Thermodynamic heat-transfer phenomena in nanostructured glassy substances: a comparative study on g-As5Se95 and g-As55Se45

Nanostructurization-driven responses in calorimetric heat-transfer phenomena are compared for glassy arsenoselenides at different levels of their molecular network organization, namely in high-polymerized g-As5Se95 and low-polymerized g-As55Se45, employing multifrequency temperature-modulated DSC-TOPEM® method complemented with Raman scattering microspectroscopy. It is shown that high-polymerized network composed of AsSe3 pyramids interlinked by Se chains with small number of Se8 molecular units decoupled from this network prevails in melt-quenched and nanomilling-derived g-As5Se95. Transition to more polymerized network due to incorporation of destroyed Se8 molecules into glass backbone occurs in this glass under nanostructurization. As a result, nanostructurization-driven calorimetric response in g-As5Se95 dominates by size-induced glass-transition temperature depression. The low-polymerized structure of g-As55Se45 is built of As–Se network enriched with tetra-arsenic selenide molecular units decoupled from this network. Molecular-to-network transition owing to nanomilling-driven destruction of these cage molecules and their incorporation into newly polymerized glass-forming backbone occurs in g-As55Se45 resulting in strong increase in calorimetric glass-transition temperature.


Background
Nanostructured materials are distinguished by great variety of size-dependent phenomena which play a pivotal role in the majority of their practical applications [1]. Thus, being transferred to a nanoscale level evolving agglomerates of atoms and/or atomic groups composing nanoparticles, these nanoscopic materials demonstrate strong propensity to multiphase occurrence, modifying essentially calorimetric responses from conventional heat-transfer phenomena [1][2][3]. As a rule, clear reduction trend dominates in characteristic interphase transition temperatures in nanostructured crystalline materials below some critical nanosizes, when one of the metastable phases becomes thermodynamically more favorable over others [3][4][5][6][7].
The similar trend is character for amorphous polymeric substances, which typically reveal notable depression trend in glass-transition temperature T g when being prepared as thin and ultra-thin (having a few nm in thickness) films [8][9][10][11][12].
However, in some amorphous materials belonging to molecular network chalcogenide glass-formers [13,14] such as melt-quenched glassy arsenoselenides g-As-Se possessing structural conformations with different degrees of polymerization [15,16], the calorimetric response on transferring to nanoscale is expected to be more complicated because of possible effect from nanostructurization-driven molecularto-network transitions (as was also in the case of isotypical glassy arsenosulphides g-As-S [17,18]). Stable low-polymerized (preferentially molecular-type) and high-polymerized (preferentially network-type) atomistic structures in g-As-Se subjected to nanostructurization through highenergy mechanical milling were refined with a help of X-ray powder diffraction (XRPD) analysis [19]. It was shown that cage-like molecular entities stabilized initially in meltquenched g-As-Se were merely destroyed under high-energy mechanical milling-driven nanostructurization facilitating formation of more polymerized chain-like network structures, this process being referred to as re-amorphization [20]. The objective of the current research is to compare most plausible nanostructurization-driven responses in the glass-transition region of these arsenoselenide glasses revealed at principally different levels of their molecular network arrangement, namely in high-polymerized g-As 5 Se 95 and low-polymerized g-As 55 Se 45 specimens, employing the multifrequency temperature-modulated DSC-TOPEM ® method complemented with microstructural characterization using the Raman scattering (RS) microspectroscopy.

Materials and methods
The bulk samples of glassy g-As 5 S 95 and g-As 55 S 45 were prepared from high-purity elemental precursors (As and Se of 5 N purity) employing conventional melt quenching technological route, that is rapid cooling of the glass from melt-liquid state [13]. The melt-rocking operation was used to prepare homogeneous alloys (as was justified by Lucas et al. [21]), avoiding necessity of extra-long homogenization stabilizing these glasses from very unfavorable vertical position in the furnace [22]. The sealed ampoules with elemental precursors taken in the ratio corresponding to As 5 S 95 and As 55 S 45 compositions were heated in a rocking furnace up to 650 °C in 6 h and then homogenized at this temperature for 10 h. At the finishing stage, the ampoule was cooled down to 500 °C and finally quenched in a water. To eliminate mechanical strains appeared in bulky material after rapid cooling, the just-synthesized glasses were annealed at 120 °C for 1 h. The ingots extracted from ampoules were amorphous, as it followed from the XRPD analysis, conchlike fracture and IR transparency of freshly prepared cut sections. The room temperature densities of g-As 5 S 95 and g-As 55 S 45 samples determined by the Archimedes displacement in ethanol using the Mettler Toledo balances were (4.310 ± 0.005) g cm −3 and (4.447 ± 0.005) g cm −3 , respectively, in full agreement with the known atomic densities of compositionally similar glassy counterparts from binary As-Se system [15,16].
The melt-quenched bulk chalcogenide glasses of the above chemical compositions (hereafter referred to as unmilled samples) were subjected to nanostructurization by high-energy mechanical milling (nanomilling) in a dry mode using Pulverisette 6 mill operated at protective Ar atmosphere and 500 min −1 rotational speed. This procedure was performed for 60 min in 250 mL tungsten carbide chamber loaded with 50 balls (each having 10 mm in diameter), using ~ 3 g of coarse-grained glass sieved under 200 μm. The amorphous state of the samples was not changed under nanomilling, as it follows from diffuse peak-halos in their XRPD patterns [19,20]. The fine-grained powdered substance was finally compressed in stainless steel die (under ~ 0.7 GPa) to produce the disc-like pellets (having ~ 6 mm in diameter and ~ 1 mm in thickness) most suitable for further microstructural characterization research.
Calorimetric thermo-analytical measurements were performed employing multifrequency temperature-modulated DSC-TOPEM ® with DSC-1 calorimeter (Mettler Toledo, Switzerland), as described in more details elsewhere [17,18]. In this method, stochastic temperature modulations are superimposed on underlying rate of DSC scans, resulting in frequency dependent and independent phenomena [23]. This provides more information on thermodynamic stability of the revealed phases. The DSC-TOPEM ® instrument was equipped with FRS5 + sensor and HT100 (Huber, Germany) intracooler, the STAR e ver. 13a software being used to control experimental conditions and process the data. The calorimeter was multi-point calibrated using In and Zn standard probes. The tested samples (ca. 10-15.0) were encapsulated in sealed 20 μL Al pans kept in protective N 2 atmosphere, and scanned at the rate of 1.0 K min −1 stochastically modulated in pulses between 15 and 40 s, the pulse height being 1 K. The evaluations were adjusted using sapphire reference curve, the width and shift of calculation window being 60 s and 1 s, respectively.
The detected calorimetric events were parameterized using the DSC-TOPEM ® profiles presenting temperature variations of non-reversing (HF nrev ) and reversing (HF rev ) heat flow in the heating run, each measuring protocol being averaged in triplicate. The reversing thermal effects, resulted from second-order phase transitions such as glass transition [24], were parameterized by heat capacity step-like jump (ΔC p ) allowing characteristic glass-transition temperatures determination in onset (T g onset ) and mid-point (T g mid ) presentation [24,25]. The non-reversing thermal effects, connected with enthalpy relaxation in the glass-transition region during heating run, were parameterized by specific enthalpies difference ΔH [17,25].
Microstructural nature of nanostructurization-driven transformations in the glass was identified with micro-Raman spectroscopy using the Horiba Xplora spectrometer equipped by CCD detector operated at room temperature. The CW 785 nm laser of 90 mW output power was employed for excitation, the 10% power option being used to avoid photostructural effects. Other measurement options applied were as follows: × 100 objective, 1800 1 mm −1 grating, 500 μm hole, and 50 μm slit. The spectral resolution was maintained at around 2 cm −1 and spatial resolution was near 2 μm. A number of scans were chosen in dependence on the surface of pelletized glass samples. Numerous scans on the entire surface were performed to be sure that RS spectra processed with the Horiba LabSpec software were reasonably identical. The milled and unmilled samples were compared through normalization by matching spectral areas in the region of interest. The detected RS bands were identified using the known data for numerous glassy analogs [26,27].

Results and discussion
Thermodynamic heat-transfer responses were detected in the pellets of arsenoselenide glasses by multifrequency DSC-TOPEM ® method. The temperature-modulated DSC-TOPEM profiles showing variation of reversing (HF rev ) and non-reversing (HF nrev ) heat flow in the bulky melt-quenched (unmilled) and nanostructurized (milled, viz. subjected to high-energy mechanical milling) g-As 5 Se 95 and g-As 55 Se 45 samples are depicted on Figs. 1, 2, respectively, the calorimetric parameters derived from these curves being gathered in Table 1.
Under heating run, the principal endothermic thermal alteration event in the studied substance represents glass transition. In modulated DSC-TOPEM profiles of unmilled g-As 5 Se 95 sample depicted on Fig. 1a, the characteristic step-like jump supplemented by small peak is revealed in the temperature behavior of reversing (HF rev ) heat flow near the glass-transition region with heat capacity variation ΔC p approaching ~ 0.14 J g −1 K −1 , while distinct peak with specific enthalpies difference of ΔH ~ 3.32 J g −1 is observed in non-reversing (HF nrev ) heat flow dependence shown on Fig. 1b. The similar calorimetric thermal alteration responses are character for other families of chalcogenide glasses undergoing physical aging [28,29].
An evident depression in the glass-transition temperature T g is observed in modulated DSC-TOPEM profiles of this g-As 5 Se 95 sample after nanomilling, and this trend being accompanied by increase in the DSC-TOPEM peak amplitude. As it follows from Table 1, the T g onset value is reduced in milled g-As 5 Se 95 on 2.6 °C (from 69.3 to 66.7 °C), while specific enthalpies difference ΔH derived from temperature variation of non-reversing (HF nrev ) heat flow is enriched on more than 16% (reaching as high as 3.86 J g −1 ). Noteworthy, the glass-transition width defined from temperature dependence of reversing (HF rev ) heat flow on Fig. 1a for g-As 5 Se 95 samples nanostructurized by high-energy mechanical milling does not change essentially.
Thus, milling-driven nanostructurization of chalcogenide glasses possessing highly polymerized network (such as under-stoichiometeric g-As 5 Se 95 ) prevails by the known effect of size-induced glass-transition temperature T g depression, which has been extensively studied from the mid 1990s [8][9][10][11][12]. However, this calorimetric heat-transfer phenomenon is cardinally changed in case of glasses having more irregular molecular-like structure (such as over-stoichiometric g-As 55 Se 45 ). The bulk glass of this composition derived by melt quenching shows similar responses (albeit more reduced ones in modulated DSC-TOPEM profiles) within the glass-transition region, particularly, the characteristic steplike jump without accompanied peak in reversing (HF rev ) heat flow on Fig. 2a with ΔC p ~ 0.11 J g −1 K −1 (Table 1) and distinct endothermic peak in non-reversing (HF nrev ) heat flow on Fig. 2b with ΔH ~ 5.57 J g −1 . The value of T g onset temperature for this unmilled glass (g-As 55 Se 45 ) defined from reversing curve occurs to be 134.6 °C (Table 1), and this value being in an excellent agreement with reduced glasstransition temperatures in the As-rich arsenoselenide glasses ascribed to variance of different molecular tetra-arsenic selenide species (such as As 4 Se 4 , As 4 Se 3 , As 4 ) [16]. Under transition to more polymerized structure of g-As 55 Se 45 samples undergoing nanomilling, the bulk glass-transition temperature notably increases approaching T g onset ~ 170.0 °C (see Table 1), the value character for unmilled melt-quenched samples of stoichiometric arsenic triselenide glass g-As 2 Se 3 [15,16]. At the same time, the glass-transition temperature width in nanomilled g-As 55 Se 45 defined from reversing heat flow curve on Fig. 2a is narrowed by a factor at least 2.
The principal calorimetric response originated from nonreversing (HF nrev ) heat flow variation is changed drastically in nanostructurized g-As 55 Se 45 due to thermal relaxation of strong inner strength generated in this glass under highenergy mechanical milling. As a result, the pronounced exothermic event (depicted on Fig. 2b) with specific enthalpies difference ΔH approaching as high as -29.70 J g −1 (see Table 1) prevails in this glassy sample (nanomilled g-As 55 Se 45 ).
To shed light on the nature of nanomilling-driven microstructural transformations in the studied glasses, the micro-RS spectra were collected and compared for g-As 5 Se 95 and g-As 55 Se 45 samples in both unmilled (melt-quenched) and milled (nanostructurized) states (Fig. 3, 4).
In the micro-RS-spectrum of melt-quenched g-As 5 Se 95 (Fig. 3, black curve) only a few bands are resolved, these being weak and relatively narrow band near ~ 107 cm −1 and ~ 132 cm −1 , weak but evidently broader band positioned near ~ 490 cm −1 and very strong band near ~ 248 cm −1 . The latter is due to overlapping of bond-stretching vibrational modes assigned to trigonal AsSe 3 pyramids at 227 cm −1 [30], and modes at ~ 230-270 cm −1 assigned to multiatomic Se fragments incorporated in various local environment [31], in part, strongly correlated chains disposed in trigonal t-Se conformations at 234 cm −1 , dissolved multi-Se chains at 250 cm −1 and molecular-type Se 8 species at ~ 260 cm −1 . The band at 107 cm −1 is probably connected with Se 8 rings contributing through A 1 mode at 112 cm −1 [32], while 132 cm −1 band is due to Se polymeric chains having RS-active E mode positioned at 138 cm −1 [32]. The broad RS-active band at 490 cm −1 is related to second-order Raman scattering from homonuclear Se-Se bonds [33]. Thus, the structure of unmilled g-As 5 Se 95 represents highly polymerized network built of AsSe 3 pyramids interlinked by Se chains with very small number of molecular units (such as Se 8 ) decoupled from this network.  Only slight changes occur in the RS spectrum of this glass subjected to nanostructurization through milling (also shown on Fig. 3). The RS band at ~ 132 cm −1 ascribed to E mode of Se chains is increased in nanomilled g-As 5 Se 95 . Strong band at ~ 248 cm −1 does not change its position after nanomilling, while being slightly depressed, especially from the high-frequency side corresponding to spectral domain of preferential location of intramolecular bond-stretching modes ascribed to Se 8 molecules [31,32]. Due to incorporation of atomic remainders of destroyed molecular units into glass network, the milled specimen becomes more polymerized, and this effect being associated with slight increase in the respective glass-transition parameters on this glass (see Table 1).
In the milled g-As 55 Se 45 samples, all the above features observed in the micro-RS spectra are essentially broadened and depressed (see Fig. 1, red curve), testifying in favor of nanostructurization-driven destruction of respective tetraarsenic selenide molecular units and their incorporation in newly polymerized arsenoselenide glass-forming network. Thus, the structure of this nanostructurized chalcogenide glass becomes notably stressed (as compared with iso-compositional unmilled sample), being affected by a variety of structural defects generated under high-energy mechanical milling. Atomic remainders of destroyed molecules are reincorporated into a glassy network, completely changing calorimetric response from the glass-transition event.

Conclusions
Nanostructurization-driven thermodynamic responses in calorimetric heat-transfer phenomena are compared for glassy arsenoselenides at principally different levels of their molecular network organization, namely in high-polymerized under-stoichiometric (preferentially network-type) g-As 5 Se 95 and low-polymerized over-stoichiometric (preferentially molecular-type) g-As 55 Se 45 , using multifrequency temperature-modulated DSC-TOPEM ® complemented with microstructure research based on Raman scattering (RS) microspectroscopy.
It is shown that high-polymerized network composed of AsSe 3 pyramidal fragments interlinked by long Se chains with small number of Se 8 molecular units (decoupled from this network under melt quenching) prevails in both meltquenched and nanomilling-derived g-As 5 Se 95 samples. Transition to more polymerized network occurs in this glass under nanostructurization by nanomilling, being caused by incorporation of destroyed Se 8 molecules into the glass backbone. As a result, nanostructurization-driven calorimetric  Fig. 4 Micro-RS spectra collected from unmilled (black curve) and milled (red curve) g-As 55 Se 45 . The most prominent RS-active modes in spectrum of bulk sample are positioned by arrows, and traced by dotted lines to micro-RS spectrum of nanostructured sample (see text for more details) response in this glass dominates with the known effect of size-induced glass-transition temperature depression.
The low polymerized preferentially molecular-type structure of g-As 55 Se 45 is built of arsenoselenide network enriched with tetra-arsenic selenide molecules decoupled from this network under melt quenching. Molecular-tonetwork transition owing to nanomilling-driven destruction of the above molecular units and their incorporation into newly polymerized glass-forming backbone occurs in this glass undergoing nanostructurization, thus resulting in strong increase in calorimetric glass-transition temperature.