Self-Forming 3D CoreâShell Ceramic Nanostructures for Halogen

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Self-Forming 3D Core-Shell Ceramic Nanostructures for Halogen-Free Flame Retardant Materials Elena Palacios, Pilar Leret, Maria J. De La Mata, Jose F Fernandez, Antonio H. De Aza, Miguel A. Rodriguez, and Fernando Rubio-Marcos ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01379 • Publication Date (Web): 23 Mar 2016 Downloaded from http://pubs.acs.org on March 28, 2016

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ACS Applied Materials & Interfaces

Self-Forming 3D Core-Shell Ceramic Nanostructures for HalogenFree Flame Retardant Materials Elena Palacios†, Pilar Leret∥, Maria J. De La Mata§, Jose F. Fernández†, Antonio H. De Aza†, Miguel A. Rodríguez†, Fernando Rubio-Marcos†,*

†

Instituto de Cerámica y Vidrio, ICV-CSIC, Kelsen 5, 28049, Madrid, Spain

∥Advanced

Dispersed Particles S.L. Research & Development Department. Oro 45, nave 14, P.I.Sur, 28770, Colmenar

Viejo, Madrid, Spain §

Laboratorio de RMN Sólidos, Servicio Interdepartamental de Investigación, Universidad Autónoma de Madrid,

28049, Madrid, Spain *Correspondence and requests for materials should be addressed to F.R-M (email: [email protected])

ABSTRACT: The synthesis of aluminum phosphates-based composites has been widely studied during the past decade because of the promising industrial application of these materials. Here we show a simple one-pot heterogeneous precipitation approach to fabricate a Sepiolite-Phosphate (SepP) composite with adequate control of the size and dispersion of the phosphate nanoparticles. This coupling between aluminum phosphate and sepiolite nanofibers results in the development of a novel three-dimensional rigid supported phosphate structure, which is generated during the thermal treatment. According to our results, this phenomenon can be explained by a migrationcoalescence mechanism of phosphate nanoparticles over the sepiolite support, assisted by a liquid phase. It is worth pointing out that this stimulant behavior observed here could have potential technological applications such as halogen-free flame retardant materials.

KEYWORDS: Phosphate; Synthesis; Precipitation; Halogen-Free; Flame retardant

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1. INTRODUCTION As new technologies emerge, so too does the demand to demonstrate their safety. The advent of plastics, for example, has witnessed a transformation in the way we live. A whole range of colourful and innovative products have entered our homes. Simultaneously, there has been a need to ensure that these products are not only functional and attractive but also safe from the danger of fire. Flammable materials present within furniture, and electrical and electronic products for example, require protective measures to safeguard us. Enter flame retardants, substances which, by their nature, delay ignition and thereby the development of a fire.1 The issues and concerns associated with such questions highlight the need to develop flame retardant materials that, additionally, comply with strict safety regulations. Moreover, nowadays, there is general agreement in respect of environmental concerns, which continue to grow regarding the most common flame retardant chemistries like halogenated small molecule compounds.2,3 Toxic halogenated flame retardants can bioaccumulate even in isolated locations, raising environmental concerns. So, the European Community proposed to restrict the usage of brominated diphenyl oxide flame retardants due to the highly toxic and potentially carcinogenic brominated furans and dioxins formed during combustion.4 Nowadays, more attention is paid to developing halogen-free flame retardants to replace the halogenated ones. Especially, intumescent systems containing phosphorus and nitrogen are welcomed. Hence, the need to develop flame retardant materials that combine efficiency with environmentally friendly aspects is undeniable.5-10 Aluminum phosphates are extensively used in many fields of research. These phosphates are used as binders in refractory materials, metal protective coatings and flame retardant systems due to their thermal and chemical stability properties.11-13 These applications are due to the ability of acid phosphate to polymerize at low temperature.14 It is well known that phosphate species containing P-OH groups have the ability to form polymeric phosphates by condensation.14-15 When these acid phosphates are subjected to a heat treatment at low temperature ( 95%, Pangel S9, TOLSA S.A.) were homogeneously dispersed in deonized water by high shear mixing obtaining a 6 wt% sepiolite final concentration, followed by the addition of a HCl solution (37%, Panreac) in order to adjust pH of the solution around 2 under stirring. On the other hand, 233 g of Al(OH)3 (90.5%, Panreac) were dissolved in aqueous solution of H3PO4, 17.33 g, (85%, Sigma-Aldrich) leaving an Al/P molar ratio of 1:3, and stirred at room temperature (RT) to obtain a uniform dissolution. Then, this last solution was added to the initial sepiolite dispersion, and the reaction was continued for 30 min under vigorous stirring. Then, a NH4OH solution (30%, Merck) was added dropwise until pH 4.8 with magnetic stirring resulting in the phosphate precipitation. The reaction mixture was kept for different ageing times at RT and, in order to obtain a phosphate (P hereafter) with small particle size, ageing time zero was chosen. Synthesized powders were filtered under vacuum, washed with deionized water and dried at 60ºC, obtaining a powder as final product. Thermal Characterization: Powders have been characterized by Differential Thermal Analysis-Thermogravimetric Analysis (DTA-TGA) following a heating rate of 10ºC/min (SETARAM SETSYS Evolution). The DTA-TGA curves were recorded over the temperature range 30–1200 °C with a flow rate of 0.04 L/min and under an air atmosphere, respectively. In addition, thermal treatments at 600ºC and 900ºC during 2 hours with similar heating rates were performed in order to study the crystalline phases generated during the thermal processes. Structural Characterization: The crystalline structure was determined by X-ray diffraction analysis (Powder diffractometer Bruker D-5000) and 27Al and 31P Solid state MAS-NMR (Bruker AV-400-WB) on the powders obtained by milling the composites at room temperature. The XRD patterns were recorded over the angular range 5–60° (2θ) with a step size of 0.0334° and a time per step of 100 seconds, using Cu Kα radiation (λ = 0.154056 nm) with a working voltage and current of 40 kV and 100 mA, respectively. 31 P

MAS (Magic Angle Spinning) NMR (Nuclear Magnetic Resonance) spectra were recorded on a Bruker AV-400-WB

spectrometer operating at 161.96 MHz. The pulse length was 2 µs and 10 s delay time was used. For the 1H-31P crosspolarization experiments, the 1H pulse width was 3 µs, the delay time was 4 s and a 1H decoupling strength of 80 KHz


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with a contact time of 2 ms was used. A total number of 256 scans were accumulated with a spinning rate of 10 KHz. Solid (NH4)H2PO4 was used as secondary reference with a 0.82 ppm chemical shift with respect to H3PO4 (85%). 27 Al

MAS NMR spectra were recorded on a Bruker AV-400-WB operating at 104.26 MHz. The pulse length was 3µs

and 2 s delay time was used. A total number of 1024 scans were accumulated with a spinning rate of 10 KHz. Al(SO4)2(NH4)12H2O was used as secondary reference with a -0.4 ppm chemical shift with respect to Al(NO3)3 0.1 M. The relative volume fractions of the different phases involved in the system can be calculated using the integrated intensities of

31 P

and 27Al MAS-NMR signals obtained from line profile analysis. Peaks positions were fit assuming a

Lorentz peak shape. Finally, the chemical composition of SepP composite was determined by X-Ray Fluorescence (XRF). X-ray fluorescence analyses were carried out using a Spectrometer (Model MagiX PW 2424, Philips, Eindhoven, Netherlands) with rhodium X-ray tube, 2.4 kW generator and five-position crystal charger. The spectrometer is also equipped with the IQ® analytical software for analysis, which includes fundamental parameters for inter-elemental correction calculations. The operating conditions were LIF200 and PE analyzing crystals, no primary beam filter, 150 µm sample collimator, vacuum atmosphere and flow-proportional and scintillation detector. The content of all oxides was determined using the melting method with Li2B4O7. For that, 0.3 g of dry samples and 5.5 g of Li2B4O7 were homogeneously mixed and placed inside a pellet box and were compacted using a hydraulic press for 1 min at a pressure of 5.1103 kg. Pellets were stored inside desiccators to avoid hydration from air humidity. (More information about the chemical analysis by XRF can be found in Table S1 of the Supplementary Information) Morphological Characterization: The powders were also morphologically characterized measuring their specific surface area (Ss) (Monosorb Surface Area Analyser MS-13). The particle size and morphology of the powders were evaluated using both a Field Effect Scanning Electron Microscope (FE-SEM) (Hitachi S-4700) and a JEOL 2100F Transmission Electron Microscope (TEM/HRTEM) operating at 200 KV and equipped with a field emission electron gun providing a point resolution of 0.19 nm. For TEM sample preparation, the composites were carefully suspended in ethanol. The suspension was dropped on a copper TEM grid with carbon film support. The particles were kept at the grid after ethanol evaporation.

SepP hybrid material preparation for its technological applications as flame retardant: In order to evaluate the flame retardant behavior a simple test was performed. To prepare the composite test samples, an inorganic mixture of 0, 5 and 10 wt% SepP, 10 wt% alumina trihydrate (ATH-104LE, Martinal®, Abermale Co.) and a bi-component epoxy


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(EpoxiCure® 2 Epoxy System, Buehler) were mixture in a tricylindrical miller EXAKT 50i during 10 minutes and then, poured into a cylinder mold of 10 mm in diameter and 50 mm in length. Finally, the composite was cured for 24 hours at RT before demolding and 1 h at 160 °C. 10 wt% alumina trihydrate was added to all the compositions because it improves flame retardant properties as well as char formation in polymeric compounds. Flame retardant behavior was evaluated igniting the composite cylinder during 10 second under a laboratory flame burner and then removing it from the flame. In addition, a pure dry mixture of 50 wt% SepP and 50 wt% of ATH was pressed in a cylinder at 100 MPa and then thermally treated at 600ºC and 900ºC for 1 minute with a heating rate of > 50ºC/min. Mechanical compression load tests were performed to evaluate the mechanical response of thermally treated samples.


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3. RESULTS AND DISCUSSION 3.1. Morphological and structural characterization of the Sepiolite-Phosphate (SepP) composites: Revealing the acid and amorphous character of aluminum phosphates. In order to clarify the dispersion state of the phosphate (P) supported on the sepiolite support (Sep), FE-SEM characterization has been carried out on the Sepiolite-Phosphate (SepP) composites. FE-SEM micrograph of SepP composite is shown in Figure 1a. The FE-SEM micrograph on Fig 1a shows the typical geometry of the Sep fiber which in terms of diameter is ~ 20-50 nm. The micrograph shows that our direct precipitation process of aluminum phosphate (P) on Sep nanofibers allows the uniform distribution of the P on the Sep fibre surface. As expected, the Brunauer–Emmett–Teller (BET) surface area values of the Sep support (284 m2/g) decreases with the aluminum phosphate (124 m2/g) coverage, because of a partial blockage of the Sep support pores by P nanoparticles. In order to observe the phosphate nanoparticles in detail, low-resolution TEM (LR-TEM) micrograph of this same sample is shown in Figure 1b. Fibres with and without phosphate nanoparticles are observed. The SepP composites, Fig. 1b, show Sep fibers covered by P nanoparticles with an average particle size of ~ 80 nm. Surprisingly, a High Resolution TEM (HRTEM) image, Fig. 1c, of the SepP composite prepared by the precipitation method, reveals that the P nanoparticles observed at low resolution (Figs. 1a-b) are made up of primary nanoparticles with sizes below 20 nm. Thus, these basic units form larger nanoparticles by aggregation processes.

Figure 1  Microstructural and structural characterization of the Sepiolite-Phosphate (SepP) composites: (a) FE-SEM and (b) TEM micrographs of the SepP composite. From the panels a and b, the SepP composite structure can be clearly observed. Isolated phosphates (Ps), signaled with yellow arrows in Figs. a and b, are highly and uniformly monodispersed on the Sepiolite (Sep) support. The Fig. (c) shows a HRTEM image where the morphology of the phosphates can be observed, which is composed of spherical-like nanoparticles with diameter sizes between 20 and 10 nm. (d) XRD patterns corresponding to the Sepiolite support (top) and the Phosphate/Sepiolite composites (middle) prepared by the heterogeneous precipitation method. The circle symbols are associated with the sepiolite support. The figure also shows the difference between the SepP and Sep diffractograms (bottom). The blue region under the curve corresponds to the amorphous phase of the aluminum phosphate.


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A comparative XRD characterization has been done between the Sep support and SepP composite, studying the XRay diffractograms (Figure 1d). SepP composite pattern corresponds to the sepiolite phase although differences in the crystallinity grade are found between both spectra. The SepP sample crystallinity is lower than the sepiolite one although no peaks of any phosphate are observed. A previous study of the homogeneous synthesis of aluminum phosphate shows that the synthesized phosphate is amorphous at the same reaction conditions.25 The amorphous character of phosphate is observed by the presence of a broad band under the XRD patterns of the SepP composite. Another relevant issue observed from Fig. 1d is the decrease in the intensity ratio of the characteristic peaks corresponding to the planes (110)/(131) of the sepiolite, which is clearly related to the lower crystalline character of the sample due to both the initial acid treatment

34,35

and the presence of amorphous phosphate. The pattern at the

bottom of the figure is obtained as the difference between SepP and Sep diffractograms. By this way, it is possible to isolate the approximate aluminum phosphate diffractogram. Then, the aluminum phosphate diffractogram corroborates its amorphous character.

Figure 2  Revealing the amorphous character of aluminum phosphates by NMR: 31P signal observed on the SepP composite by CP-MAS NMR (a), and MAS- NMR (b). Spinning side bands are indicated by asterisks. The panel (c) shows the 27Al MAS NMR spectra of the SepP composites. The spectrum is fitted to the sum of three Lorentzian peaks, which are indexed as octahedral, pentahedral and tetrahedral Al3+ environments, respectively.


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The amorphous nature of the synthesized phosphate by the precipitation process makes difficult its characterization and the evaluation of its acid character. In order to solve this issue,

31P

and

27Al

MAS-NMR study have been done.

Moreover, 1H-CP 31P MAS-NMR has also been used in order to observe the phosphate acid characteristic. In this cross polarization technique, 1H is excited and its energy is then transferred to the observed nucleus, in this case the Therefore, an increase of

31P

31P.

signal is observed if both nuclei are close. By this method it is possible to estimate the

strength of the dipolar interaction between

31P

and 1H and, accordingly, to see the hydroxyl groups or hydrogens

bonded to the phosphorus in the PO43- groups.36 Given that the sepiolite has not phosphorus or aluminum in the structure, signals of both nuclei are only due to phosphate. The

31P

MAS and CP-MAS spectra of SepP samples are

shown in Figures 2a and b, respectively. 31P spectrum shows a very broad signal at ca. -18 ppm and a shoulder at zero ppm. The width of these two bands indicates that phosphate is amorphous. In CP-MAS measure (Fig. 2b), this peak centered at zero ppm is resolved and presents an important intensity increase. This fact involves that H atoms are close to phosphorus ones, indicating the presence of hydroxyl and/or ammonium groups bonded to these P atoms and therefore, that the nanoparticles precipitated on to sepiolite are an acid phosphate. In Figure 2c 27Al MAS NMR spectra of SepP is shown. Three broad signals are observed centered at 40, 4 and -14 ppm, which correspond to octahedral, pentahedral and tetrahedral Al3+ environments.37 As in the case of

31 P

spectra, the

width of signals is attributed to the amorphous character. As a relevant result, the presence of these three kinds of Al3+ is attributed to an aluminum phosphate precursor where the final structure is not yet developed, which is in accordance with the XRD results shown in Fig. 1d.

3.2. Reaction mechanism by thermal analysis studies. Thermal behavior of Sep support and SepP composite is shown in Figure 3. DTA and TGA curves of the Sep support show the usual thermal behaviour of this clay. TGA (Fig. 3a) shows four overlapping events at ∼ 150°C, from 150°C to 350°C, from 350°C to 550°C, and from 550°C to 850°C, which are marked as 1, 2, 3 and 4 in Fig. 3a. Three are ascribed to dehydration processes (points marked from 1 to 3 in Fig. 3a), which specifically correspond to the loss of adsorbed water31 (point 1), the loss of hydration water31 (point 2) and the loss of co-ordination water31 (point 3), respectively, while, the last one corresponds to sepiolite dehydroxylation process31 (point marked as 4 in Figs. 3a). These dehydration processes are also observed in the DTA curve (Fig 3b) although only the last one is well resolved and correspond to enstatite formation.31, 38


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DTA curve of SepP composite (blue curve of the Fig. 3b) is characterized by overlapped endothermic processes up to 300ºC which are accompanied by an important weight loss and correspond to free and structural water losses. The exothermic peak around 330ºC, which has an associated weight loss, is attributed to a polymerization process by condensation of acid phosphate to form a polymeric phosphate (region contained in the gray area marked on the Figs. 3a and 3b). Hydroxyl groups of acid phosphate react with other hydroxyl groups releasing a water molecule in this reaction. This peak is broad due to the process is overlapped with the sepiolite dehydration. The tangents of both TGA curves have been drawn in order to observe the water losses differences (see Fig. 3a). As TGAs show, the slope of the SepP curve is steeper than the sepiolite one in the 100-280ºC and 280-420ºC. Then, the first dehydration processes are due to sepiolite and phosphate and the second one is attributed to sepiolite dehydration and phosphate condensation.

Figure 3  Thermal behaviour of the Sepiolite and Sepiolite-Phosphate (SepP) composites: (a) TGA curves of Sepiolite support (Sep) and the Phosphate/Sepiolite composites (SepP) prepared by heterogeneous precipitation method. Four weight losses can be observed on the TGA curve. The Fig. (b) shows the DTA curves of Sep and SepP. The gray rectangle evidences the exothermic peak around 330ºC.

As mentioned in the introduction section, we believe that the SepP composites can be used as fillers in flame retardant systems. For hybrid materials manufacturing with potential technological applications such as flame retardant materials, an essential requirement is to know their thermal behavior at high temperatures, being the thermal behavior of the fillers a key factor. Therefore, it is necessary to determine how thermal treatment affects the SepP material due to the amorphous nature of the phosphate. So, a structural characterization of Sep support and SepP composite samples thermally treated at high temperature, 900ºC, must be done. Additional characterization of heat-treated powders at 600ºC will be also displayed in order to understand the evolution with the temperature. A detailed explanation of these features can be followed below.


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X-Ray diffractograms of Sep support and SepP composite thermally treated at 600ºC and 900ºC are shown in Figure 4. In the case of samples heated at 600ºC (Figure 4a) the Sep diffractogram corresponds to an anhydrous sepiolite that corresponds to magnesium silicate hydroxide (Mg8Si12O30(OH)4).31 In the case of SepP sample, this phase is accompanied by a poor crystalline AlPO4 phase. At 900ºC (Fig. 4b) sepiolite residue is quite amorphous and the diffractogram corresponds to a poor crystalline enstatite phase, Mg2(Si2O6). Although crystallinity is not so high, it is possible to distinguish two other crystal phases with higher crystallinity degree that correspond to aluminum orthophosphate (AlPO4) and magnesium orthophosphate, Mg3(PO4)2. The low crystallinity of the SepP sample is related to the amorphous nature of the sepiolite residue.

Figure 4  Influence of the thermal treatment in the crystalline structure of the Sepiolite (Sep) and Sepiolite-Phosphate (SepP) composites: The Figs show X-ray diffraction patterns of Sep and SepP samples thermally treated at 600ºC (a) and 900ºC (b). Structural evolution confirmation of the Sepiolite-Phosphate (SepP) composite by NMR: (c) 31P MAS NMR and (d) 27Al MAS- NMR spectra of the SepP composite thermally treated at 600ºC and 900ºC. Spinning side bands are indicated by asterisks.

The unexpected presence of magnesium phosphate, Mg3(PO4)2, is attributed to the sepiolite support. In the experimental procedure, Sep support is treated with acid until pH 2 and it is known that a fraction of Mg2+ cations are leached from Sep structure when it is acid treated 39 according to: Mg8Si12O30(OH)4(H2O)4·8H2O + 2xH+↔ Mg8−xH2xSi12O30(OH)4(H2O)4·8H2O+xMg2+


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This Mg2+ reacts with the phosphate anions to make an amorphous magnesium phosphate, Mg3(PO4)2. As Fig. 1d showed, where a non-good crystallized sepiolite diffractogram appears, it is no possible to distinguish the nature of the as-synthesized phosphates, and therefore, it is necessary to thermally treat the sample in order to know the presence of the different products. Figures 4c-d show the 31P and 27Al MAS-NMR of SepP composite, which were thermally treated at 600ºC and 900ºC. 31 P

spectrum of the sample treated at 600ºC (at the bottom of the Fig. 4c) shows two main signals centered at -0.4 and -

29.5 ppm. An additional signal, obtained by fitting the complex band, is centered at -20.3 ppm. The broad peak of -0.4 ppm is well defined when temperature of thermal treat is increased up to 900ºC, see spectrum at the top of the Fig. 4c. At 600ºC, phosphate structure is still amorphous, in accordance with the XRD results shown in Fig. 4a. At 900ºC, the crystal structure is completely formed and both signals are well resolved due to the crystallization increase. In the case of 27Al spectra, only one signal appears at both temperatures centered at 39.2 ppm corresponding to a tetrahedral Al3+ coordination. According to XRD characterization, only one aluminum phosphate phase appears at 600 and 900ºC that correspond to orthophosphate phase (AlPO4). The

27Al

NMR spectra (Fig. 4d) show a well resolved signal centered at 39.2 ppm at

both temperatures which is assigned to this orthophosphate phase. In the case of

31 P

spectra, the signal centered at -

29.3 ppm is well resolved not only or the 600ºC sample but also for the 900ºC one. Thereby, this signal is assigned to AlPO4. The signal centered at -0.3 ppm, which increases its intensity and its resolution from 600ºC up to 900ºC and which is not still observed by XRD, has been assigned to magnesium orthophosphate (Mg2(PO4)3) that crystallizes at higher temperature than aluminum phosphate. Due to the difficulty to resolve the diffractogram derived from the amorphous sepiolite residue, the contribution of aluminum and magnesium phosphate to the sample has been estimated fitting the well resolved 31P spectrum of the 900ºC thermally treated sample. The final product is a mixture 21 ± 1 % of Mg2(PO4)3 and 79 ± 1 % of AlPO4.

3.3. Revealing the 3D ceramic nanostructure formation by HR-TEM. Figure 5a shows the LR-TEM micrographs of SepP composite, which was thermally treated at 900ºC. Phosphate particles are still over the sepiolite fibres and have still nanometric sizes. From Figure 5a, it is possible to distinguish a connection between fibres that produce a 3D sepiolite network formation. To investigate also the nanostructure formed during the heat treatment on the SepP composite, HRTEM was employed (Figure 5b). Noteworthy, we note from the HRTEM image in Figure 5b that the SepP composite presents a “core–shell” structure. From this unexpected


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nanostructure (Fig. 5b), we can determinate that the shell consists of a phosphate coating of 17 nm thick, while the core is formed by a sepiolite fiber. As a relevant result, it can be observed that the sepiolite fiber possesses a continuous nanocoating and a well-preserved fiber morphology. As it has been shown, phosphate continuous coating is developed over sepiolite fibres with thermal treatment at 900ºC. Nevertheless, phosphate nanoparticles are not precipitated uniformly on the Sep surface support. Moreover, the thickness of the phosphate coating is lower than the particle aggregates obtained by the precipitation reaction. According to this, these phosphate aggregates are broken and primary nanoparticles (NPs) coalesce during thermal treatment in order to develop the continuous coating. These coarsening phenomena have been studied mainly related to metallic nanoparticles.40, 41 In our composite, two main coarsening pathways have been considered; firstly, Ostwald ripening, in which individual atoms detach from small clusters and diffuse over the support surface until they join larger NPs, and secondly, diffusion coalescence, in which entire NPs diffuse across the support surface until they coalesce with other NPs.40, 42,43

Figure 5  Self-forming nanostructured ceramic network based on Aluminum Phosphate on the Sepiolite fibers: (a) TEM images of SepP composites thermally treated at 900ºC. From panel a, it is possible to observe a characteristic interconnection between SepP composites. Panel (b) shows a HRTEM image where the morphology of a SepP composite is observed. We note from the HRTEM image in panel (b) that the fibres are present as a core-shell structure. The shell is formed by a phosphate continuous coating, due to the exposure of the SepP composite to a 900ºC thermal treatment. The coating thickness, of about 17 nm, is signaled with red arrows. Meanwhile, the core composed of a Sepiolite fibre with diameter of 20 nm, is signaled with white arrows.


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Figure 6 (a) TEM images of SepP composites thermally treated at 600ºC. The Fig. (b) shows HRTEM image where it can be observed that the SepP composite morphology is formed by phosphate nanoparticles, which are embedded in an amorphous phase arising from a liquid phase formed during thermal process.

In order to explain the coating formation mechanism, a TEM micrograph of the 600ºC thermally treated SepP is shown in Figure 6. At this temperature, aggregates are dissolved and particles are bigger than the precipitated ones, which are embedded in a liquid phase. First of all, to understand the formation mechanism of the nanostructured ceramic network (NsCN) based on aluminum phosphate on the sepiolite fibers, it is necessary to know the SepiolitePhosphate (SepP) composite´s composition. The analyzed composition of SepP composite is shown in Table S1, evidencing that the SepP composite contains a 0.2 wt% of Na2O. This fact plays a relevant role in the formation mechanism of the NsCN because Na+ cations form phosphates with low melting points, which generate a liquid phase during the thermal treatment. This behavior can be notably verified through the analysis of the phase diagram. Figure S1 displays the phase diagram corresponding to the (NaPO3)3‐Na3PO4 binary sub-system of the Na2O‐P2O5 system,44 which shows that the first liquid phase formation occurs at 490°C. So, the small amount of Na in the Sep would result in the formation of these phosphates. The liquid phase on these samples is hard to detect, implying it could be a transient liquid phase formed during the thermal process with a high solubility in the system. This would lead to its eventual disappearance with time and/or temperature. We have been able to find the presence of an amorphous phase arising from the liquid phase formed during the thermal process, which is located over the surface support for the SepP composite thermally treated at 600°C, see Fig. 6b. According to our TEM results, the formation of a limited amount of liquid phase during the thermal process promoted the phosphate particle growth by increasing solutionprecipitation mechanisms that resulted in isolated particles bigger than the precipitated ones. In general, migration of particles occurs before coalescence. Small particles migrate towards the biggest ones. Thereby the small particles energy decreases and the global system is stabilized.45 In this case, phosphate nanoparticle migration along fibres by


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superficial diffusion is supposed. Particle aggregates are broken when temperature rises due to thermal agitation and liquid phase formation. By this way, the system energy is reduced. The phenomenology described here, could be explained by a simple model based on the liquid phase appearance that evolves towards the formation of the phosphate ceramic three-dimensional structure over the surface support, during the thermal treatment. The presence of Na+ cations on the Sep structure plays a relevant role in the formation mechanism of the 3D ceramic structure, because Na+ cations form phosphates with low melting points which generate the formation of the liquid phase during the thermal treatment.44 Figure 7 shows a scheme to explain the phosphate coating formation onto sepiolite fibres during thermal treatment. When phosphate nanoparticle aggregates are supported on sepiolite fibres, these aggregates tend to dissolve due to thermal agitation, assisted by the liquid phase (point marked as 1 in the Fig. 7). This liquid phase favors the nanoparticles migration along fibres (point 2). When temperature rises (around 600ºC) particles coalesce (point signaled as 3 in Fig. 7) as it has been shown in the micrographs of the Figure 6. The subsequent increase in temperature results in the ceramic coating formation (point 4 of the Fig.7). Micrographs of this ceramic coating are collected in Figs. 5 and 6.

Figure 7 Schematic representation of the formation mechanism of the 3D nanostructured ceramic network during thermal treatment. A detailed explanation of the scheme can be followed in the text of the main body.

Summarizing, heterogeneous synthesis of acid aluminum phosphates using sepiolite as support favors the dispersion control and reduces the particle size. The relative amorphous nature of the SepP composite is maintained up to 900ºC where a sepiolite-phosphate rigid structure is developed. The acid treatment of sepiolite that favors the acid phosphate formation, produces that partial phosphate fraction reacts with the Mg2+ leached from sepiolite forming a magnesium phosphate. Sepiolite acts as the support on which acid phosphate precipitates and polymerizes by condensation reaction releasing a water molecule by each reaction. This generates a 3D nanostructured ceramic network in which the skeleton is provided by the sepiolite fibres, core structure, being the nanocoating the shell


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structure. In this network the fibers are temperature “glued” by the phosphate shell because of the high reactivity of the formed amorphous acid phosphate. Therefore, this unique 3D nanostructure is self-formed in a wide temperature range due to the nature of the inorganic materials involved. Potential applications in which a temperature regime is required, as flame retardant materials for polymers, could benefit from the self-forming ability of the Sep-phosphate composites here shown. The SepP composites can be incorporated into both thermostable and thermoplastic polymers during the conformation step and the self-forming structure could contribute to the char stability during combustion.

3.4. Finding a potential technological application of the Sepiolite-Phosphate (SepP) composites: Flame retardant materials. As has been mentioned above, the self-forming ability of the Sep-phosphate composites could be used for potential applications such as flame retardant on polymeric materials. To prove that the SepP composites develop a flame retardant behavior, we have carried out a simple test at laboratory scale. (The reader can find more information about the making-off of the laboratory scale prototype in the experimental section). To that end, the composite cylinders with various SepP loads, between 0 wt% and 10 wt% (step marked as t0 in Figures 8a-c), were ignited by direct exposition to a laboratory flame during 10 second in air atmosphere (step indicated as t1 in Figs. 8d-f). After ignition, the cylinder was removed from the flame and in the composite with 10 wt% of SepP the flame extinguished in just 17 seconds, showing its potential as flame retardant (steps marked as t2→3 in Figs. 8g and 8j). Thus, the composite is capable of selfextinguishing a flame in air atmosphere, with less than 10 wt% of SepP. For the cylinder test with 5 wt% of SepP the cylinder burns off during 5 minutes (steps indicated as t2→3 in Figs. 8h and 8k). In this case, it is worth noting that the burning cylinder keeps its integrity without causing the release of particulate matter (as shown Fig. 8k). Finally, pure epoxy with standard ATH flame retardant burns off in an uncontrollable way (steps designated as t2→3 in Fig. 8i and 8l).


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Figure 8  Sequence of photographs of the polymeric cylinders ignition with a 10 wt% (a, d, g, j), 5 wt% (b, e, h, k), and 0 wt% (c, f, i, l) of SepP composite, showing the detailed mechanism for the flame retardant activity of the SepP additive. This mechanism is associated to the 3D nanostructure self-formed. Additionally, on the top of each image the burning time value is indicated.

In addition, compression mechanical tests performed on pressed cylinder show an interesting behavior, Figure 9. Meanwhile the mixture of inorganic component 50 wt% SepP and 50 wt% ATH heated at 600ºC breaks at ca 3.5 MPa (see insert of the Fig. 9) the thermally treated sample at 900ºC shows nearly one order of magnitude higher in compression resistance, > 35 MPa. However, the more exciting behavior comes from the fact that the fracture behavior is not critical and the composite shows a plastic deformation curve, in which the load produces a large deformation before the structure is collapsed. This behavior is consistent with the self-formed 3D network in which the sepiolite fibers acted as skeleton and the phosphate material as gluing agent. The 3D network is flexible enough to absorb part of the load and thereafter deforms plastically without collapsing. This fact endows the char with a capability to absorb the high stresses that occur during the polymer burn out. These 3D materials, having both flame retardant characteristics and high load plastic deformation, open a new path for the design of nanocomposites with superior fire resistance.


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Figure 9 Mechanical properties: the figure shows the compression mechanical test of the SepP-ATH systems, thermally treated at 600 and 900ºC. This figure evidences that the thermally treated sample at 900ºC exhibits a huge increase in compression (> 35 MPa), of one order of magnitude, compared to the sample thermally treated at 600ºC (~ 3.5 MPa). The insert shows a detail of the stress in compression for the SepP-ATH systems.

4. CONCLUSIONS In summary, we report a facile method to prepare an amorphous and nanometric acid aluminum phosphate (P) using sepiolite (Sep) as support through a simple one-pot heterogeneous precipitation approach, which is economic, facile and easily scaled-up. The use of a magnesium silicate such as Sep and the chemical process produces a partial formation of magnesium phosphate from the Mg2+ leached from Sep. The synergistic effect between P and Sep fiber may contribute to the formation of a 3D nanostructured ceramic network during the thermal treatment, which can be explained through a migration-coalescence mechanism assisted by a liquid phase. It is worth pointing out that this exotic mechanism observed here could have potential technological applications such as halogen-free flame retardant on polymeric materials. For that end, we carried out a simple test at laboratory scale, in which we undoubtedly demonstrated that the controlled incorporation of SepP composites in a polymer matrix is capable of selfextinguishing a flame during flammability tests and improves the mechanical performance of a reference epoxy resin.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:.


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Phase diagram of the system (NaPO3)3‐Na3PO4 andanalyzed composition of Sepiolite-Phosphate (SepP) composites by X-Ray Fluorescence (XRF).

Acknowledgements This work has been financially supported by the projects MAT2013-48009-C04-01-P, MAT2013-48426-C2-1-R and CENIT DOMINO. The authors like to thank TOLSA S.A. for sepiolite supply. E. Palacios acknowledges the ﬁnancial support of the JAE (CSIC) fellowship program. Dr. F. Rubio-Marcos is also indebted to MINECO for a ‘‘Juan de la Cierva’’ contract (ref: JCI-2012-14521), which is co-financed with European Social Fund. Dr. P. Leret is also indebted to MINECO for a “Torres Quevedo” contract (ref: PTQ-12-05470), which is co-financed with European Social Fund. Additional information Competing financial interests: The authors declare no competing financial interests.


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The Table Of Contents (TOC) Keyword Phosphate; Synthesis; Precipitation; Halogen-Free; Flame retardant

Title Self-Forming 3D Core-Shell Ceramic Nanostructures for Halogen-Free Flame Retardant Mate

ToC figure


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Self-Forming 3D CoreâShell Ceramic Nanostructures for Halogen

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