Industrially relevant nanopowder was synthesised by microwave hydrothermal synthesis to obtain well-controlled composition (ZrO2–AlO(OH) system) which was found to determine a number of physical and thermal characteristics. This study reports variation of particle size, density, specific surface area (SSABET), as well as thermal behaviour of nanopowder mixtures of ZrO2–AlO(OH) in the whole range of compositions. It was found that the onset temperature (T on) of physically and chemically bounded water desorption depends on the Al3+/or AlO(OH) content. The lower content of Al3+ in the ZrO2–AlO(OH) system, the higher T on of physically bound water desorption. There are three distinct temperature regions for water decomposition for nanomaterials investigated in air (at approximately 50, 250 and 450 °C). These temperature ranges depend on particle size and chemical composition of ZrO2–AlO(OH) nanopowders. Materials were divided into three groups characterised by different properties: (1) ZrO2 with 2–12% of Al3+, where particle sizes are from 4 to 8 nm, (2) ZrO2 with 30–67% of AlO(OH), where particle sizes are from 10 to 13 nm, and (3) ZrO2 with 80–99% of AlO(OH), where particle sizes are from 13 to 23 nm. AlO(OH) content determines thermal and physico-chemical properties of synthesised ZrO2–AlO(OH) nanopowders.
It is well known that nanoparticle size, shape, surface chemistry, dispersibility or sinterability are dependent on external factors such as temperature, humidity or level of contamination as well as being time dependent. This has implications for industry, standardisation community and regulators. Fundamental questions of sell-by date for nanomaterial, environmental fate, toxicology and usability in applications are being asked. It is known that nanomaterials have a large specific surface area and that their properties are often governed by surface processes. The dynamics of such processes (agglomeration, dispersion or sintering) are determined by external factors as well as internal energy transitions. This presents a complex problem where chemistry of material as well as its surface, particle size and shape affect dynamics and practical applications. One of such commercial applications is the use of sintered ZrO2-based ceramics which are industrially relevant nanomaterials in medical implants and dentistry. Amongst other factors, thermal behaviour of nanopowders is one of the most influential factor in this material system governing material parameters. Chemical bonding of –OH groups and physisorption of H2O on the surface depend on material composition and its particle size. Their presence may influence densification and sintering processes .
Nanomaterials used in biomedical applications, especially the ones in implant and prosthetics applications area received a lot of attention in recent years . One of the most promising bio-compatible materials systems is a ceramic based on ZrO2. Synthesis of such ceramics presents a number of challenges . For a good sinterability, synthesis of ZrO2-based ceramic powders with narrow particle size distribution is of considerable interest . Relative to their conventional micron-sized crystalline counterparts, nano-ZrO2-based ceramics demonstrate better materials properties and superplastic behaviour [5,6,7,8]. Therefore, nanograined ceramics hold promise of unique mechanical properties that are not commonly found in coarse-grained counterparts .
Generally, ZrO2 in the equilibrium state exists in three polymorphic forms: monoclinic (m-ZrO2), tetragonal (t-ZrO2) and cubic (c-ZrO2) . The amount and type of dopant and the synthesis route may determine the phases of the crystalline product . From existing crystallographic forms of ZrO2, the tetragonal phase is the most demanded phase because of its mechanical properties. From crystallographic X-ray studies, it is not possible to distinguish t-ZrO2 and c-ZrO2 phases  due to the overlap of their characteristic diffraction lines.
The required t-ZrO2 phase is very often stabilised by additional compounds such as yttria (Y2O3) . However, it was found that t-ZrO2 can be stabilised at room temperature by doping divalent or trivalent cations such as Ca2+ or Al3+ which are much less expensive than Yttrium ions [12, 13]. The XRD studies show that the structure remains unaltered even after 10 mol% of Al3+ doping, confirming the stability of such crystalline structure . This opens a perspective for cost-effective synthesis and sintering of bulk materials made from zirconia–alumina alloys. Zirconia–alumina nanomaterials amongst other advanced ceramics show superior mechanical properties, chemical inertness and biocompatibility. They are used for a wide range of applications such as biomedical implants [14, 15] and structural ceramics [16, 17].
The production process of ZrO2–Al2O3 (ZrO2–AlO(OH)) nanopowders follows various chemical routes, such as sol–gel method [4, 18,19,20], hydrothermal synthesis [21, 22] or co-precipitation method [23, 24]. It was found  that the main benefit of hydrothermal synthesis is the generation of weakly agglomerated nanopowder. However, some research suggests difficulties in reproducing and controlling properties of these nanopowders [25, 26]. That is because the synthesis of ZrO2–Al2O3 ceramics is highly sensitive to synthesis parameters, including concentration, temperature, pH and drying method . Also, synthesis conditions influence density, specific surface area, phase composition of ZrO2–Al2O3 [26, 27] and the amount of adsorbed water.
On the other hand, sintering properties of nanomaterials depend on the green body micro- and nanostructure . It is also required that the pores are in the nanometre size range, since large pores may grow during synthesis . The green body nanostructure depends strongly on the viscosity of the slurry used for its production, which in turn depends on the amount of water physically or chemically bounded to the nanoparticles. Cinar et al.  showed that nanopowder suspensions based on ZrO2–Al2O3 are characterised by higher viscosities compared to micron size powders. The lower viscosity indicates poor dispersion of nanopowder with a lot of aggregates. Higher viscosity indicates that all particles are contributing to the dynamics of the liquid due to the increased solid–liquid contact around particles . Cinar et al.  also showed that high viscosity of powders is linked to the bound water on particle surface. It was explained that bound water, which exists around the nanoparticles, does not function as a solvent in the system, but behaves as a part of the powder. This fact leads to the change of viscosity. Thus, for optimal green body formation technology, it is required to control existence and amount of bound water around the nanoparticles.
This work evaluates the impact of composition (in ZrO2–AlO(OH)) on physical and thermal parameters such as water decomposition, onset temperature of ZrO2–AlO(OH) phase transformations, as well as zeta potential, density, particle size, and SSABET.
The novelty of this paper is the discussion of characterisation results and thermal behaviour of ZrO2–AlO(OH) nanopowders (synthesised by MHS method) with different amounts of AlO(OH) in a whole range of compositions (from 2 to 99% of Al3+). Pure ZrO2 and χ-AlO(OH) were used as a references. The boehmite (χ-AlO(OH)) phase transforms into Al2O3 after annealing which we already reported . However, in this work we describe results obtained only for as-synthesised powders in order to preserve water content and analyse it in detail. Applied synthesis method allows to create fully crystalline product in contrary to the traditional sol–gel synthesis. Our results demonstrate the complexity of processes taking place in nanomaterials. This work may be helpful to track nanomaterials ageing process and material selection for various applications.
Nanopowders synthesis and characterisation methods
Various compositions of ZrO2–χ-AlO(OH) nanopowders were synthesised using microwave- hydrothermal synthesis. The reagents used in the process were: zirconyl chloride octahydrate [ZrOCl2·8H2O Sigma-Aldrich (≥ 99.5%)], sodium hydroxide (CHEMPUR, analytically pure) and aluminium nitrate non-ahydrate [Al(NO3)3·9H2O CHEMPUR, analytically pure]. The reagents were used without additional purification. Deionised water with specific conductance below 0.1 µS cm−1 was obtained using a deioniser (HLP 20UV, Hydrolab, Poland). Microwave reactions took part in a MAGNUM II ERTEC microwave reactor (2.45 GHz, 600 W). The reaction parameters were set as follows: T = 258–263 °C, P = 50–56 atm, heating time = 20 min. The details of the hydrothermal synthesis method were reported previously . Calculations for all compositions were done for mass% of AlO(OH) needed to obtain such amount of Al2O3 (in ZrO2–Al2O3) after annealing.
Scanning electron microscopy (SEM) analysis for ZrO2–AlO(OH) nanopowders was performed on carbon-coated samples using Zeiss Ultra Plus scanning electron microscope.
The low-temperature differential scanning calorimetry and thermogravimetry (LT-TG–DSC) experiments were carried out on a STA 449 F1 Jupiter by Netzsch using stainless steel furnace. The experiments were performed with a heating rate of 10 °C min−1 from − 150 °C up to 800 °C. Constant flow of helium at 60 mL min−1 was applied. For each experiment, we used approximately 30 mg fine (previously mortared) powder which was pressed into crucible prior each measurement. In order to obtain reproducible results, samples were outgassed without heating before the start of each experiment.
The high-temperature differential scanning calorimetry and mass spectrometry (HT-DSC–MS) analyses were performed using a STA 449 F1 Jupiter by Netzsch equipped with SiC furnace. The experiments were carried out with a heating rate of 10 °C min−1 from ambient temperature up to 1450 °C, where a constant flow of air (60 mL min−1) was applied. The volatile products emitted during heating were detected with a mass spectrometer (QMS 403C Aeolos) coupled in line with the STA instrument.
X-ray diffraction patterns of nanopowders were obtained on X’Pert PRO, PANalytical diffractometer equipped with a copper anode (CuKα1) and an ultra-fast PIXcel1D detector. The analysis was performed at room temperature in the 2θ range 10°–100° with a step of 0.03°. The Scherrer equation was used to calculate the average crystallite diameter for selected nanopowders .
Density measurements were taken using the helium pycnometer (AccuPyc II 1340, FoamPyc V1.06, Micromeritics, USA). The measurements were taken in accordance with ISO 12154:2014 at temperature of 25 ± 2 °C.
Specific surface area (SSA) of nanopowders was determined using the surface analyser (Gemini 2360, V 2.01, Micromeritics, USA) by gas (nitrogen) adsorption method based on the linear form of the BET (Brunauer–Emmett–Teller) isotherm equation in accordance with ISO 9277:2010. Prior to performing measurements of density and specific surface area, the samples were subject to 2-h desorption in a desorption station (FlowPrep 060, Micromeritics, USA), at temperature of 220 °C with the flow of helium. Based on the determined specific surface area and pycnometric density, an average equivalent spherical particle diameter was determined. In this case, the assumption was that all particles are spherical and identical. The following equation was used for calculating the average particle size:
where D—average size (diameter) of particles/nm, N—shape coefficient being 6 for the sphere, SSABET—specific surface area/m2 g−1, d—density/g cm−3.
The zeta potentials (ζ) of samples were measured at 23 °C using laser Doppler electrophoresis analyser [Zetasizer Nano ZS (ZEN3600), Malvern Instruments Ltd]. Each sample powder (10 mg) was dispersed in 50 mL of deionised water (0.07 µS cm−1, HLP 20UV, Hydrolab, Poland) by ultrasonication for 10 min in ultrasonic washer (30 W, Elma Schmidbauer GmbH, Germany). In order to obtain the isoelectric point (IEP), the pH of the suspension was first adjusted to pH 11 by adding 0.2 M NaOH solution and changed several times to pH 2 by adding 0.2 M HCl solution using an automated titration system (titrator MPT-2, pH electrode type MV 114-S.C. SEN 0106, Malvern Instruments Ltd; vacuum degasser, P/N 0001-6353, Systec). The zeta potential of each sample was measured two times at every pH value, and the average zeta potential at a certain pH value was plotted against the pH value. The pH value where the zeta potential was zero was taken as the IEP.
Results and discussion
Figure 1 shows XRD diffraction patterns for chosen ZrO2–AlO(OH) compositions. The XRD patterns for different compositions are presented before . Here we show only some results in order to demonstrate how the phase composition changes with increasing amount of AlO(OH). Powders which contained 99 and 100% of AlO(OH) are characterised by pure boehmite [χ-AlO(OH) (JCPD 21-1307)] structure. ZrO2 nanopowder without Al3+ addition is a mixture of tetragonal (JCPD 17-0293) and monoclinic zirconia (JCPD 86-1449). Samples containing between 5 and 15% of Al3+ addition show diffraction patterns for only t-ZrO2. Higher amount of AlO(OH) (between 30 and 90%) besides t-ZrO2 phase also contains boehmite phase.
Our previous results  confirmed that boehmite phase appears in as-synthesised ZrO2 samples from 30% of AlO(OH) additive. Below this value (up to 25% of Al3+), ZrO2–Al3+ creates solid solution (Al3+ doped ZrO2) . The temperature of microwave synthesis conducted by us was in the range from 258 to 263 °C, which was not high enough to obtain Al2O3 phase. α-Al2O3 is formed in a reactor with minimum crystallisation temperatures of 380 °C or higher . In this work, microwave synthesis conditions (T = 80–270 °C) could only form χ-AlO(OH)  phase. However, according to hydrothermal Al2O3–H2O phase diagram in the temperature range 310–380 °C, and after applying pressure higher than 17 MPa, α-AlO(OH)  is formed. It was mentioned above that boehmite phase transforms into Al2O3 after annealing in at least 800 °C, which was shown in  and .
Figure 2 shows SEM images for selected compositions in order to demonstrate the change of powders’ morphology. It can be seen that all samples are rather uniform. With the higher content of AlO(OH) particles became more elongated. This fact is due to presence of Boehmite which is characterised by plate-like, elongated morphology .
Figure 3a shows SSABET results for all synthesised nanopowders. The spread of the values is from approximately 80 up to 326 m2 g−1. However, the biggest differences are observed for ZrO2 with 12% Al3+ and 90% of χ-AlO(OH) (Table 1). Higher SSABET from 90% of AlO(OH) is due to apparent non-sphericity of particles as shown in Fig. 2. The increase in the specific surface area for the ZrO2 and ZrO2 with 2–12% of Al3+ is due to increasing amount of aluminium ions. Stable t-ZrO2 is leading to the formation of smaller particles . For the compositions ranging from 30 to 67% of AlO(OH), SSABET values are relatively constant declining slowly from 30 to 67% of AlO(OH). The SSABET decline could be attributed to an increased amount of non-outgassed water in the χ-AlO(OH) which remains in the system during the BET outgassing procedures. We think that it results in a small reduction in apparent surface of the sample as –OH groups occupy some available sites. Change in SSABET is also visible when shape of the particles changes ]with the composition of ZrO2 from 80% AlO(OH) to the pure AlO(OH)].
As it was described above, the SSABET values show 3 different regions approximately 0–12% Al3+, 30–67 and 80–100% of AlO(OH) addition. Based on the combined measurements of SSABET and density (1) it was possible to report the variation of mean equivalent sphere size of particles in the nanopowder for all compositions investigated (Fig. 3b) . It should be noted that particle density was found to monotonically decrease with % AlO(OH) in the system (Fig. 3c). This is as expected from the compound composition. It is possible to distinguish nanopowders with 3 different particle size ranges: form 4 to 8.5 nm, from 10 to 13 nm and from 13 up to 23 nm (Fig. 3b). The particle size increases, while the amount of Al3+ in the composition increases. Interestingly, for 0–12% compositions the SSABET increased sharply due to the rapid decrease in particle equivalent diameter. From 30 to 67% of AlO(OH), particle size increases slowly. However, for particles of 80% or higher the SSABET increases rapidly leading to a steep decrease in the equivalent particle diameter. From the SEM images of these samples (Fig. 2e, f), it is evident that the particle shape is not spherical. Particle aspect ratio or the non-sphericity increases with increasing AlO(OH) content for this composition range (from 80 to 100%). In order to test the assumption of particle size evaluation using SSABET, another method (Scherrer method) was used [30, 36]. The particle size range for selected compositions was following: 80% of AlO(OH): 18–25 nm, 90% of AlO(OH): 17–33 nm, 96% of AlO(OH): 9–17 nm, 99% of AlO(OH): 7–13 nm, and AlO(OH): 6–12 nm. Obtained results from Scherrer equations are in agreement with particle size calculated using Eq. (1).
Variation of density with composition is shown in Fig. 3c. The theoretical density of m-ZrO2 and χ-AlO(OH) is 5.68 g cm−3 [37, 38] and 3.01 g cm−3 [31, 38], respectively. Pycnometric density of nano-ZrO2, measured as a reference sample, is 5.17 g cm−3. The difference between literature value, and that obtained in our examination maybe due to nanosized particles. At the same time, that difference could be caused by surface defects and hydroxides . The density value of χ-AlO(OH) is very close to the literature (Table 1; Fig. 3c). As it could have been predicted, values decrease when Al content in synthesised powder increases.
The isoelectric point (IEP) for ZrO2 is 6.0 (Table 1; Fig. 4), which is in agreement with previously reported works [28, 39]. The IEP value for χ-AlO(OH) is 9.5 and is slightly higher than reported in the literature (9.1–9.2 [27, 40]). The difference may be caused by specific features of nanomaterials obtained by MHS. In case of ZrO2 and χ-AlO(OH), the positive charge on the surface exists for pH < pH(IEP) while the negative charge appears in pH > pH(IEP) range. The increase of Al3+ content in ZrO2 causes an increase in IEP values. This result is due to changes in nanopowder composition.
Figure 5 presents results of low-temperature DSC (conducted from − 100 up to 800 °C) for all synthesised nanopowders. We chose such a low starting temperature in order to observe clearly the onset temperature of released water. It is known that for this type of system the first endothermic transition is located at approximately 100 °C which corresponds to the evaporation of the physically adsorbed water . The second one, between 200 and 500 °C, takes place due to the loss of chemically bonded water (hydroxides transformation) together with carbon dioxide [29, 39]. It can be seen that nano-ZrO2 and ZrO2 with AlO(OH) content below ~ 30% are characterised by only one endothermic transition due to releasing of physically absorbed water (Fig. 5a). There is no signal from chemically bonded water. The reason for such behaviour is the increasing amount of χ-AlO(OH) in the ZrO2–AlO(OH) . These findings are in agreement with results described above. It should be noted that such behaviour is very sensitive to composition. We found that the limit for the presence of chemically bonded water for ZrO2–AlO(OH) materials investigated in helium is between approximately 12 and 30% of AlO(OH). Above 30% both physically bound and chemically bonded water is present. The type of released gasses during ZrO2–AlO(OH) thermal treatment will be discussed further.
Obtained onset temperature (T on) from DSC experiment for ZrO2–AlO(OH) nanopowders is shown in Fig. 6 and Table 1. Figure 6a presents change in the onset temperature for the first endothermic event as a function of composition. Taking into account that estimation of starting point of the transition (T on1) is quite difficult, we use the same approach and software (Proteus, Netzsch) in order to extract these values. Analysis of Fig. 6a shows that onset temperature for the evaporation of the physically adsorbed water differs for samples with lower content of Al. One possible explanation of such behaviour might be a reduced particle size. Figure 6b confirms a dependence of T on1 on the particle size. The smaller the average particle size, the higher the T on1. Therefore, we would expect to obtain the lowest onset temperature for the powders with the smallest particle size and the highest specific surface area, especially while we are referring to the physically adsorbed water.
On the other hand, T on2 for chemically bonded water shows rather linear dependence on composition (Table 1). The higher amount of AlO(OH), the lower onset temperature, which is most likely due to slower kinetics. There is clear exception for ZrO2 nanopowder with 90% of AlO(OH) additive. Analysis of data collected in Table 1 shows such a high onset temperature for chemically bonded water desorption may be due to particle size and SSABET. The ZrO2–90%-AlO(OH) sample is characterised by very small SSABET ~ 78 m2 g−1 (compared to other samples) and relatively big particles size (~ 24 nm). These values differ from other compositions by nearly 200%. Grabis et al.  have characterised ZrO2–Al2O3 nanopowders produced by plasma techniques. They synthesised materials with very small SSABET (29–46 m2 g−1) which increased with alumina content. Authors suggest that the described behaviour is coupled with the reduction in m-ZrO2 present.
Figure 7 shows sample mass loss (TG) during thermal treatment which is attributed to water and carbon dioxide release as we discussed above. In addition, Fig. 8 shows correlations between compositions and sample mass losses during the first (Fig. 8a) and second event (Fig. 8b). The desorption of physically absorbed water is the highest for the compositions at the ends of composition range [up to approximately 12% of Al3+ and AlO(OH)-10% ZrO2]. Sample mass loss during second transition increases, while the content of AlO(OH) in the composition increases (Fig. 8b). This fact confirms our previous findings .
The high-temperature DSC was prepared in air in order to analyse thermal stability in broader temperature range (Fig. 9). Experiment curves were split into 3 figures: Fig. 9a: ZrO2 with 5–12% Al3+, Fig. 9b: ZrO2 with 30–67% of AlO(OH) and Fig. 9c: ZrO2 with 80–96% of AlO(OH). In this way, it is highlighted how particle size and materials’ chemical composition change thermal characteristic of ZrO2–AlO(OH). The onset temperatures for event 2, 3 and 4 are listed in Table 2. Transitions from RT up to approximately 450 °C are associated with the H2O and CO2 release (Fig. 10) [1, 41, 42], while the following endothermic transition is due to χ-AlO(OH) transformation and water release . The forth, weak endothermic event above 1000 °C is related to particle growth and/or further (t → m) ZrO2 phase transformation .
The interdependence of composition, particle size, specific surface and thermal properties due to water adsorption has been demonstrated on an industrially relevant nanomaterial.
We showed how density, particle size and SSABET values are changing for ZrO2–AlO(OH) nanomaterials synthesised by MHS method in the whole range of compositions. The chosen synthesis method allowed to create crystalline ZrO2–AlO(OH) nanopowders in simple and repeatable way. Obtained results, especially investigation of thermal behaviour, allow to establish suitable conditions for densification and sintering process of ZrO2–AlO(OH) nanopowders.
Our research showed that nano-ZrO2 and nano-ZrO2 with 2–12% of Al3+ are characterised by only one endothermic transition due to releasing of physically adsorbed water (T on1). There is no signal from chemically bonded water. The onset temperature for the evaporation of the physically adsorbed water was found to be relatively low and varied between the samples with lower content of AlO(OH). We found that T on1 depends on the particle size and boehmite presence. The smaller the particle size, the higher the T on1. Specific surface area SSABET has a direct influence on T on1. The composition limit for the presence of only chemically bonded water was found to be between approximately 12 and 30% of Al3+. Above 30% of AlO(OH) both physically bound and chemically bonded water is present. Experimental results show that ZrO2–AlO(OH) nanopowders properties change above 30% of AlO(OH). For the composition below this value, Al3+ exists in solid solution (Al3+ doped ZrO2).
Presented results allow to distinguish 3 groups of ZrO2–AlO(OH) nanopowders, with distinct properties and thermal behaviour. The first group (I) of materials is ZrO2 with Al3+ content up to 12% characterised by very small particles size (~ 4–10 nm), relatively high T on1, and a visible endothermic transition at approximately 1050 °C. As-synthesised powders from group (I) indicate m + t-ZrO2 phase composition. DSC–MS of these nanopowders conducted in air shows that water is release in T on1 and T on2 regions.
The second group (II) of materials is placed in the 30–67% range of AlO(OH) additive. This group is characterised by particles size in the range 10–13 nm, lower than (I) group value of T on1, and lack of or shift of T on4 to the ~ 1320 °C. As-synthesised powders of group (II) show AlO(OH) and m + t-ZrO2 phases. DSC–MS of these nanopowders conducted in air is characterised by water release in T on1, T on2 and T on3 regions.
The third group (III) of ZrO2–AlO(OH) materials contains from 80 to 99% of AlO(OH). This group is characterised by larger particles size up to 23 nm, lower than (I) group value of T on1, and shift of T on4 to the ~ 1320 °C. As-synthesised powders of group (III) show AlO(OH) and m + t-ZrO2 phases. DSC–MS of these nanopowders conducted in air is characterised by water release in T on1 and T on3 regions.
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Authors are grateful to the Polish National Science Centre for financial support under Contract No. UMO-2013/11/D/ST8/03429-“Sonata 6”. The research subject was partly carried out with the use of equipment funded by the Project CePT, reference: POIG.02.02.00-14-024/08, financed by the European Regional Development Fund within the Operational Programme “Innovative Economy” for 2007–2013.
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Koltsov, I., Prześniak-Welenc, M., Wojnarowicz, J. et al. Thermal and physical properties of ZrO2–AlO(OH) nanopowders synthesised by microwave hydrothermal method. J Therm Anal Calorim 131, 2273–2284 (2018). https://doi.org/10.1007/s10973-017-6780-8
- Low-temperature DSC
- Microwave hydrothermal synthesis
- ZrO2–AlO(OH) nanopowders
- Thermal stability
- Water decomposition
- High-temperature DSC–MS