Introduction

Al–Cr–phosphate composites have been used as binders in refractory ceramics for many years. It is also well-known that they can be used as a potential wave-transparent material for high-temperature environments because of their low thermal conductivity, low dielectric constant, good chemical stability and high melting point, which are similar to aluminum phosphate binder [1–4]. However, as a high-temperature wave-transparent material, it should also possess a thermally stable dielectric constant [as temperature increases to 373 K, relative dielectric constants (ε r) change less than 1 %], thermal stability and high thermal shock resistance [5–7]. Han et al. [1, 2, 8] once studied the chemical structure and crystalline phase changes of aluminum–chromium phosphates when cured at temperatures up to 1,473 K. They concluded that a starting phase of the aluminum–chromium phosphates binder is transformed to amorphous at 473 K, and finally to a cyclic Al(PO3)3 beginning at 973 K. However, the internal structure changes, thermal transformation, thermal decompositions, and relevant dielectric constant at various temperatures have not been investigated. Thus, it is necessary to study the high-temperature thermal transformation and dielectric property of aluminum–chromium phosphates.

In this article, high-temperature thermal transformations of aluminum–chromium phosphates have been studied using thermalgravimetric–differential scanning calorimetry, infrared absorption spectra and X-ray diffraction analysis. The relative dielectric constants (ε r) were measured using Vector Network Analyzer, and thermal decompositions were discussed through aforementioned analysis.

Experimental

Preparation of samples

Aluminum–chromium phosphates binder was obtained from reagent grade Al(OH)3 (Kelong Chemical, AR), CrO3 (Galaxy Chemical Co. Ltd., 99 %), and H3PO4 (Kelong Chemical, 85 % aqueous solution) using mixing ratio at Al:Cr:P = 3:1:9 (mol%). A typical preparation is described here. 10 g of Al(OH)3 was dissolved in 57.98-g (65 mass%) aqueous H3PO4 at 353 K for 30 min until it became clear. Immediately, 4.27 g of CrO3 was added to the solution, which was quickly dissolved by adding 1.37 g of methanol (Cuicr, AR). The aluminum–chromium phosphates binder obtained is described as the ACP binder. Reaction equations are as follows [3, 8, 9]:

$$ {\text{Al(OH)}}_{ 3} + 3 {\text{H}}_{ 3} {\text{PO}}_{ 4} \mathop{\longrightarrow}\limits^{{ 3 5 3 {\text{ K}}}}{\text{Al(H}}_{ 2} {\text{PO}}_{ 4} )_{ 3} + 6 {\text{H}}_{ 2} {\text{O}} $$
(1)
$$ {\text{Al(H}}_{ 2} {\text{PO}}_{ 4} )_{ 3} \mathop{\longrightarrow}\limits^{{ 3 5 3 {\text{ K}}}}[ ( {\text{H}}_{ 2} {\text{PO}}_{ 4} )_{ 2} {\text{Al(HPO}}_{ 4} ) ]^{ - } {\text{ + H}}^{ + } $$
(2)
$$ 4 {\text{CrO}}_{ 3} {\text{ + 12[(H}}_{ 2} {\text{PO}}_{ 4} )_{ 2} {\text{Al(HPO}}_{ 4} ) ]^{ - } {\text{ + 12H}}^{ + } {\text{ + 3CH}}_{ 3} {\text{OH}}\mathop{\longrightarrow}\limits^{{353\;{\text{K}}}} 4 {\text{Cr}}^{{ 3 { + }}} {\text{ + 12[(H}}_{ 2} {\text{PO}}_{ 4} )_{ 2} {\text{Al(HPO}}_{ 4} ) ]^{ - } {\text{ + 3HCOOH + 9H}}_{ 2} {\text{O}} $$
(3)
$$ 4 {\text{Cr}}^{{ 3 { + }}} + 1 2 [ ( {\text{H}}_{ 2} {\text{PO}}_{ 4} )_{ 2} {\text{Al(HPO}}_{ 4} ) ]^{ - } \mathop{\longrightarrow}\limits^{{ 3 5 3\;{\text{K}}}} 1 2 [ ( {\text{H}}_{ 2} {\text{PO}}_{ 4} )_{ 2} {\text{Al(HPO}}_{ 4} ) {\text{Cr}}_{\frac{1}{3}} ] $$
(4)

Characterization

As the samples were treated at various temperatures, the crystalline phases were identified by means of X-ray diffraction analysis (Model D/max-RB, Rigaku, Japan) between 10° and 80° (2-θ) using a graphite monochromatic Cu Kα radiation, with a step of 0.02° and a scanning rate of 0.5° min−1. Diffraction peaks were indexed with Jade 6 software. Thermal behaviors of specimens were observed using Thermogravimetric analysis–differential scanning calorimetry (TGA–DSC SDT Q600, TA Instrument) with a heating rate ranging from 283 K min−1–1,573 K under dry air. Infrared absorption spectra (AVATAR370, Nicolet, USA) were obtained within the range of 400–4,000 cm−1. The relative dielectric constants (ε r) were measured within the range of 8–12 GHz at room temperature, using an E5071C vector network analyzer (Agilent).

Results and discussion

Figure 1 shows the XRD patterns of ACP binder with different heat treatments from 293 to 1,873 K. In the samples heated at 1,173 K, there were no obvious crystalline phase but still amorphous. As temperature increased to 1,273 K, amorphous aluminum–chromium phosphates began to crystallize, the dominant phases were Al(PO3)3,Cr(PO3)3, and a few AlPO4. After heating up to 1,473 K, the peak intensity of AlPO4 is much more than before with decreasing intensity of Al(PO3)3 until it completely disappeared at 1,573 K. Based on the analysis from DSC data (seen in Fig. 2), it can be due to the thermal decomposition of Al(PO3)3. After heating at 1,673 K, the peak of Cr4(P2O7)3 appeared, but Cr(PO3)3 wasn’t detectable. At 1,773 K, Cr2O3 was detected by the XRD measurement, and eventually leaving a Cr2O3- and AlPO4-rich residue at 1,873 K.

Fig. 1
figure 1

XRD patterns of ACP binders heated at various temperatures. Filled circle Al(PO3)3, filled star AlPO4, open diamond Cr(PO3)3, open triangle Cr4(P2O7)3, and Cr2O3

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Fig. 2
figure 2

Thermal analysis curves of ACP binder: a DSC–TG patterns of ACP binder; b DSC–DTG patterns at the second DSC endothermic stage; c DSC–DTG patterns at the third DSC endothermic stage

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Following a previous study on chromium phosphates’ thermal stability [10], the series changes of chromium-containing compound as mentioned above can be ascribed to the thermal decomposition of Cr(PO3)3 occurring as follows:

$$ 4 {\text{Cr(PO}}_{ 3} )_{ 3} \mathop{\longrightarrow}\limits^{{ 1 6 7 3\;{\text{K}}}}{\text{Cr}}_{ 4} ( {\text{P}}_{ 2} {\text{O}}_{ 7} )_{ 3} {\text{ + 3P}}_{ 2} {\text{O}}_{ 5} {\uparrow}$$
(5)
$$ {\text{Cr}}_{ 4} ( {\text{P}}_{ 2} {\text{O}}_{ 7} )_{ 3} \mathop{\longrightarrow}\limits^{{1773{-}1873\;{\text{K}}}} 2 {\text{Cr}}_{ 2} {\text{O}}_{ 3} {\text{ + 3P}}_{ 2} {\text{O}}_{ 5} {\uparrow}$$
(6)

Figure 2 shows both the DSC and the TG curves of ACP binder from room temperature to 1,573 K. The curves indicated the relationship between temperature change and mass loss of the sample. In the DSC curve, four endothermic stages were recorded with their onset temperatures amounting, respectively, to: 1, 2, 3, and 4, accompanied by varying mass losses. It was observed that the first stage was due to the dehydration and condensation of ACP binder as supported by some earlier studies [8, 11]. The mass loss associated with this endothermic stage was equal to 20 mass%.

For the second endothermic stage, there were no obvious mass loss but a small DTG peak (Fig. 2b). Based on the XRD analysis from 1,173 to 1,273 K, it can be attributed to the complete crystalline transition of amorphous aluminum–chromium phosphates forming metaphosphates like Al(PO3)3 and Cr(PO3)3. The small DTG peak is due to the Al(PO3)3 decomposition forming AlPO4 and P2O5 gas because a few AlPO4 has been detected by XRD measurement, so as to the third endothermic stage (Fig. 2c).

The last endothermic stage, beginning at the onset temperature of 1,423 K, is accompanied by a wide mass loss. Considering the XRD analysis from 1,373 to 1,573 K, it can be concluded that these endothermic and mass losses are associated with the complete decomposition of Al(PO3)3 that takes place probably according to the equations [11–14]:

$$ {\text{Al}}\left( {{\text{PO}}_{ 3} } \right)_{ 3} \mathop{\longrightarrow}\limits^{{1473{-}1573\;{\text{K}}}}{\text{AlPO}}_{ 4} {\text{ + P}}_{ 2} {\text{O}}_{ 5} \uparrow $$
(7)

The IR spectra of ACP binder between 293 and 1,473 K are given in Fig. 3. Five infrared absorption bands at 1240, 1097, 940, 776 cm and 480 cm−1 gradually changed with high-temperature treatment. The 1,097 cm−1 band is converted to broad absorption band in the range 1,240–940 cm−1, after 473 K heating. Then, the 1,240 cm−1 band (bridging O=P–O) shifts up to 1,280 cm−1, the absorption maximum at 940 cm−1 (P–O–P) moves to 1,150 cm−1 (Al–O–P). Simultaneously, the 776-cm−1 band moves right with the increasing intensity.

Fig. 3
figure 3

IR spectra of ACP binders at various temperatures

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IR reflectance spectra of Al(H2PO4)3 and ACP binder have been reported by Waleed Mekky [10] and Han et al. [4]. Their spectra show the same trend. Following the numerous spectral studies on phosphate [15–28], the bands observed can be assigned as follows. The principal band at 1,240 cm−1 in the IR spectra is due to the asymmetric stretching of bridging PO2 (O=P–O–Al/Cr), the observed band in 1,097 cm−1 are attributed to the PO4 3− bending and stretching vibrations. Bands at 1,150 and 1,280 cm−1 was assigned to the stretching vibration of PO3 groups. In addition, bands at 940 and 776 cm−1 belong to the asymmetric stretching vibration of the linear P–O–P chain.

After heating at 473 K, the band at 1,097 cm−1 disappeared, converting to broad absorption band in the range 1,240–940 cm−1 with the vast decreasing intensity of the hydroxyl group, –OH, at 1,638 cm−1. It can be attributed to the transformation of dihydric phosphates to polymerized Phosphates, a condensation reaction between P–OH and P–OH. From temperatures 473 to 973 K, the bands at 776 and 1,240 cm−1 gradually increased with decreasing intensity of the hydroxyl group, –OH, at 1,638 and 3,467 cm−1, respectively. This was presumably due to an internal condensation reaction of amorphous aluminum–chromium phosphates, condensation reactions between O=P–OH and –O–Al/Cr–O–, P–O–P–OH and P–O–P–OH. Figure 4 shows the reactions with a simple model of aluminum phosphate glasses [29–31]. In this figure, hydroxyl groups, P–O–P–OH, polymerized to form longer linear P–O–P chain, O=P–OH and –O–Al/Cr–O– groups polymerized to form more bridging PO2 (O=P–O–Al/Cr), finally, the bridging of the chains and three-dimensional structures of amorphous aluminum–chromium phosphates have been strengthened. Moreover, this fact is also confirmed by DSC-TG and dielectric constant testings. Because of the water losing constantly in those condensation reactions, the dielectric constant (seen in Fig. 5) and TG curve (seen in Fig. 2) decreased below 1,073 K. After 1,273 K thermal treatment, 776-, 940-, and 1,240-cm−1 bands, respectively, are converted into three new sharp absorption peaks, 750, 1150, and 1280 cm−1. The appearance of these new bands, which was assigned to the asymmetric stretch of the P–O–P and PO2 rings, suggests that the P–O–P and O=P–O bonds began to change from a linear or bridging structure to a small (three- or four-member) metaphosphate Al(PO3)3 ring structure. This also can be confirmed by means of XRD patterns at 1,273 K.

Fig. 4
figure 4

Schematic representation of internal condensation reaction of amorphous aluminum–chromium phosphates

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Fig. 5
figure 5

Relationship between wave frequency and relative dielectric constants (ε r) of ACP binder heated at various temperatures

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Figure 5 shows the relationship between the heating temperature and the relative dielectric constants with different temperature treatments. The data reveal a fluctuant dielectric constant with the increasing heating temperature. For instance, at 10 GHz, the dielectric constant of the sample drops from 4.2 to 3.3 when temperature increases from 673 to 873 K, but increases to 3.8 when temperature increases to 1,373 K, and finally decreases to 3.5 at 1,573 K. The decrease in dielectric constant from 673 to 873 K is possibly related to the further internal condensation dehydration of amorphous aluminum–chromium phosphates. The increase in dielectric constant to 1,373 K is probably due to the metaphosphate phase formed from amorphous phase which has a higher electric dipole moment. This leads to the increase of ionic polarizability which is the key factor affecting the dielectric constant. The last decrease in dielectric constant to 1,573 K is attributed to the thermal decomposition from Al(PO3)3 to AlPO4 because of a lower refractive index than that of the metaphosphate [32].

Conclusions

The results of the conducted research allow for the following statements:

  • About 443 K, the starting phase of the ACP binder transformed to amorphous via condensation.

  • At temperatures from 473 to 1,173 K, the only phase is amorphous. However, IR and DSC–TG analyses show that amorphous structure is variational: residual hydroxyl groups, P–O–P–OH polymerized to form longer linear P–O–P chain, and O=P–OH and Al/Cr–OH groups polymerized to form more bridging PO2 (O=P–O–Al/Cr).

  • Approximately at 1,273 K, amorphous phase began to crystallize, forming metaphosphates like Al(PO3)3 and Cr(PO3)3.

  • At 1,473 K, Al(PO3)3 began to decompose into AlPO4 and P2O5 gas according to Eq. (7).

  • Between 1,673 and 1,873 K, Cr(PO3)3 has undergone two decompositions, first, forming Cr4(P2O7)3 and P2O5 gas, and then transforming into Cr2O3 and P2O5 gas according to Eqs. (5) and (6).

  • Thermal transformation of Aluminum–chromium Phosphates from 473 to 1,873 K lead to a fluctuant dielectric constant, but finally stabled at 3–4 because of the final products AlPO4 and Cr2O3.