve the viscosity of supercooled liquids of the Zr-based bulk metallic glasses (BMGs) Vitreloy 105 and AMZ4 for temperatures from standard glass transition down to ~0.78Tg/T. Characteristic temperatures of the glass transition were translated into viscosity values by means of composition-independent shift factors based on the equivalency between structural relaxation and viscous flow. The extended MYEGA model with a fragile term dominant at high-temperatures and a strong term dominant at low-temperatures describes the entire viscous range. The analysis revealed that Vitreloy 105 and AMZ4 are strong liquids for log10η ≥ 4.9–5.5. In turn, the fragile-to-strong crossover is centred on 0.69Tg/T for Vitreloy 105 and on 0.66Tg/T for AMZ4. The extent of the fragile-to-strong transition was found to be larger for Vitreloy 105 than for AMZ4, while their values agreed well with the inverse relation between transition factor and kinetic fragility of the strong regime established for BMG-forming liquids.

Calorimetric and Dielectric Investigations of Epoxy-Based Nanocomposites with Halloysite Nanotubes as Nanofillers

Epoxy nanocomposites are promising materials for industrial applications (i.e., aerospace, marine and automotive industry) due to their extraordinary mechanical and thermal properties. Here, the effect of hollow halloysite nanotubes (HNT) on an epoxy matrix (Ep) was the focus of the study. The structure and molecular mobility of the nanocomposites were investigated using a combination of X-ray scattering, calorimetry (differential (DSC) and fast scanning calorimetry (FSC)) and dielectric spectroscopy. Additionally, the effect of surface modification of HNT (polydopamine (PDA) and Fe(OH)3 nanodots) was considered. For Ep/HNT, the glass transition temperature (Tg) was decreased due to a nanoparticle-related decrease of the crosslinking density. For the modified system, Ep/m-HNT, the surface modification resulted in enhanced filler–matrix interactions leading to higher Tg values than the pure epoxy in some cases. For Ep/m-HNT, the amount of interface formed between the nanoparticles and the matrix ranged from 5% to 15%. Through BDS measurements, localized fluctuations were detected as a β- and γ-relaxation, related to rotational fluctuations of phenyl rings and local reorientations of unreacted components. A combination of calorimetry and dielectric spectroscopy revealed a dynamic and structural heterogeneity of the matrix, as confirmed by two glassy dynamics in both systems, related to regions with different crosslinking densities.

Nucleation behaviour and microstructure of single Al-Si12 powder particles rapidly solidified in a fast scanning calorimeter

The understanding of rapid solidification behaviour, e.g. the undercooling versus growth velocity relationship, is crucial for tailoring microstructures and properties in metal alloys. In most rapid solidification processes, such as additive manufacturing (AM), in situ investigation of rapid solidification behaviour is missing because of the lack of accurate measurement of the cooling rate and nucleation undercooling. In the present study, rapid solidification of single micro-sized Al-Si12 (mass%) particles of various diameters has been investigated via differential fast scanning calorimetry employing controllable cooling rates from 100 to 90,000 K s−1 relevant for AM. Based on nucleation undercooling and on microstructure analysis of rapidly solidified single powder particles under controlled cooling rates, two different heterogeneous nucleation mechanisms of the primary α-Al phase are proposed. Surface heterogeneous nucleation dominates for particles with diameter smaller than 23 μm. For particles with diameter larger than 23 μm, the nucleation of the primary α-Al phase changes from surface to bulk heterogeneous nucleation with increasing cooling rate. The results indicate that at large undercoolings (> 95 K) and high cooling rates (> 10,000 K s−1), rapid solidification of single particle can yield a microstructure similar to that formed in AM. The present work not only proposes new insight into rapid solidification processes, but also provides a theoretical foundation for further understanding of microstructures and properties in additively manufactured materials.

Insight into crystallization paths in Au–Si eutectic alloy through the energy-temperature diagram

An energy/temperature (E/T) diagram is a powerful representation of the stability relations between various solid-states with the same composition if they melt at a single temperature instead of a temperature range, such as eutectic microstructures in addition to allotropes and polymorphs. The characteristic melting point and melting enthalpies of two metastable eutectic microstructures and one metastable single phase in Au–18.6at%Si alloy have been measured using fast differential scanning calorimetry (FDSC) which is used as a phase identification technique. An E/T diagram for the eutectic Au–Si alloy has been prepared using these thermodynamic parameters, which gives all possible crystallization paths in this simple binary alloy. In the present contribution, we give a roadmap to understand FDSC curves that demonstrate the characteristics of different crystallization paths in this alloy. A strong phase competition between a rich variety of metastable and stable solid-states present in Au–Si eutectic alloy has been investigated using the E/T diagram. The stochastic nature of nucleation has also been demonstrated as a critical phenomenon on phase competition between metastable solids.

Metallographic Preparation of Single Powder Particles

Abstract This work developed a systematic method for a metallographic preparation of single powder particles with diameters of approx. 20 to 40 μm. It was motivated by the objective of understanding additive manufacturing processes such as Laser Powder Bed Fusion. A fundamental aspect of the relationship between manufacturing, structure, and properties is the correlation of rapid solidification and resulting microstructure. During powder-based additive manufacturing processes, cooling rates up to 1 MK/s are attained. A thermal analysis determining the characteristics of solidification at such rapid cooling rates can be performed with the aid of chip sensor-based, dynamic Differential Fast Scanning Calorimetry, DFSC. For this purpose, the heat flow during the solidification of single powder particles is measured and, for instance, the solidification onset temperature is evaluated as a function of cooling rate. It is thus possible to estimate the undercooling which has a significant impact on the resulting structure. Subsequently, cross sections of single powder particles must be prepared for the analysis of the resulting structure.

Assessment of AlZnMgCu alloy powder modification for crack-free laser powder bed fusion by differential fast scanning calorimetry

Additive manufacturing, e.g. by laser powder bed fusion (LPBF), is very attractive for lightweight constructions, as complex and stress-optimised structures integrating multiple functions can be produced within one process. Unfortunately, high strength AlZnMgCu alloys tend to hot cracking during LPBF and thus have not so far been applicable. In this work the melting and solidification behaviour of AlZnMgCu alloy powder variants with particle surface inoculation was analysed by Differential Fast Scanning Calorimetry. The aim is to establish a method that makes it possible to assess powder modifications in terms of their suitability for LPBF on a laboratory scale requiring only small amounts of powder.Therefore, solidification undercooling is evaluated at cooling rates relevant for LPBF. A method for the temperature correction and normalisation of the DFSC results is proposed. Two ways of powder modification were tested for the powder particles surface inoculation by titanium carbide (TiC) nanoparticles: via wet-chemical deposition and via mechanical mixing.A low undercooling from DFSC correlates with a low number of cracks of LPBF-manufactured cubes. It appears that a reduced undercooling combined with reduced solidification onset scatter indicates the possibility of crack-free LPBF of alloys that otherwise tend to hot cracking.

Physical Aging Behavior of a Glassy Polyether.

The present work aims to provide insights on recent findings indicating the presence of multiple equilibration mechanisms in physical aging of glasses. To this aim, we have investigated a glass forming polyether, poly(1-4 cyclohexane di-methanol) (PCDM), by following the evolution of the enthalpic state during physical aging by fast scanning calorimetry (FSC). The main results of our study indicate that physical aging persists at temperatures way below the glass transition temperature and, in a narrow temperature range, is characterized by a two steps evolution of the enthalpic state. Altogether, our results indicate that the simple old-standing view of physical aging as triggered by the relaxation does not hold true when aging is carried out deep in the glassy state.

Critical crystallization properties of an industrial-grade Zr-based metallic glass used in additive manufacturing

The major challenge to overcome when processing metallic glasses (MGs) is to avoid crystallization. Therefore, time-temperature-transformation (TTT) diagrams are used, but they are often derived from experiments in which heating or cooling rates are limited and do not cover the full range of processing conditions, especially those encountered in additive manufacturing (AM) where the rates are very high. Here, an industrial-grade Zr-based MG (AMZ4) is investigated via fast differential scanning calorimetry (FDSC). The critical cooling and heating rates of AMZ4 are experimentally measured and TTT diagrams are determined upon heating and cooling. The critical heating rate of 45,000 K/s is 18 times the critical cooling rate, which indicates the presence of a self-doped glass (SDG) that includes quenched-in nuclei. The results illustrate that AMZ4 is very sensitive to crystallization, even in laser-based AM conditions, where heating and cooling steps need to be distinguished.