Rom 77k to 159 K Phase Change Process Then Temperature Change Again From 159 to 300k

• Periodical List
• Materials (Basel)
• 5.13(v); 2020 Mar
• PMC7085018

Materials (Basel). 2020 Mar; 13(5): 1162.

Study of the Phase Transitions in the Binary Arrangement NPG-TRIS for Thermal Free energy Storage Applications

Sergio Santos-Moreno,^{1, 2, 3} Stefania Doppiu,^{one, *} Gabriel A. Lopez,³ Nevena Marinova,² Ángel Serrano,¹ Elena Silveira,² and Elena Palomo del Barrio^{1, 4}

Sergio Santos-Moreno

¹Middle for Cooperative Research on Culling Energies (CIC energiGUNE), Basque Research and Technology Alliance (BRTA), Alava Technology Park, 01510 Vitoria-Gasteiz, Espana; moc.enugigrenecic@sotnass (S.S.-Thou.); moc.enugigrenecic@omolape (E.P.d.B.)

²TECNALIA, Basque Enquiry and Technology Brotherhood (BRTA), Parque Tecnológico de San Sebastián, 20009 Donostia-San Sebastián, Spain; moc.ailancet@avoniram.aneven (Northward.1000.); moc.ailancet@arievlis.anele (East.South.)

³Practical Physics II, University of the Basque State UPV-EHU, 48940 Leioa, Spain; sue.uhe@zepol.ordnajelaleirbag

Stefania Doppiu

ⁱCentre for Cooperative Research on Culling Energies (CIC energiGUNE), Basque Research and Technology Alliance (BRTA), Alava Technology Park, 01510 Vitoria-Gasteiz, Kingdom of spain; moc.enugigrenecic@sotnass (S.S.-Thousand.); moc.enugigrenecic@omolape (Eastward.P.d.B.)

Ángel Serrano

¹Centre for Cooperative Research on Alternative Energies (CIC energiGUNE), Basque Research and Technology Alliance (BRTA), Alava Applied science Park, 01510 Vitoria-Gasteiz, Spain; moc.enugigrenecic@sotnass (South.S.-K.); moc.enugigrenecic@omolape (Due east.P.d.B.)

Elena Palomo del Barrio

ⁱCentre for Cooperative Research on Culling Energies (CIC energiGUNE), Basque Research and Engineering science Brotherhood (BRTA), Alava Engineering Park, 01510 Vitoria-Gasteiz, Kingdom of spain; moc.enugigrenecic@sotnass (Due south.South.-One thousand.); moc.enugigrenecic@omolape (East.P.d.B.)

⁴Ikerbasque, Basque Foundation for Scientific discipline, 348013 Bilbao, Spain

Received 2020 Feb 11; Accustomed 2020 Mar 3.

Abstract

Neopentylglycol (NPG) and tris(hydroxymethyl)aminomethane (TRIS) are promising stage change materials (PCMs) for thermal energy storage (TES) applications. These molecules undergo reversible solid-solid phase transitions that are characterized by high enthalpy changes. This work investigates the NPG-TRIS binary arrangement every bit a way to extend the utilise of both compounds in TES, looking for mixtures that cover transition temperatures different from those of pure compounds. The phase diagram of NPG-TRIS system has been established by thermal analysis. It reveals the being of two eutectoids and ane peritectic invariants, whose main properties equally PCMs (transition temperature, enthalpy of stage transition, specific heat and density) have been determined. Of all transitions, only the eutectoid at 392 K shows sufficiently high enthalpy of solid-solid phase transition (150–227 J/g) and transition temperature significantly dissimilar from that of the solid-state transitions of pure compounds (NPG: 313 K; TRIS: 407 Thousand). Special attention has besides been paid to the analysis of metastability bug that could limit the usability of NPG, TRIS and their mixtures every bit PCMs. It is proven that the improver of minor amounts of expanded graphite microparticles contributes to reduce the subcooling phenomena that characterizes NPG and TRIS and solve the reversibility bug observed in NPG/TRIS mixtures.

Keywords: phase change fabric, thermal energy storage, latent heat storage, neopentyl glycol, TRIS, plastic crystals, globular polyols, subcooling

one. Introduction

Thermal energy storage (TES) is a key element in the energy transformation that our order must undergo in guild to alleviate the furnishings of climate change and the scarcity of fossil resources. Traditionally, TES has been used to better thermal direction and energy efficiency in the sector of heating and cooling in buildings, as well as in industrial heat processes. Nowadays, thermal energy storage might go an essential tool for increasing the use of renewable energies, characterized by their temporal variability. It has experimented a especially of import development coupled to depression-medium temperature solar thermal collectors, as well equally in concentrating solar thermal ability plants. Due to the possibility of storing large amounts of energy at relatively low cost, it has started to be considered as an alternative/complement to the massive storage of free energy on the electricity grid, which needs to be provided with greater flexibility to absorb the growing proportion of renewables (photovoltaic (PV), wind) continued to the filigree. Also, the recent willingness to decarbonize the building and industrial sectors through electrification opens upwards new prospects for the use of TES.

Currently, sensible heat storage technologies dominate the market. However, latent estrus storage based on stage change materials (PCMs) are particularly bonny technologies for applications where energy has to be stored/delivered over a narrow temperature range or/and when space is a limiting factor. Indeed, PCMs can reversibly blot or release heat during their phase change processes at well-nigh constant temperature. Moreover, they enable compact TES systems with volumetric storage capacity five to ten times greater than that of sensible estrus storage systems.

The research on PCMs has been increasing tremendously over the terminal 25 years. Many different types of PCMs have been considered for their use in TES systems [1,two,3]. Over half of the materials studied have transition temperatures below 120 °C, with a large number of possible material categories (i.e., salt hydrates, paraffins, fatty acids, sugar alcohols, etc.). Higher up 300 °C, simply anhydrous salts and metallic alloys have been investigated. At medium temperatures (120–250 °C), which dominates in industrial heat processes, there is a significant drop-off in the number and types of PCMs. Moreover, the vast majority of proposed materials are solid-liquid PCMs with relatively high latent heat of fusion. Still, the leakage of the liquid phase of these materials at temperatures above their melting point can hinder their application. In that way, prior to their implementation in the final appliance, solid-liquid PCMs are usually confined in a supporting matrix or encapsulated [2], which implies considerable production costs. Solid-solid PCMs can be an attractive alternative to solid-liquid PCMs, avoiding the leakage problem and offering a wide range of possibilities for integration into the TES system, as they can be easily shaped and sized [four,five].

Timmermanns [half-dozen] already in 1961 proposed a class of organic molecules referred to as "globular molecules" or "plastic crystals" as solid-solid PCMs. These molecules undergo reversible phase transitions from a depression temperature ordered layered structure (tetragonal, orthorhombic, monoclinic etc.) to a high temperature orientationally disordered (FCC or BCC) phase, referred to as "plastic or orientationally disordered crystal (ODIC)". These polymorphic changes are characterized past high enthalpy of solid-solid stage transition, which results from the reversible breaking of nearest-neighbor hydrogen bonds in the molecular crystals [7]. Even so, very few alcohol and amine derivatives form these plastic crystals. They are pentaerythritol (PE), pentaglycerin (PG), neopentylglycol (NPG), tris(hydroxymethyl)aminomethane (TRIS) and 2-amino-two-methyl-1,iii-propanediol (AMPL), whose crystallographic and thermodynamic backdrop have been extensively investigated [7,8,9,10,11,12,13,14,15,16,17]. They exhibit solid-solid transition in the range of temperatures from 44 °C (NPG) to 185 °C (PE) with enthalpies of transition from 110 J/g (NPG) to 300 J/g (PE), which make them particularly bonny for TES applications in the industrial sector.

In guild to increase the possibilities of application of these materials, the miscibility of their solid phases has as well been investigated. The study of the phase diagrams of the binary systems NPG-PG [18,nineteen,20], AMPL-NPG [21], NPG-TRIS [20,22,23], PG-TRIS [18,24,25] and TRIS-AMPL [26] has revealed the beingness of invariant reactions (eutectoids, peritectoids) and/or solid-solid solutions over wide concentration ranges, which should let the apply of this course of molecules to shop energy at temperatures dissimilar from the transition temperatures of pure materials.

This study focusses on NPG-TRIS system. The objective is to assess its usability in TES applications. This involves not only identifying compositions displaying solid-solid transitions at temperatures different from those of NPG and TRIS, but likewise evaluating their storage capacity. Although previous experimental work carried out by Barrio et al. [22] uses X-ray diffraction and differential scanning calorimetry to establish the phase diagram of NPG-TRIS system, the enthalpy of solid-solid stage transitions has only been reported for pure NPG [seven,15,16,17,21] and TRIS [11,15,16,17,18]. The aforementioned is true for specific heats and densities [14,21,27,28,29]. Likewise, whereas metastable phases are normally observed in plastic crystals when cooling, this is not a gene that has been considered in their evaluation as TES materials although it tin limit their usability. Moreover, there are no previous studies trying to avoid metastability. On the i hand, the present study provides total assessment of NPG-TRIS system for TES applications. On the other manus, it also includes a offset endeavor to reduce metastability issues by doping the NPG-TRIS mixtures with expanded graphite.

2. Experimental

2.i. Materials

Neopentylglycol (NPG; C_fiveH₁₂O_ii) and tris(hydroxymethyl)aminomethane (TRIS; C_fourH₁₁NO_three) with a purity of 99 and 99.viii%, respectively, were purchased from Sigma-Aldrich (St. Louis, Missouri, Usa). To avoid eventual degradation or hydration, both NPG and TRIS were stored in closed glass containers within a glove box with an argon atmosphere and levels of oxygen and humidity beneath 0.one ppm. Highly conductive expanded graphite powder (SIGRATHERM^®GFG) was purchased from SGL Carbon SE (Wiesbaden, Germany). The average particle size (D₅₀) and powder density are 75 μm and 0.120 g/cmⁱⁱⁱ, respectively. The powder's carbon content is more than 95% and the moisture content less than 5%.

NPG_1−xTRIS_x (0 < 10 < 1, tooth fraction of TRIS) samples were prepared past a uncomplicated three-step method consisting in: (1) grinding and mixing of NPG and TRIS in the right proportion; (ii) heating up of the sample up to 180 °C (453 G) in hermetically closed drinking glass container (x mL) to avert eventual sublimation of NPG; and (3) slow cooling down of the sample followed by annealing at room temperature for at to the lowest degree 1 h in order to avoid freezing of metastable phases commonly observed otherwise. The same procedure was practical to prepare NPG_ane−xTRIS_x samples doped with expanded graphite. In all cases, NPG, TRIS and expanded graphite were used as received. Regarding the first step of the preparation method, hand-grinding in a mortar and brawl-milling were tested and compared. An 8000 M Mixer/Mill^® High-Free energy Ball Mill, from SPEX SamplePrep LLC (Metuchen, NJ, USA), was used. The samples (2 g) were grinded in stainless steel vials during 15 min at 875 rpm, employing three stainless steel balls of 1 g each. No differences were observed between the thermal behavior of hand-grinded samples and that of ball-milled ones. Therefore, ball-milling was systematically used due to its efficacy.

ii.2. Methods

two.2.1. Thermal Assay and Density

Differential Scanning Calorimetry (DSC) studies were performed with a Q2500 Calorimeter from TA Instruments (New Castle, DE, USA) using T-zero airtight aluminum crucibles and samples of about fifteen mg. All experiments were performed with argon (l mL/min) as purge gas.

To make up one's mind the phase diagram of NPG-TRIS system, 21 NPG_one−xTRIS_x (0 ≤ x ≤ 1) samples were tested with heating/cooling charge per unit of 1 K/min from 293 Grand to 453 K. The DSC is calibrated every month for heat period and temperature using high purity (>99.99%) reference materials indium, can, zinc and aluminum. In this work, the calibration was checked using indium standard earlier tests. The thermograms obtained (compensation heat flux vs. temperature) were used to determine phase transition temperatures and corresponding enthalpies of phase transition. In the example of an isothermal phenomenon, the transition temperature is considered to exist given by the onset temperature; whereas the shape factor method [xxx] was used to make up one's mind the transition temperature when the phenomenon is no longer isothermal. The enthalpies of phase transition are calculated past integration of the endothermic peaks bold a linear baseline. The accuracy in the determination of transitions temperatures is virtually ±1 Thousand, whereas that of the enthalpies of phase transition is ±5%. The aforementioned testing conditions were applied to perform the thermal analysis of NPG, TRIS and NPG_one-tenTRIS_ten samples doped with expanded graphite. Scanning rates between 0.5–x M/min were employed to study eventual furnishings of the cooling rate on freezing of metastable phases in NPG, TRIS and NPG_one-10TRIS_ten mixtures.

The specific heat of NPG_ane-xTRIS_x (0 ≤ x ≤ 1) samples was measured over broad range of temperatures using modulated heating method (accurateness ±5%). The samples were heated past superimposing periodic temperature variations of ±0.2 or ±0.5 Thou aamplitude and 1/60 Hz frequency on a temperature ramp of 1 K/min. The DSC was previously calibrated using sapphire as standard material.

The truthful density of NPG_1−xTRIS_x (0 ≤ 10 ≤ 1) samples was determined by a helium pycnometer (Accupyc II 1340, Micrometrics; Norcross, Georgia, USA) at room temperature.

2.2.two. X-Ray Diffraction (XRD)

X-ray diffraction measurements were performed to verify that during the preparation of NPG_1−xTRIS₁₀ (0 < ten <1) samples no degradation or structural changes occur later grinding or heating processes. The diffractograms were obtained with a D8 Discover diffractometer from BRUKER (Billerica, MA, USA) equipped with a LYNXEYE XE detector for ultra-fast diffraction measurements and with the Vario1 monochromator, using a CuKα_ane radiations with a 1.5419 Å wavelength. Diffractograms accept been recorded in the 2θ athwart range x–80°, with a pace size of 0.02° and a pace time of ane.0 s, using a tube voltage of twoscore kV and a tube current of twoscore mA. All the samples were tested at room temperature.

2.2.iii. Liquid-Country NMR

The ¹H-NMR spectra of NPG_i−tenTRIS_x (0 < x <one) samples were obtained to determine their composition with high accuracy before and after applying heating treatments. This is peculiarly relevant to check that the composition of the samples does not change during heating considering of NPG evaporation. ^aneH-NMR spectra were recorded using a Bruker^® 500 Advance Three spectrometer (500 MHz for ¹H and 125 MHz for ¹³C) in the perdeuterated solvent D₂O in a concentration of ca. ten mg/mL. The values of chemical shifts (δ) in ppm are referred to tetramethylsilane (Me4Si, TMS) every bit standard (δ = 0.00 ppm).

3. Results and Discussion

3.i. Pure NPG and TRIS

NPG and TRIS are tetrahedral molecules derived from neopentane whose crystallographic and thermodynamic parameters take been extensively investigated [11,18,21,22,23]. The crystal construction of the depression temperature phase of NPG is monoclinic P2_i/n, whereas that of TRIS exhibit an orthorhombic lattice with infinite grouping Pn2_onea. High temperature orientationally disordered crystals (ODIC) are confront-centered cubic (FCC) and body-centered cubic (BCC) structures for NPG and TRIS, respectively. The results of the thermal analysis carried out in this study are depicted in Effigy 1. 2 kind of DSC tests take been performed. In the offset one, the samples of NPG and TRIS are submitted to three sequent heating and cooling cycles at a heating/cooling charge per unit of 5 K/min to check the repeatability of the results (Figure 1a). In the second one, the samples are too cycled three times but using different heating/cooling rates (1, 5 and ten K/min) to analyze the eventual influence of this parameter on metastability (Effigy 1b).

DSC thermograms (compensation heat flux vs. temperature) for (a) pure NPG and (b) pure TRIS obtained at heating rates of (a) v K/min for NPG and (b) 1, 5 and 10 Thousand/min for TRIS.

For NPG (Figure ia), the solid-country phase transition from the low temperature monoclinic crystal [M] to the loftier temperature FCC plastic phase [C_F] happens at 313.v K. The loftier temperature plastic phase [C_F] melts at 400.5 K. The enthalpy of phase transitions [One thousand] → [C_F] and [C_F] → [50] are 119.four J/one thousand (12.43 kJ/mol) and 37.6 J/g (3.91 kJ/mol), respectively. Figure 1b shows that the low temperature orthorhombic solid phase [O] of TRIS stabilized below 407.iii K, whereas the high temperature BCC phase [C_B] is stable from 407.3 K to 445 K. The liquid phase [L] appears above 445 Thousand. The enthalpy of the phase transitions [O] → [C_B] and [C_B] → [L] are, respectively, 280.7 J/g (33.99 kJ/mol) and 26 J/yard (3.fifteen kJ/mol). As shown in Table 1, these results are in good understanding with those of previous studies.

Table ane

Crystal structures, temperature and enthalpy of transition from the low temperature ordered structure to the high temperature ODIC (T_TR, ΔH_TR), and melting point and latent rut of fusion of the ODIC (T_m, ΔH_{one thousand}) for pure NPG and TRIS.

Chemical compound	Depression Temp. Phase	TTR (Yard)	ΔHTR (kJ/mol)	High Temp. Phase	T1000 (K)	ΔHk (kJ/mol)	Ref.
NPG	Monoclinic	315.0	13.six	FCC	399.0	four.6	[23]
		315.0	12.ane		403.2	4.four	[21]
		314.6	12.viii		401.3	iv.4	[22]
		313.v	12.4		400.five	4.0	This written report
TRIS	Orthorhombic	408.0	32.7	BCC	445.0	3.iii	[23]
		407.3	32.9		446.0	three.0	[11]
		406.8	34.0		442.7	3.seven	[18]
		407.3	34.0		445.0	iii.1	This written report

Moreover, the enthalpy of transition from the low temperature ordered construction to the OCID is almost three times college for TRIS than for NPG. Indeed, the unusually large enthalpies of these solid-state transformations have been explained in terms of a rotational/vibrational disorder transformation. Benson et al. [14] suggested that hydrogen bonding in polyhydric alcohols held the nearly spherical molecules rigidly in the depression temperature crystal stage until, at the transition temperature, all these bonds are cleaved permitting molecular vibration and rotation. This hydrogen bonding hypothesis was tested past infrared absorption spectroscopy, then further by examining the dependence of the solid-state transition enthalpies (ΔH_TR) on the number of hydrogen sites able to make hydrogen bonding (acid hydrogens) per molecule (due north). The results demonstrated that in that location is a perfect linear correlation between ΔH_TR and due north², the transition enthalpy increasing with the number of acid hydrogens per molecule. Accordingly, NPG, with two hydroxyl groups, shows much lower solid-state transition enthalpy than TRIS, that has 3 hydroxyl groups (-OH) and ane amine group (-NH₂) per molecule.

Equally shown in Figure i, whereas undercooling is negligible in the solid-liquid transitions of NPG and TRIS, their solid-solid transitions (NPG: [M] → [C_F], TRIS: [O] → [C_B]) display a significant degree of undercooling. Within the range of tested cooling rates (1-ten Yard/min), the observed undercooling is almost 15 K for NPG and 65 One thousand for TRIS. This is typical in polyhydric alcohols and its amine derivatives and could be explained by the degree of disorder and molecular motion in the plastic phase. Indeed, the existence of a loftier degree of orientational freedom is the almost characteristic feature of the plastic crystalline country. According to Rao [31], amid possible rotational motions in crystals (free rotation, rotational improvidence and jump reorientation), standoff-interrupted molecular rotation is the most likely one in plastic crystals. Preferential orientations of tetrahedral, or neopentane-like, molecules have been studied past Guthrie et al. [32] from steric and symmetry considerations. The results of this theoretical study were subsequently confirmed by molecular dynamic simulations [33] and configurational entropy calculations [34]. According to Guthrie's work, the molecules of NPG display i unmarried configuration in the depression temperature ordered crystal while they exhibit threescore configurations (10 molecular orientations that each possesses half dozen possible hydroxymethyl conformations) in the plastic phase. Therefore, below but shut to the solid-state transition temperature, the probability that a particular hydroxyl group take a juxtaposed hydroxyl group from a nearest neighbor molecule with which to form a hydrogen bail is very low (p = 1/60 × 1/60 < 0.028%). Past further reducing the temperature, the reduction in volume experienced by the plastic stage should probable reduce the number of preferential molecular orientations and thus facilitate the appearance of the stable crystalline stage. This interpretation of undercooling in plastic crystals is quite speculative and further research will be needed to accomplish a well-established theory to explicate this phenomenon.

Another measured thermal property of NPG and TRIS is the specific heat. The results depicted in Figure 2 bear witness that the college the phase disorder, the higher the specific oestrus. The highest values represent to the liquid phases, while the lowest are those of the ordered monoclinic (NPG) and orthorhombic (TRIS) crystal structures. In the scanned temperature range, the specific heat of the solid phases of NPG increases linearly with temperature. The same applies for the orthorhombic crystal structure of TRIS, while the specific rut values in the plastic phase are practically insensitive to temperature.

Measured values of specific heat for pure NPG (black) and TRIS (cherry).

Table two summarized the results obtained at temperatures feature of the low temperature ordered phase, the plastic phase and the liquid. The results in the table show that there is a quite good agreement betwixt the values of specific rut measured for NPG in this study and those reported in the literature [28]. However, the values for TRIS are 15–twenty% lower than those measured by Suresh et al. [29].

Table two

Specific heat value (C_p) for temperatures feature of the depression temperature ordered phase, the plastic stage and the liquid of NPG and TRIS.

Compound	Crystal Construction	T (Chiliad)	Cp (J/mol/K) This Study	Cp (J/mol/Thou) Refs. [28,29]
NPG	Monoclinic crystal structure	310	169.3	178.iii
	FCC plastic phase	379	281.9	285.0
	Liquid	401	296.6	313.3
TRIS	Orthorhombic crystal construction	402	179.6	230.0
	BCC plastic phase	421	311.4	363.3
	Liquid	450	319.1	396.6

3.2. Phase Diagram of NPG-TRIS System

The phase diagram of NPG-TRIS binary arrangement was established for the first time by Barrio et al. [22]. Using both crystallographic and thermal assay, they identified two eutectoid invariants and one peritectic invariant. The commencement eutectoid ([M + O + C_F]) was reported at 310 ± one K with composition of iii.5 mol% of TRIS, the second one ([O + C_F + C_B]) was observed at 392.5 ± 1 M and 57 mol% of TRIS, and the peritectic invariant ([C_F + C_B + L]) was identified at 410.seven ± ii K with peritectic composition of 48.4 mol% of TRIS. Based on the experimental results of Barrio et al. [22], the NPG-TRIS binary system was calculated by Shi et al. [20] using regular and sub-regular solution models and CALPHAD method and proven to exist in very good agreement with experimental data.

In this study, the NPG-TRIS arrangement has been investigated by thermal assay. Xx-1 dissimilar compositions have been studied. Compared to the previous work of Barrio et al. [22], a effectively exploration of the peritectic region has been carried out including 11 compositions within the composition range from 10 = 0.45 to ten = 0.55. The DSC tests were performed by heating the samples from 20 °C (most of them) or 100 °C (those used to refine the peritectic plateau) upwards to 180 °C at a heating rate of 1 Yard/min. XRD and liquid NMR were used to check that neither samples grooming nor their thermal treatment leads to degradation, unexpected structural changes or compositional changes due to NPG sublimation (Appendix A).

The results of the thermal analysis carried out are summarized in Table three and Figure three. Table 3 shows the temperature values of the phase transitions observed for each of the 21 tested compositions. The DSC thermograms for 8 selected compositions are depicted in Figure threea. Finally, Figure 3b displays the phase diagram of NPG-TRIS system proposed by Barrio et al. [22] together with the experimental points obtained in this study.

(a) DSC thermograms for eight selected compositions and (b) phase diagram of the NPG-TRIS system proposed past Barrio et al. [22] (discontinuous lines), with the experimental points of these authors and those obtained in this report.

Tabular array 3

Characteristic temperatures for the different phase transitions in the NPG-TRIS organization: T_e1, eutectoid invariant [M + O + C_F]; T_sv1, first superior solvus corresponding to the change [O + C_F] to [C_F]; T_e2, eutectoid invariant [O + C_F + C_B]; T_sv2, second superior solvus corresponding to the change [O + C_B] to [C_B]; T_p, peritectic invariant [C_F + C_B + L]; T_liq, liquidus.

Mole Fraction of TRIS	Te1 (K)	Tsv1 (K)	Te2 (K)	Tsv2 (Chiliad)	Tp (K)	Tliq (K)
0 (NPG)	(313.five) one					400.viii
0.ten	310.5					402.ii
0.20	310.5	356.8				403.4
0.35	310.2	378.1				407.2
0.45	north.e 2		389.2			410.5
0.46	309.0		391.4		412.i	414.1
0.47	n.eastward 2		390.5		409.ii	413.half-dozen
0.48	309.6		393.5		409.6	413.ii
0.485	311.7		391.8		413.i	413.1
0.49	308.7		394.9		409.8	413.4
0.50	n.e ii		391.viii		411.1	414.vii
0.51	northward.due east 2		392.2		411.1	413.4
0.52	n.e 2		392.i		410.i	414.6
0.53	311.5		392.0			413.two
0.54	n.e 2		391.8			412.8
0.55	north.e 2		392.3	392.three		413.1
0.60	310.iv		392.ane	392.1		415.three
0.70	309.half dozen		391.9	395.7		423.0
0.80	309.2		392.0	398.3		424.two
0.90	309.3		397.six	402.seven		437.7
1 (TRIS)	(407.3) one					445.0

Every bit shown in Figure 3a (see as well Tabular array 3) the calorimeter bespeak of all the samples (with the exception of pure compounds) shows an endothermic consequence at 310 ± 1.0 K, that corresponds to the eutectoid [Thousand + O + C_F] which extends over the whole range of studied compositions. For the samples with TRIS mole fraction beyond 0.45 (with the exception of pure TRIS), a second endothermic indicate is observed at 392 ± 2.0 K, corresponding to the 2nd eutectoid [M + O + C_B]. The peritectic plateau is evidenced by a third endothermic effect observed at 410 ± 1.0 Yard in the samples with TRIS mole fraction between 0.45 and 0.53. The last endothermic elevation in each composition corresponds to the melting.

Autonomously from already mentioned isothermal transitions, the calorimeter signal for samples with TRIS mole fraction below 0.45 show a progressive endothermic consequence but after the depression temperature eutectoid (Figure 3a), that results from the slow diffusion of TRIS molecules toward the cubic lattice C_F [22]. As shown in Effigy 3b, the temperature-composition dependence of the purlieus solid solution C_F between the two eutectoids (start superior solvus line) ranges from 310 1000 to 392 M. Similarly, a progressive endothermic effect is observed just after the high temperature eutectoid in the samples with TRIS mole fraction beyond 0.55 (with the exception of pure TRIS). This is due to the progressive transformation of orthorhombic crystal structure of TRIS molecules into BCC matted stage. As information technology can be seen in Effigy 3b and Table 3, the 2d superior solvus line ranges from 392 Thou to 403 K.

In summary, it is worth to note that there is a very good agreement betwixt the results obtained in this report and those reported past Barrio et al. [22]. The main difference appears in the first superior solvus (Figure threeb), which shows lower temperature values in this study only improve agreement with the computational model by Shi et al. [20]. Some differences can too exist appreciated in the transition temperatures associated to the melting processes. Barrio et al. [22] propose an isomorphous phase diagram in which solid phases [C_F] and [C_B] are completely miscible, thus forming a continuous solution. Therefore, they requite temperature values for both the solidus and the liquidus lines. Notwithstanding, the DSC thermograms obtained in this study practise non allow to make a articulate distinction between both lines so that but the onset temperature of the endothermic peaks of melting is given. As shown in Figure 4a, the shape of the melting/crystallization peaks obtained in this study does not differ from that typically observed in melting/crystallization processes without solid solutions. Nevertheless, information technology is remarked that the starting crystallization temperature is ever higher than the onset melting temperature, which would support the hypothesis of Barrio et al. [22] about the miscibility of [C_F] and [C_B] phases. The similarity between the crystalline structures of both phases would also support this hypothesis. In improver, for peritectic transformation to occur, there must exist a region where phases [C_B] and [50] (cf. [C_F] and [L]) coexist. Consequently, the apparent differences betwixt our results and those of Barrio et al. [22] do not reverberate any key contradiction, simply only different ways of exploiting the DSC thermographs.

(a) Zoom of the DSC thermogram obtained for NPG-TRIS mixture with 0.seven mole fraction of TRIS showing the melting/crystallization procedure; (b) Zoom of the DSC thermogram obtained for NPG-TRIS mixture with 0.485 mole fraction of TRIS showing the peritectic transition and melting/crystallization process.

three.three. Enthalpies of Transition and Specific Heats

The enthalpy changes associated to the unlike transitions observed in NPG-TRIS mixtures have been determined from DSC thermograms and are analyzed here. We focus on isothermal or quasi-isothermal transformations, which are those of interest for the storage of thermal energy by latent heat.

In the 2d column of Table iv the enthalpy of transition from the ordered crystal structure to the plastic phase of NPG and TRIS is reported. The values of enthalpy associated to the low temperature eutectoid reaction ([M] + [O] → [C_F] at 310 ± i.0 Yard) are given in the third cavalcade, whereas those related to the high temperature eutectoid are in the next cavalcade. For compositions 0.45 < x < 0.55, the values reported represent to the isothermal transition [C_F] + [O] → [C_F] + [C_B] at 392 ± ii.0 K; while at compositions beyond x = 0.55, they represent to the whole transition [C_F] + [O] → [C_B] which take place betwixt the eutectoid plateau (392 ± two.0 K) and corresponding superior solvus (from 392 K upward to 403 Grand depending on the limerick). The enthalpy of melting or the sum of the enthalpies of melting and peritectic transition is given in the last column. It must be noticed that within the peritectic region, the liquidus line and the peritectic plateau are very close in temperature, then as the peritectic reaction and the melting appear as overlapping endothermic peaks in the DSC thermograms (see Figure 4b). This is the reason why the corresponding enthalpies of transition are not given separately but as a sum.

Table 4

Enthalpies of stage transition in NPG-TRIS system: ΔH_pure, pure NGP and TRIS, ΔH_e1, depression temperature eutectoid reaction; ΔH_e2, high temperature eutectoid transformation; ΔH_p+melting, peritectic reaction (when applies) and melting.

Mole Fraction of TRIS	ΔHpure	ΔHe1	ΔHe2	ΔHp+melting
	J/g; (J/cm3)	J/thousand; (J/cm3)	J/g; (J/cm3)	J/one thousand; (J/cm3)
0 (NPG)	119.4 (125.4)	-	-	37.6 (39.5)
0.1	-		-	35.7 (38.2)
0.2	-	87.6 (97.3)	-	30.half-dozen (34.0)
0.35	-	threescore.ix (69.8)	-	25.6 (29.iii)
0.45	-	n.e 1	94.5	25
0.46	-	53.0 (63.1)	114.ix (163.8)	26.viii (31.nine)
0.47	-	n.east ane	104.8	25.1
0.48	-	57.iv	103.five	27.3
0.485	-	54.8 (65.6)	108.6 (129.9)	26.6 (31.8)
0.49	-	57.6	104.7	26.5
0.5	-	n.east 1	102.3	19.seven
0.51	-	n.e ane	122.4	26.7
0.52	-	n.e i	106.ii	24.8
0.53	-	45.7 (55.i)	126.5 (152.5)	26.7 (32.2)
0.54	-	n.e 1	110.7	26
0.55	-	due north.e two	91.6	14.vi
0.6	-	41.3 (50.four)	141.9 (173.ane)	26 (31.7)
0.7	-	29.5 (36.7)	171.one (212.8)	26.5 (33.0)
0.8	-	18.4 (23.vi)	192 (246.one)	22.ix (29.3)
0.9	-	one.5	227.2	26.four
ane (TRIS)	280.7 (376.4)	-	-	26 (34.9)

The experimental values of enthalpy in Table 4 are depicted in Effigy v against the mole fraction of TRIS. The discontinuous lines are calculated linear trends. In the case of the depression temperature eutectoid transition, the regression line calculated on the experimental data (symbols) has been used, together with the eutectoid [Grand + O + C_F] limerick of 0.035 mole fraction of TRIS proposed by Barrio et al. [22], to determine the and then called Tammann plot (dependence of enthalpy related to the eutectoid effect on tooth fraction). We remind that as a outcome of the lever rule, the enthalpy of transition should descend linearly on either side of the eutectoid point. The maximum value appears at the eutectoid betoken (0.035 mole fraction of TRIS) and, by extrapolating the regression line, is estimated to exist most 105 J/g.

Enthalpy changes related to the different transitions in the NPG-TRIS arrangement: experimental data (symbols) and calculated linear trends (discontinuous lines).

A shown in Figure five, the enthalpy of melting (including the enthalpy of the peritectic transition when applies) is quite depression: it ranges from 37 J/yard to 18 J/g, depending on the composition. This is a typical characteristic of plastic crystals. Indeed, although the plastic phase has a cubic crystal structure with the centre of mass of the molecules at fixed positions (FCC for NPG, BCC for TRIS), it is characterized past highly rotationally and vibrational disorder, and then the entropy change when melting is relatively low. Therefore, exploiting either melting or peritectic transition or both transformations together in storage applications makes no much sense.

Higher storage energy capacity can be obtained through the low temperature eutectoid transition. As it tin can exist seen in Figure five, the enthalpy of this solid-solid transition shows a maximum value of 105 J/g for 0.035 mole fraction of TRIS (eutectoid point [Thou + O + C_F]), which is quite high. Yet, using this transition for TES has no articulate advantages compared to pure NPG. Indeed, the eutectoid transition takes place at 310 K, only three One thousand approx. below the solid-solid transition of pure NPG, and the enthalpy of transition is 14 J/g lower (105 J/g vs. 119 J/g). It should also exist noted that the enthalpy of transition at the eutectoid point is lower (10 J/thousand approx.) than that obtained by multiplying the [M] phase initial content in the mixture by the enthalpy of the eutectoid reaction [M] + [O] → [C_F] (119 J/yard). This comes from the fact that NPG and TRIS have different number of hydroxyl groups. As a upshot, the construction of hydrogen bridges in the depression temperature crystal structure of NPG changes when TRIS molecules are incorporated, so as some -OH or -NH₂ not form hydrogen bonds [17].

The high temperature eutectoid transition has greater interest. Indeed, the enthalpy associated to this transition ranges from 95 J/g (at 0.45 mole fraction of TRIS) upwardly to 245 J/thousand (close to pure TRIS). Again, the observed enthalpy changes are lower than that of pure TRIS solid-solid transition. They are even beneath the enthalpy values corresponding to the solid-solid transition of the fraction of TRIS in the mixture. Equally already explained, this reflects the effect of incorporating NPG molecules in the low temperature crystal structure of TRIS, which makes the number and free energy of hydrogen bonds to be reduced. In spite of information technology all, high enough enthalpy values (>150 J/g), comparable to those of paraffin waxes and salt hydrates in the same range of temperature, tin can be obtained by using NPG-TRIS mixtures with composition ≥0.6 mole fraction of TRIS. Moreover, the whole transition [C_F + O] → [C_B] takes identify in a narrow temperature range at 39-53 Chiliad below the solid-solid stage transition of pure TRIS (Figure 3b).

In practice, not only the gravimetric energy density is important, but also the volumetric free energy density, which determines the volume of the storage organization. Figure six shows the true density values of unlike NPG-TRIS mixtures measured at room temperature.

Measured values of true density (symbols) and calculated linear trend (discontinuous line).

As expected by the general rule of mixing, the density value increases linearly with the TRIS content in the mixtures, from i.05 m/mL (NPG) to 1.34 thou/mL (TRIS) at a rate of 0.293 g/mL per mole fraction unit of TRIS. The estimated values of volumetric energy density (enthalpy of phase transitions in kWh/thousand^three) from measured values of enthalpies of transition and densities are reported in Table 5.

Table 5

Estimated volumetric free energy density associated to the different stage transitions in NPG-TRIS system.

Phase Transition	Volumetric Energy Density (kWh/m3)
NPG solid-solid transition [Grand] → [CF]	34.vii
Low temperature eutectoid transition	0–38.8 depending on the composition
High temperature eutectoid transition	31–91.iii depending on the composition
Melting (including peritectic transition)	xi–9.half-dozen depending on the limerick
TRIS solid-solid transition [O] → [CB]	104.5

Although specific heat is a secondary parameter in thermal storage by isothermal or quasi-isothermal stage modify phenomena, we have also analyzed it. Measured information of specific heat in liquid phase, including stable and metastable liquid phases, are depicted in Figure 7. As expected, the specific rut increases with temperature for all compositions. Moreover, for a given temperature, the specific heat tends to slightly decrease when the content of TRIS in the mixture increases.

Specific heat as part of temperature for the liquid phase of different NPG-TRIS mixture (0.two to 0.viii mole fraction of TRIS). The data at temperatures to a higher place 400 K stand for the specific rut of stable liquid phase, whereas those beneath 400 K represent to metastable liquid phase achieved during cooling.

The measurements in the solid phases are more difficult because of the different phase transitions taking place, whose endothermic effects interfere with the calorimeter bespeak of the specific rut. The specific heat values obtained just below and to a higher place the low temperature eutectoid transition (electric current phases: [G] + [O]), besides as that shut below the high temperature eutectoid (current phases: [C_F] + [O]), are depicted in Figure 8.

The specific oestrus values obtained just below (303–307 Chiliad) and above (317–321 K) the low temperature eutectoid (E1), likewise every bit that close below (362–363 Thousand) the high temperature eutectoid (E2).

The effigy shows that, in all cases, the specific heat decreases increasing the TRIS content in the mixture. This behavior was expected considering that the orthorhombic ([O]) crystal structure of TRIS has lower specific rut than the monoclinic ([K]) and plastic phase ([C_F]) of NPG (see Figure two).

3.iv. Metastability Issues

We take already identified undercooling equally a cistron limiting the usability of the solid-solid transitions taking place in pure NPG ([1000] → [C_F]) and pure TRIS ([O] → [C_B]) in thermal free energy storage applications (Department 3.ane). Boosted metastability bug affecting the stage transitions in NPG-TRIS arrangement are hither discussed. To illustrate them, let us consider NPG-TRIS mixture with 0.485 mole fraction of TRIS, which exhibit all possible transitions taking identify in NPG-TRIS arrangement (except [M] → [C_F] and [O] → [C_B]) as well as all metastability problems observed in other compositions.

Figure ix represents the DSC thermograms obtained for unlike NPG_0.515TRIS_0.485 samples submitted to different heating/cooling rates (0.v to 2 Yard/min) betwixt 293 M and 453 K. At the lowest heating rate (0.5 M/min), the DSC thermogram shows four endothermic peaks that correspond to the following transitions (see Figure 4): [M + O] → [C_F + O] at 310 Chiliad (low temperature eutectoid), [C_F + O] → [C_F + C_B] at 392 1000 (high temperature eutectoid) and [C_F + C_B] → [C_B + L] (peritectic) followed past [C_B + 50] → [L] (melting) at 410 Grand. Still, on cooling, only two exothermic peaks appear in the thermogram. They correspond to the phase transitions [50] → [C_B + L] and [C_B + Fifty] → [C_F + C_B], which happen with negligible undercooling. The high temperature eutectoid reaction [C_F + C_B] → [C_F + O] does not take identify and, therefore, the low temperature eutectoid transition [C_F + O] → [M + O] neither. A metastable state in which [C_F] and [C_B] phases coexist is reached at the end of the cooling stage. As the heating/cooling rate increases, non only does this metastability persist, simply in that location is besides a progressive decrease of the endothermic peaks respective to the low temperature eutectoid transition. The enthalpies of phase transition measured over heating are reported in Table 6. It tin can be seen that the enthalpies of the high temperature eutectoid reaction and that of the peritectic transition and melting are not affected by the heating rate, whereas the enthalpy of the low temperature eutectoid reaction is significantly reduced for heating rates beyond one K/min. This is likely due to the tedious improvidence of NPG molecules toward the cubic lattice C_F [22].

DSC thermograms obtained for different NPG_0.515TRIS_0.485 samples submitted to different heating/cooling rates (0.5 to 2 One thousand/min) between 293 K and 453 Thou.

Table half dozen

Enthalpy of stage transition (J/g) associated to the endothermic phenomena appearing in the DSC thermograms in Figure 10.

Phase Transition	Heating/Cooling Charge per unit (M/min)
	0.5	one.0	2.0
Low temp. eutectoid	58.0	52.viii	17.6
High temp. eutectoid	120.3	119.seven	119.viii
Peritectic + Melting	28.ii	28.5	27.7

Let united states come up back to the stage transitions [C_F + C_B] → [C_F + O] and [C_F + O] → [G + O]. To investigate which, or if both, is responsible for the observed metastability, a new DSC experiment was carried out on a NPG_0.515TRIS_0.485 sample. The sample was first submitted to heating and cooling betwixt 293 K and 433 M with a heating/cooling rate of one K/min. After the cooling stage, the sample was kept at 293 M for 1h before subjecting it to a second heating and cooling bicycle betwixt 293 K and 323 G at a charge per unit of ane K/min. The calorimetric signals corresponding to this experiment are depicted in Figure ten. As expected, the stage transitions [C_F + C_B] → [C_F + O] and [C_F + O] → [M + O] are not observed during the cooling step of the start DSC bicycle. Nevertheless, during annealing (one h isotherm at 293 K-Times 300–400 min in Figure ten), a progressive weak exothermic effect can be appreciated respective to the transition from metastable stage [C_F + C_B] to stable [G + O]. This is confirmed by the appearance of the endothermic peak associated to the depression temperature eutectoid during the 2d DSC cycle. The latter also shows that the phase transition [C_F + O] → [M + O] takes identify over cooling, although information technology shows a significant caste of undercooling (c.a. 12 Chiliad). All this indicates that the apparent lack of reversibility in the transition from [C_F + C_B] to [M + O] is mainly adamant by slow kinetic of the phase transition [C_F + C_B] → [C_F+O].

DSC results obtained for the NPG-TRIS sample with limerick equal to 0.485 mole fraction of TRIS with heating/cooling charge per unit of 1 Thousand/min and annealing for one h at 293 K between the first and 2nd cycles.

As a starting time attempt to solve metastability issues, pure NPG and TRIS as well as NPG - TRIS mixtures have been doped with expanded graphite (EG) microparticles (average particle size = 75 μm). Figure 11 shows the DSC thermograms obtained for pure TRIS doped with different EG weight percentages. It tin be seen that the addition of EG micro-particles has a benign consequence on undercooling, although still insufficient for the apply of TRIS as stage change material in thermal energy storage applications. Like results were obtained for pure NPG doped with EG microparticles.

DSC thermograms obtained at a heating/cooling charge per unit of i K/min for TRIS samples doped with EG micro-particles. EG content: 0%wt, 5%wt, 10%wt, fifteen%wt and twenty%wt.

Tabular array 7 summarizes the observed effects of added microparticles on undercooling also equally on the temperature and enthalpy of solid-solid transition.

Tabular array vii

Summary of the influence of EG microparticles on the solid-solid transitions of pure NPG and TRIS: T_TR = solid-solid transition temperature at equilibrium; ΔH_TR = measured enthalpy of solid-solid transition on heating (referred to the total mass of the composite); ΔH_TR (cal.) = enthalpy of the solid-solid transition calculated by applying the dominion of mixtures.

Compound	EG Content (%Wt.)	Undercooling (K)	TTR (K)	ΔHTR (Exp.) (J/chiliad)	ΔHTR (Calc.) (J/thousand)
NPG	0	14.3	312.half dozen	119.4	119.4
	10	9.0	312.7	106.0	107.4
	xx	9.5	312.5	95.0	95.v
	30	nine.ane	313.three	78.9	83.6
	50	ten.vii	313.4	53.0	59.7
TRIS	0	65.0	406.8	281.0	281.0
	5	55.iv	406.1	230.8	267.0
	10	50.0	404.6	210.eight	252.9
	xv	47.0	401.1	199.2	238.8
	20	41.6	399.8	185.5	224.eight

Tabular array seven shows that the undercooling of pure TRIS is progressively reduced when increasing the EG content, from 65 One thousand for pure TRIS to 41.6 M for TRIS doped with 20%wt of EG. That is, the percentual reduction of undercooling goes from fifteen% to 36% approx. In the case of NPG, EG particles yield to an undercooling reduction of about 5 K (35% reduction) for all tested composites. It tin can as well be observed that EG microparticles accept meaning effect on the temperature and enthalpy of the transition from the orthorhombic crystal structure of TRIS to its plastic phase. The transition temperature (T_TR) is moderately lowered every bit the EG content is increased, whereas the enthalpy (ΔH_TR) is abnormally reduced. Indeed, measured values of ΔH_TR are 30–50 J/g approx. less than the enthalpy modify calculated by applying the rule of mixtures (ΔH_TR (cal.) = (1 − weight fraction of EG) × ΔH_TR). These furnishings are progressively amplified by increasing the amount of added EG, and, to a lesser extent, they can also be appreciated in NPG. The reduction of T_TR and ΔH_TR likely indicates that EG microparticles alter the hydrogen bonds structure of the low temperature crystal structure of TRIS and NPG. Added microparticles probably act every bit structural defaults that reduce the average number of hydrogen bonds by molecule. This leads to lower enthalpy of the solid-solid transition toward the plastic phase, on the one side; and facilitates the transition from the plastic phase to the ordered crystal structure, on the other side.

The effect of EG microparticles in the stage transitions [C_F + C_B] → [C_F + O] and [C_F + O] → [M + O] has besides been investigated. Figure 12 shows the DSC thermogram obtained for a NPG_0.515TRIS_0.485 sample doped with x%wt of EG microparticles. The figure shows that, contrary to the case of undoped mixture, the phase transitions [C_F + C_B] → [C_F + O] and [C_F + O] → [K + O] accept place within the scanned temperature range, although they display high undercooling caste which makes them of few interest for TES applications.

DSC thermograms obtained at heating/cooling rate of ane G/min for NPG_0.515TRIS_0.485 mixture with 10%wt. of EG.

4. Conclusions

The polyhydric alcohols neopentylglycol (NPG) and tris(hydroxymethyl)aminomethane (TRIS) exhibit solid state transformations in which hydrogen bonds between molecules are broken and rotational and vibrational disorder is introduced. These transitions from an ordered crystal structure to a matted i are characterized past high enthalpy changes, which makes NPG and TRIS very bonny phase alter materials for thermal free energy storage applications. NPG enables energy storage effectually 313 Thou with gravimetric free energy density of 119 J/g, whereas TRIS stores heat at 407 Chiliad with 281 J/g storage capacity. NPG-TRIS binary system has been investigated in this study looking for mixtures that comprehend transition temperatures different from those of pure compounds, thus extending their potential utilize in TES applications. The phase diagram of NPG-TRIS system has been studied by thermal analysis. The results show that NPG-TRIS mixtures exhibit similar solid-land transformations than pure compounds, plainly following the same mechanisms. The phase diagram presents 3 invariants (two eutectoids and 1 peritectic) that could be used to store rut under isothermal or near-isothermal weather condition. However, simply the eutectoid at 392 Grand shows sufficiently high enthalpy of solid-solid phase transition and transition temperature significantly different from that of the solid-country transitions of pure compounds. This eutectoid extends over wide range of compositions (mole fraction of TRIS above 0.45) providing increasing free energy storage chapters values equally the TRIS content increases, from 95 J/g (0.45 mole fraction of TRIS) upwardly to 245 J/k (close to pure TRIS).

In spite of numerous assets of NPG and TRIS, undercooling has been identified as a limiting gene for their use in TES applications. The observed undercooling caste is about 15 K for NPG and 65 M for TRIS. This is typical in polyhydric alcohols and its amine derivatives and could exist explained past the existence of a high degree of orientational freedom in their plastic stage, which leads to very low probability that hydroxyl groups from 2 neighbor molecules juxtapose and form a hydrogen bond. It has been proven that adding expanded graphite (EG) microparticles to NPG and TRIS has a beneficial effect on undercooling, which is significantly reduced. For NPG, the reduction is of most 5 K regardless of the EG content. In the instance of TRIS, the higher the EG content, the greater the reduction the undercooling (65 K of TRIS to 41.6 K of TRIS doped with 20%wt of EG). In both cases, this result on undercooling is accompanied by abnormal reduction of the enthalpy of the solid-country transition, which indicates that EG microparticles modify the hydrogen bonds structure of the ordered crystal construction of TRIS and NPG, so as some -OH or -NH_two not form hydrogen bonds. The resulting reduction of the boilerplate number of hydrogen bonds per molecule should facilitate the transition from the plastic phase to the ordered crystal structure and could explain the undercooling mitigation observed by adding EG particles.

NPG-TRIS mixtures non only have subcooling problems, but also certain solid-land transformations bear witness ho-hum kinetics. In particular, the transitions [C_B] → [C_F + O], [C_F + C_B] → [C_F + O] and [C_F] → [C_F + O] showroom huge undercooling and need long annealing times to be completed. It has been proved that the improver of EG microparticles (10%wt) has not only meaning effect on undercooling, which is strongly reduced, merely also significantly ameliorate the kinetics of these transitions.

Although the addition of EG microparticles improves the behavior of NPG, TRIS and their mixtures, the reduction of subcooling accomplished is still bereft for the apply of these materials in thermal storage applications. A better agreement of the mechanisms behind the phenomenon of undercooling in plastic crystals volition be needed to guide the search for more efficient doping nanoparticles.

Acknowledgments

The authors express their sincere thanks to Yagmur Polat and Leticia Martinez for their technical back up.

Appendix A

NPG_1−xTRIS_x (0 < x <one) samples were submitted to several preliminary tests to ensure the pertinence of the results. XRD at room temperature was used to check that neither samples grooming nor their thermal treatment leads to degradation or structural changes compared to NPG and TRIS. An example of the results accomplished is depicted in Figure A1. This effigy shows the diffractograms of as received NPG (monoclinic structure) and TRIS (orthorhombic construction), likewise every bit those of an NPG_0.515TRIS_0.485 sample but later on milling/mixing and later on melting and slow cooling down upwardly to room temperature. The diffractograms of NPG and TRIS show the typical diffraction peaks of NPG monoclinic construction and TRIS orthorhombic structure, respectively. Regarding NPG_0.515TRIS_0.485, the diffraction pattern after milling/mixing is like to that obtained after melting and solidification of the sample. It tin be seen that the diffraction peaks of NPG and TRIS can be clearly recognized in the NPG_0.515TRIS_0.485 diffractograms as expected (run across Figure 4). Moreover, none of them present new diffraction peaks. All this indicates that the crystalline structures of NPG and TRIS remain unchanged afterward milling/mixing and melting/solidification processes, proving that none of those processes induce degradation or new phases formation.

Figure A1

XRD diffractogram obtained at room temperature for (i) monoclinic construction of NPG (black line), (two) orthorhombic structure of TRIS (dark-brown line), (iii) NPG_0.515TRIS_0.485 subsequently ball milling and mixing (red line) and (iv) NPG_0.515TRIS_0.485 after heating and cooling treatment (orangish line).

The limerick of NPG_1-xTRIS_x (0.45 ≤ x ≤ 0.55) was measured past liquid NMR before and after the heating handling in order to verify that eventual NPG evaporation does not significantly change the initial limerick of the sample. This technique was selected because both polyols (NPG, TRIS) are soluble in h2o and take distinct, nonoverlapping NMR spectra, thus leading to loftier accurate conclusion of their mixture limerick. Indeed, as shown in Figure A2, the ratio between the number of methyl protons of NPG (chemical shift = 0.798 ppm) and methylene protons of TRIS (chemic shift = 3.450 ppm) can be hands determined. The signals at chemical shifts 3.338 ppm and four.702 ppm correspond, respectively, to the methylene protons of NPG and water. The measurements of the limerick of NPG-TRIS mixtures are reported in Tabular array A1. It tin exist seen that in that location are no significant changes in the composition of the samples after heating handling, thus concluding that evaporation of NPG is negligible under applied training and testing weather condition.

Tabular array A1

Composition (molar fraction of TRIS) in NPG-TRIS mixtures determined by ¹H NMR.

Earlier Thermal Handling:	0.45	0.46	0.47	0.485	0.485	0.485	0.49	0.5	0.51	0.52	0.53
Subsequently Thermal Treatment:	0.454	0.471	0.472	0.479	0.485	0.478	0.495	0.507	0.518	0.53	0.538

Figure A2

¹H-NMR spectrum of the NPG-TRIS mixture with 0.485 molar fraction of TRIS.

Author Contributions

Conceptualization, Southward.S.-G., Due east.P.d.B., South.D. and Due east.Due south.; methodology, Due south.Southward.-Chiliad., E.S, Southward.D. and E.P.B.; investigation, Southward.Due south., G.A.L., N.1000.; information curation, S.S.-Thou. and Due south.D.; writing—original draft preparation, S.Southward.-M., Northward.One thousand. and E.South.; writing—review and editing, Southward.S.-Grand., E.P.d.B., Southward.D. and Á.S.; supervision, S.D., E.S., G.A.L. and E.P.d.B.; project administration, S.D.; funding conquering, Due east.P.d.B and S.D. All authors have read and agreed to the published version of the manuscript.

Funding

This enquiry was funded by the Ministry of Science Innovation and Universities of Spain, grant number RTI2018-099557-B-C21, and by the Basque Government through Elkartek18 R&D program.

Conflicts of Interest

The authors declare no conflict of interest.

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