Properties of -O-Cu-O- Bridged Copper Phosphate-Based Thermal Insulation Materials.

P-O-H polycondensation -O-Cu-O- ion-bonded bridges were formed in copper phosphate thermal insulation materials by mixing Al(OH)3 dissolved in H3PO4 with CuO filler and Al2O3, SiC, ZrC, and Cr2O3 as curing accelerators, alone or in combination. The effects of different additive combinations on the curing behavior and thermal stability of the copper phosphate thermal insulation material matrixes were compared using thermogravimetry/differential scanning calorimetry, X-ray diffractometry, and scanning electron microscopy. The copper phosphate materials exhibit good thermal stabilities and low thermal conductivities. The thermal weight losses before and after ceramic reinforcement were 4-19.8 and 3.8-9.4%, respectively, and the thermal conductivities of the P-O-H polycondensation -O-Cu-O- ion-bonded bridges formed in the copper phosphate thermal insulation materials were in the range of 0.656-1.824 W/(m·K).

Introduction

With the rapid development of science and technology, a large number of new energy technologies have been researched and applied.[1−5] At the same time, improving energy efficiency is also an important part of future sustainable energy planning.[6] Among them, the development of insulation materials has played a vital role. It has been widely used in aerospace, automotive, construction, household appliances, petrochemical industries, and so forth.[7] For example, excellent thermal insulation is a vital requirement for hypersonic aircraft enclosures to protect the interior of the aircraft from surface ultrahigh temperatures.[8]

At present, common insulation materials include ceramic heat-insulating tiles,[9] organic insulating materials,[10] and ceramic aerogels.[11] Although such materials have produced ideal results in certain applications, their disadvantages include low useable temperatures, poor thermal matching, and complicated production processes. In addition, the integrity of the bonding between a heat-insulating and heat-resistant material is difficult to ensure in practical applications, which complicates the fabrication and repair processes, particularly in the context of large structures.

Phosphate-based materials are preferable to the above-mentioned insulating materials because of their low curing temperatures, structural design flexibilities, and high bonding strengths.[12−15] When used to develop insulation materials, phosphate-based materials are inexpensive and easy to apply. In addition, phosphate materials provide good load distributions, which reduce stress concentrations and avoid defects, such as cracks and holes.[16] Second, phosphate materials can accommodate the strain caused by differences in thermal expansion coefficients and temperature changes, which minimizes excessive damaging stress in the ceramic.[16] Third, the performance of phosphate materials can be tuned through the use of additives and reinforcing materials to satisfy a variety of requirements.[16] Finally, phosphate materials have very low thermal conductivities and can act as strong barriers to heat transfer; consequently, they are a promising new type of insulation material for a range of applications.[17−19]

Phosphates are formed by the reaction of metal oxides with phosphoric acid or phosphate. Kingery reacted various oxides with a phosphoric acid solution and recorded the condensation times and elevated temperatures required to form the reaction product;[20] however, the product produced by that method was mainly hydrogen phosphate, which has limited applications.[21] Chung reacted metal cations in different molar ratios with phosphate and found that the properties of the phosphate varied with the metal/phosphate molar ratio.[22] Research by Wagh found that oxides dissolve highly exothermically in phosphoric acid to form saturated solutions that solidify to form well-crystallized phosphate materials.[23] On the basis of these results, the performance of a phosphate material appears to depend on the ratio of oxide to phosphoric acid. In addition, the amounts and combinations of additives, such as acid inhibitors, accelerators, and reinforcing materials, also impact phosphate performance.[24−26]

While copper phosphate materials have attracted attention because of their simple fabrication, cost effectiveness, and strong binding forces, there are few reports on this material in the literature to date. To address this, in this study, we focused on determining the ideal composition ratio for a copper phosphate heat insulation material suitable for use in C/C-ultra-high-temperature ceramics. We accomplished this by investigating the microstructures of copper phosphates with different composition ratios, characterizing the thermal properties of these copper phosphate materials by thermal-analysis and other testing techniques, and studying the thermal stabilities and diffusion behavior of these materials.

Results

Samples A–J were prepared according to the molar ratios of reactants listed in Table . Full sample-preparation details are provided in Section .

Table 1

Molar Ratios of the Phosphate Material Reactants

			mixing ratio 0028mol %)
sample	CuO	Al(OH)3	H3PO4	Al2O3	SiC	ZrC	Cr2O3
A	20		8
B	20	1	8
C	16	5	8
D	2.3	2.3	8
E		11	8
F	20	1	8	0.8	2
G	20	1	8	1.6	4
H	20	1	8	0.5	1.3	0.6
I	20	1	8	0.4	1	0.4	0.3
J	20	1	8	0.5	1.3		0.4

Curing Characteristics

The curing times and density changes of the copper phosphate materials before and after modification are listed in Table . As shown, the material density was observed to decrease gradually with increasing amounts of Al(OH)3, and the material had difficulty forming at 25 °C when 50% Al(OH)3 was added. The sample solidified within a few seconds but contained many macroscopic pores when a large amount of Al(OH)3 was reacted with H3PO4. The resulting material exhibited a lower density when a ceramic filler was added, compared to the material prepared without the filler, and a lowed curing time was required. This was especially true as the proportion of Al2O3 was increased, as was observed for sample G, which had a corresponding curing time of only 262 s.

Table 2

Full Curing Time and the Corresponding Density

sample	A	B	C	D	E
density (g/cm3)	4.49 ± 0.05	3.06 ± 0.05	2.67 ± 0.05
setting time (s)	1020 ± 10	780 ± 10	763 ± 10
sample	F	G	H	I	J
density (g/cm3)	2.83 ± 0.05	2.84 ± 0.05	2.94 ± 0.05	2.95 ± 0.05	2.89 ± 0.05
setting time (s)	778 ± 10	262 ± 10	452 ± 10	708 ± 10	592 ± 10

Microstructural Characterization

Phase Analysis

X-ray diffraction (XRD) patterns of the copper phosphate materials prepared according to the composition ratios listed in Table are shown in Figure , which reveals that the proportions of Al3+ and PO43– in the physical phase increase with decreasing molar ratios of CuO and Al(OH)3. In addition, when only CuO was added to phosphoric acid, as in Sample A, the product was primarily composed of Cu2(PO4)(OH) and CuO. In this case, the intensities of the diffraction peaks corresponding to CuO were higher than those of Cu2(PO4)(OH), which indicate that some unreacted CuO remained in the sample. Cu3Al4(PO4)2(OH)12·2H2O appeared when Al(OH)3 was added, as evidenced by comparing the peaks of the sample B phase with those of sample A. Due to the addition of metal cations with higher potential-series numbers (such as Al3+) to CuO and H3PO4, H3PO4, they took precedence over more active metal cationic reactions,[27] resulting in a stronger bond between CuO and phosphoric acid and a denser material microstructure (as shown in Figure b). The components in sample C, which has further reduced molar ratios of CuO and Al(OH)3, were found to be Cu2PO4OH·0.2H2O, CuO, and a small amount of Al(OH)3. The appearance of Cu2PO4OH·0.2H2O indicates that the addition of Al(OH)3 promotes the reaction between CuO and phosphoric acid.

Figure 1

XRD patterns of copper phosphate samples A–F–J.

Figure 2

SEM image of copper phosphate samples A–F. (a1) Magnified image of area (a1) marked in (a); (b1) magnified image of area (b1) marked in (b); (c1) magnified image of area (c1) marked in (c); (d1) magnified image of area (d1) marked in (d); (e1) magnified image of area (e1) marked in (e); (f1) magnified image of area (f1) marked in (f).

With regard to the promoting ability of Al(OH)3 in the reaction of CuO with phosphoric acid, sample D, which has a 1:1 molar ratio of CuO to Al(OH)3, is primarily composed of AlH3(PO4)2·H2O, Al(H2PO4)(HPO4)·H2O, and Cu3Al4(PO4)2(OH)12·2H2O. The diffraction peaks corresponding to CuO in this sample were extremely weak. At this ratio, most of the CuO reacted to retain PO43–. The XRD pattern of sample E reveals that AlPO4 is only formed when Al(OH)3 reacts with phosphoric acid; the reaction was fast, and the curing time was short, which indicates that Al(OH)3 plays a catalytic role in the copper phosphate material.

As shown in Figure f, the microstructure of sample B was the densest among the samples because of its level of unreacted CuO. XRD revealed the appearance of small amounts of Al(OH)3 when different ceramic fillers were added to sample B (i.e., samples G–J), which is indicative of the good chemical stabilities of SiC, ZrC, and Cr2O3 at room temperature (RT). As shown in the XRD pattern, Al2O3 possibly reacted earlier with phosphoric acid than with Al(OH)3. From a microscopic point of view, the addition of Al2O3 is likely to accelerate the condensation reaction of phosphoric acid to produce water vapor, which is why the proportion of Al(OH)3 is larger and the overall structure is looser before modification.[27]

Structure Characteristics

Since phosphoric acid continuously polycondenses to form long-chain macromolecular polyphosphoric acids upon addition of CuO, Cu–O ions bond to form a phase with alternating covalent and ionic bonds. Scanning electron microscopy (SEM) images of the copper phosphate materials are shown in Figure , which reveals that the microstructure of sample A is loose and contains both large and small holes. This observation is possibly ascribable to the large amount of water vapor generated during solidification of the CuO/H3PO4 mixture during discharge of the resulting gas; the lower number of bubbles leads to higher sample density. As was observed by XRD, the main elements in the white areas in the SEM image in Figure a are Cu and O, whereas the main elements in the gray areas are Cu, O, and P. Upon further consideration, we conclude that the white areas correspond to CuO, whereas the gray areas correspond to Cu2(PO4)(OH). The microstructure of sample B, in which Al(OH)3 was added, is denser than that of sample A. As shown in Figure b,b1, sample B has a much higher surface density than sample A, which may be the result of the added Al(OH)3 that accelerates the rate of phosphoric acid polymerization, as the solution contains more water than before. This, in turn, reduces the amount of water vapor generated during curing, which increases the surface density of sample B. A more uniformly distributed white phase surrounded by gray is clearly observed in the SEM image, and a black phase is also present. Qualitative energy-dispersive X-ray spectroscopy (EDS) revealed that the white phase in Figure b is primarily composed of Cu and O in the form of CuO; the gray phase contains O, Al, P, and Cu; and the black phase contains Al and O. The distribution of elements can be intuitively understood in Figure S1. As was observed by XRD, the phases in sample B are primarily composed of Cu2(PO4)(OH), Cu3Al4(PO4)2(OH)12·2H2O, CuO, and a small amount of Al(OH)3.

The addition of Al(OH)3 accelerates the rate of phosphoric acid polymerization. The microstructure was observed to loosen as the Cu2+/Al3+ molar ratio was reduced from 20/1 to 16/5. As shown in Figure c, the microstructure of sample C contains large distinct particles, with smaller white CuO phases compared to those in Figure b, and the holes are clearly visible. This may be due to the fact that the rate of phosphoric acid polycondensation was too fast; when combined with a lower CuO content, there may be insufficient CuO remaining after the reaction to fill the pores produced by phosphoric acid polycondensation, which indicates that the remaining CuO acts as a reactant for the formation of the copper phosphate material. In other words, the remaining CuO acts as a filler. The appearance of the Al(OH)3 phase is evident in Figure c, which may be the result of a reaction that is too fast such that some of the Al(OH)3 does not participate in the reaction process and solidifies. Therefore, as shown in Figure d, the surface of sample D is looser and is primarily composed of a gray sheetlike structure and fine white particles. At the same time, the white granular CuO phase is not visible and large pores are evident.

On this basis, unreacted CuO fills the pores formed after the polycondensation of phosphoric acid. As shown in the SEM image, a large number of pores appear when only Al(OH)3 is reacted with phosphoric acid. The microstructure after reaction at a 11:8 Al(OH)3-to-phosphoric-acid molar ratio is shown in Figure e, which reveals that the resulting structure is irregularly connected by a plurality of irregular gray particles, with many large and small pores throughout. EDS qualitatively revealed that the interiors of the particles are mainly composed of Al and O, whereas the surfaces are composed of O, P, and Al. Consistent with the XRD data, the former is Al(OH)3, and the latter is AlPO4, which suggests that the particles are linked by AlPO4.

A SEM image of the material produced when ceramic particles were added to sample B is shown in Figure f, which reveals that Al(OH)3 and SiC particles are surrounded by phosphoric acid polycondensation reaction products. As shown in the enlargement of the indicated region (Figure f1), the filler particles are closely connected with the copper phosphate material to provide an improved interface.

In summary, copper phosphate materials form by the continuous polycondensation of phosphoric acid to give long-chain macromolecular polyphosphoric acids that bond to Cu–O ions to form a phase containing alternating covalent and ionic bonds. Any remaining unreacted CuO then fills the pores after polycondensation.

Thermal Properties

Thermal Stability

The copper phosphate material decomposes into copper pyrophosphate and then into copper metaphosphate upon heating from RT. Figure shows the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves for the copper phosphate materials fabricated using different molar ratios of CuO and Al(OH)3; these materials appear to have three distinct temperature stages. The first stages for samples A–E occurred between RT and ∼300 °C (sample A), in the 200–300 °C range (samples B and C), from RT to ∼200 °C (D), and from RT to ∼250 °C (E). As shown in Figure b, the five systems exhibit wide exothermic peaks at around 100 °C, which are produced by unreacted phosphoric acid at RT and the residual reactions of CuO with Al(OH)3 to form phosphate; crystallization therefore resulted in the release of heat. However, the exothermic reaction had not yet completely ended at this point. An endothermic peak was next generated by the decomposition of the amorphous compound, which released some water. As shown in Figure a, samples D and E exhibited larger weight losses at this stage and lower starting temperatures. Sample D has a 1:1 molar ratio of CuO to Al(OH)3, and since it had not completely reacted after curing, as indicated by the XRD pattern in Figure , the formed phases were complicated and possibly unstable; consequently, it began to decompose at a temperature of about 80 °C. Since a large amount of the residual Al(OH)3 in sample E decomposed at about 150 °C, the thermal weight loss of sample E was large at a low temperature; an endothermic peak between 260 and 300 °C was observed in the first stage, which was the result of the dehydration of the amorphous compound to copper pyrophosphate.

Figure 3

TGA–DSC curves of copper phosphate samples A–J. (a) TGA curves of samples A–J; (b) DSC curve of samples A–J.

As the temperature increased, the copper pyrophosphate was again polycondensed and dehydrated into a cyclic or long-chain copper metaphosphate. As shown in Figure a, the second dehydration stages for samples A–E were 300–600, 300–600, 300–600, 200–500, and 250–500 °C, respectively. Samples A, B, and C exhibited endothermic peaks at about 600 °C as the copper phosphate became desorbed from the crystal water and the copper pyrophosphate was decondensed through polycondensation to form copper metaphosphate. The temperatures of samples A, B, and C were approximately 600 °C during the third stage, while those of D and E were 500 °C. The production of copper metaphosphate in this stage occurs by the removal of structural water. As shown in Figure , the thermal weight losses of the final samples A–E were 4.2, 4, 5.8, 19, and 19.8%, respectively, which indicates that the addition of a small amount of Al(OH)3 improves the thermal stability of the phosphate material. However, the thermal weight loss increases with increasing amounts of added Al(OH)3. Taken together with the XRD and SEM data, the heat performance and organizational structures of the B samples suggest that they are suitable for use in high-temperature environments.

Figure 6

Products of the reaction of macromolecules of polyphosphoric acid with CuO.

The addition of the ceramic filler was found to improve the thermal stability of the copper phosphate material at high temperatures, despite the increase in the thermal weight loss. Figure reveals that, while the TGA and DSC curves of the F samples are quite similar to those of the B samples, the curves for the other four types of samples are quite different. In addition, the temperature required for dehydration and polycondensation into copper pyrophosphate decreased, and no significant weight loss and reaction occurred above 300 °C, which may be due to the added ceramic filler that accelerates the polycondensation reaction. The thermal weight losses of samples F–J were 3.8, 8.5, 7.8, 7.5, and 9.4%, respectively, which suggests that the addition of the ceramic filler enhances thermal weight loss. In summary, among the samples with ceramic fillers, sample F exhibited a low thermal weight loss rate and a dense microscopic morphology, which is ideal for use in multicomponent systems.

Thermophysical Properties

The thermal properties of the copper phosphate materials are listed in Table , which reveals that sample A has the lowest thermal diffusivity and that the thermal conductivity of the corresponding copper phosphate material increased significantly after the addition of the ceramic filler. Since sample A is mainly composed of CuO and phosphoric acid in the form of Cu2(PO4)(OH) and CuO, which are networklike macrostructures, and the microstructure of sample A is relatively loose, its thermal diffusivity was found to be low. Sample B is mainly composed of Cu2(PO4)(OH), CuO, and a small amount of Cu3Al4(PO4)2(OH)12·2H2O. The amount of residual CuO increases with the addition of Al(OH)3. As shown in Figure , the density of sample B on the microscopic level was higher than that of sample A, which indicates that sample B has a higher thermal mass coefficient. There are many Al-containing phases in the microstructure of sample C; hence its thermal diffusivity was even higher. The thermal conductivity of samples F–J to which the ceramic filler was added was higher than that of the sample B.

Table 3

Thermal Properties of the Various Copper Phosphate Samples

sample	thermal diffusivity mm2/s	thermal conductivity W/(m K)	specific heat J/g/K
A	0.376 ± 0.015	0.656 ± 0.026	0.389 ± 0.015
B	0.458 ± 0.024	0.872 ± 0.045	0.622 ± 0.019
C	0.864 ± 0.032	1.668 ± 0.059	0.723 ± 0.045
F	0.802 ± 0.034	1.467 ± 0.053	0.646 ± 0.027
G	0.713 ± 0.034	1.702 ± 0.030	0.841 ± 0.023
H	0.588 ± 0.032	1.282 ± 0.020	0.742 ± 0.021
I	0.867 ± 0.054	1.683 ± 0.026	0.658 ± 0.025
J	0.945 ± 0.033	1.824 ± 0.060	0.668 ± 0.008

Discussion

Aspect of Heat Conduction

The copper phosphate thermal insulation materials is the composite mainly composed of phosphate and CuO, so the thermal conductivity largely depends on the number and arrangement of each of the phases and their respective thermal conductivities. For the two phases of parallel and vertical alignment in the composites, the calculation of thermal conductivity can be performed as two mechanisms of parallel and series. Specifically[28]where Km is the thermal conductivity of the composites, v1 and v2 are the volume fractions of the two phases, respectively, and k1 and k2 are the thermal conductivities of the two phases, respectively.

For a structure formed by a continuous main phase and a discontinuous second phase or by a discontinuous main phase and a continuous second phase, it can be calculated by Maxwell’s relationship[28]where Km is the thermal conductivity of the composites, k1and k2 are the thermal conductivities of the continuous phase and the dispersed phase, respectively, and v2 is the volume fraction of the dispersed phase.

It can be seen from the formula 1–3 that when adding the component, which possesses higher thermal conductivity in the composites, the overall thermal conductivity increases. In this study, the copper phosphate material was a macromolecular polymer with very low intrinsic thermal conductivity. Samples F–J were formed by adding a ceramic filler to sample B as a matrix. As shown in Table , the thermal conductivity of samples F–J was higher than that of sample B. At the same time, the higher the content of the ceramic filler, the higher the thermal conductivity, and so the thermal conductivity of sample G is higher than that of sample F. When a filler containing a higher thermal conductivity is added, the thermal conductivity is also increased. The sample with high thermal conductivity of Cr2O3 has a higher thermal conductivity under the same ceramic filler composition.

In inorganic nonmetallic materials, heat flow is mainly carried by acoustic phonons, and its thermal conductivity can be expressed by the following formula[29]where C is the specific heat capacity, v is the average sound velocity, and l is the phonon mean free path. The mean free path of phonons is a key factor affecting thermal conductivity. In practice, scattering of other phonons and various defects can also affect thermal conductivity. Therefore, for sample B with a more dense microstructure, the average free path of the phonons is larger and the thermal conductivity is higher than that of sample A. Because the microstructure of sample H was looser (as shown in Figure S2), the mean free path of the phonon decreased, resulting in the lowest thermal conductivity of the sample added to the ceramic filler.

Curing Mechanism of Cu–O Ion Bonds on Copper Phosphate Materials

The formation of reticulated inorganic polymers by phosphoric acid condensation and Cu–O ionic-bond bridges increases the viscosity of the solution and forms copper phosphate materials with high bonding strengths after curing. Since the outer electronic configuration of phosphorus is 3s23p3, concentrated phosphoric acid forms a structure of regular PO4 tetrahedrons. P–O–H is dehydrated and condensed and is connected by oxygen bridges to form a mixture of polyphosphoric acids, H (PO3). PO43– is present in the phosphoric acid solution and is polymerized by the addition of a basic oxide during the reaction. The resulting PO42– units become nodes in the polymer chain. As the reaction progresses, PO42– is converted into PO4– ions, which is an intermediate structure arising during the reaction, and is connected end-to-end to form a linear polyphosphate. In the later stage of the reaction, the branched structure formed by uncharged PO4 is continuously extended outward until a spatial three-dimensional network is formed, as shown in Figure . Note that CuO is typically a monoclinic crystal at 25 °C and is weakly basic; it exists in an ionic-bonded form as Cu–O with a bond energy of 82 kcal/mol. The interaction forces between crystals are generally van der Waals, although the Cu–O ionic bonds that form in the bodies of copper phosphate crystals have large binding forces, as shown in Figure . In this case, CuO reacts with concentrated phosphoric acid to form copper phosphate, copper pyrophosphate, and other phosphates that mostly contain Cu2+, H2PO4–, HPO42–, and PO43– plasma. The continuous distribution of the networked inorganic polymer formed by the polycondensation of phosphoric acid, −O–Cu–O– ion-bonded bridges, ionic bonding forces, and hydrogen-bonding forces cause Cu2+ and PO43– to form a continuous and interpenetrating phase before combining with the unreacted CuO. As was observed by XRD, the main product of the reaction between CuO and phosphoric acid is Cu2(PO4)(OH), as evidenced by its Fourier transform infrared (FTIR) spectrum (Figure ). The viscosity of the solution increases and the rate of ion movement decreases as the phosphoric acid continues to polymerize. To some extent, the degree of reaction between CuO and phosphoric acid decreases until the sample is completely cured. As shown in the SEM images, the samples solidify to produce strongly bonded structures.

Figure 5

Spatial structural composition of the phosphate.

Figure 4

FTIR of Cu2(PO4)(OH): the stretching vibration peak of the hydroxyl group at 3440 cm–1 is consistent with the hydroxyl group of Cu2(PO4)(OH) in the XRD phase. Near 1620 cm–1 are the P–O stretching vibration peak and the bending vibration peak, 1086 cm–1 is the symmetric stretching vibration peak of [PO4]3–, 992 cm–1 is the asymmetric stretching vibration peak of [PO4]3–, and around 628 cm–1 is the in-plane bending vibration peak of [PO4]3–.[30] The telescopic vibration peak of Cu–O is near 559 cm–1.[31]

As shown in Table and Figure , the curing time shortens and the microstructure of the material becomes denser with the addition of Al(OH)3 because the Al3+ in Al(OH)3 is more active than Cu2+; hence, phosphoric acid reacts preferentially with Al(OH)3, which increases the rate of polycondensation, accelerates the rate of curing, and further increases the chance that large numbers of bubbles are generated by the water vapor produced during the reaction; the sample is denser when fewer bubbles are generated. The equations describing the reactions of phosphoric acid are

As only a small amount of Al(OH)3 was added during the reaction, aluminum phosphate molecules formed once Al(OH)3 had completely reacted with phosphoric acid, and after it began to converge, the remaining phosphoric acid continued to react with CuO.[32] The degree of phosphate polymerization increased as the reaction progressed, and the temperature of the reaction also increased to eventually form a networklike macromolecular structure. The cohesive force of the phosphate material, along with its strength, can be seen to increase with increasing degree of longitudinal bonding in the polyphosphoric acid macromolecule. From the microscopic structure of the copper phosphate in Figure b, the unreacted CuO interspersed with the copper phosphate material to form a dense copper phosphate material. However, the density of the copper phosphate material was observed to decrease with increasing Al(OH)3 content, even at 25 °C. On the basis of these results, the optimum molar ratio of CuO/Al(OH)3/H3PO4 was determined to be 20:1:8.

Conclusions

In this work, a series of Cu/Al ratio copper phosphate insulation materials were prepared by designing variables, and the thermal stability and thermal conductivity were measured by thermal analysis. The results show that the addition of a small amount of Al(OH)3 makes the microstructure dense, but the excess will decrease. In this study, the microstructure of the most compact B combination Cu/Al molar ratio is 20/1, and the thermal weight loss rate at 900 °C is less than 5%. In addition, this bonding structure has a low thermal conductivity of 0.872 W/(m K). On the basis of the results obtained in this study, we conclude that copper phosphate is a very promising class of material for thermal insulation at high temperatures.

Experimental Section

Material Preparation

The primary raw material in this study was composed of CuO (analytical grade, AR, Sinopharm Group), Al(OH)3 (analytical grade, AR, Sinopharm Group), and phosphoric acid solution (analytical grade, AR, Sinopharm Group). The following materials were used as fillers in the ratios listed in Table : Al2O3 (analytical grade, AR, Sinopharm Group), Green SiC (analytically pure, AR, Sinopharm Group), ZrC (Jinzhou Hao Tian Titanium Powder Processing Co., Ltd., 400 mesh), and Cr2O3 (analytically pure, AR, Sinopharm Group). The copper phosphate material was synthesized using a liquid-phase reaction method. Sample A (as an example) was prepared as follows: 8 mol of phosphoric acid solution and 20 mol of CuO powder were combined in a beaker and thoroughly stirred until the mixture solidified completely.

To fully study the formation mechanism of the copper phosphate material, the performances of samples A to E were compared, from which the distribution ratio of sample B was selected. The distribution ratios of samples F–J were selected by exploring the influence of nonreactive filler additives on the properties of the copper phosphate material.

Characterization

Each sample was subjected to phase analysis using a RIKEN D/max (2550) fully automatic (18 kW) brick target X-ray diffractometer with a current of 20 mA, a voltage of 35 kV, and a scanning interval of 0.02°. Morphological, microstructural, and qualitative elemental analyses were performed using a Thermo Fisher Quanta FEG 250 field-emission scanning electron microscope and an EDS system. TGA and DSC were conducted in air using a TA Instruments SDT650 synchronous thermal analyzer at a heating rate of 10 °C/min to 900 °C. The thermal diffusivity and specific heat capacity were measured using a NETZSCH LFM457 laser thermal conductivity meter, and the thermal conductivity of the copper phosphate material was calculated using formula 8where λ is the thermal conductivity in W/(m K), α is the thermal diffusion coefficient in mm2/s, ρ is the density in g/cm3, and C is the specific heat capacity in J/g/K.

The sample was processed into a pellet sample φ 10 mm × 2.5 mm in size. Note that the thermal conductivity λ refers to the heat per unit area passing through the unit area per unit time under a unit temperature gradient and has the units of W/(m K). The thermal diffusivity indicates the ability of the material to align within the temperature gradient.