Lots of materials known to show positive thermal expansion as temperature increase. In contrast, some materials show completely different thermal expansion properties and contract upon heating. This negative thermal expansion (NTE) phenomena has been found in some A₂(MO₄)₃ compounds, where the A cation can be a trivalent main group metal, transition metal, or rare earth element ranging from Lu to Ho, while M corresponds to W or Mo (Sumithra and Umarji, 2004, 2006; Liu H. F. et al., 2012; Liu Q. Q. et al., 2012; Liu et al., 2015). In addition, compounds with aliovalent cations on the A and M site have been reported. For example, Zr₂WP₂O₁₂ has been reported to exhibit strong and stable NTE over a wide temperature range. Zr₂WP₂O₁₂ adopts the orthorhombic Sc₂W₃O₁₂ structure, which consists of ZrO₆ octahedra that share corners with two WO₄ tetrahedra and four PO₄ tetrahedra. Zr-O-W (P) linkages in this structure will lead to the volume contraction due to transverse vibration of bridging oxygen atoms as temperature increase (Isobe et al., 2008, 2009; Cetinkol and Wilkinson, 2009; Tani et al., 2010).

Thermal expansion is an important property of materials, and mismatch in thermal expansion often induces unstable performance or failure of devices in the field of microelectronics, optics and micromachines. To avoid the above problems, control of thermal expansion of materials can be necessary. An easy approach is to mix the NTE material with the positive thermal expansion material in the right proportion.

Most studies describing attempts to synthesize controllable thermal expansion composites mainly focus on ZrW₂O₈ based composites, such as ZrW₂O₈/ZrO₂ (De Buysser et al., 2004; Lommens et al., 2005; Yang et al., 2007; Khazeni et al., 2011; Romao et al., 2015), ZrW₂O₈/Cu and ZrW₂O₈/polyimide (Yilmaz, 2002; Sullivan and Lukehart, 2005; Yang et al., 2010; Hu et al., 2014). The coefficient of thermal expansion (CTE) of the composites drops with the increase of the ZrW₂O₈ filler. However, the cubic NTE phase of ZrW₂O₈ is metastable at room temperature, and has to be prepared by rapid quenching after sintering at 1,200°C. Cubic ZrW₂O₈ show a isotropic NTE over a wide temperature range, but a phase transition from α -ZrW₂O₈ to β -ZrW₂O₈ occurs around 160°C, which leads to the decrease of CTE. This change in thermal expansion may be disadvantageous for composite design. Moreover, when heated to 740°C, ZrW₂O₈ decomposes into ZrO₂ and WO₃ (Mary et al., 1996; Banek et al., 2010; Gao et al., 2016). In addition, cubic ZrW₂O₈ undergoes a pressure induced phase transition to an orthorhombic phase with a positive CTE. This transformation has been observed in composites during thermal cycling, and leads to irreproducible thermal expansion behavior (Perottoni and Jornada, 1998; Miao et al., 2004; Varga et al., 2007; Liu et al., 2014).

Zr₂WP₂O₁₂ is a new NTE material for use as a filler to adjust the CTE of ceramics, glasses, metals, and polymers. It exhibits a strong NTE over the broadest temperature range (room temperature to its sublimation point of about 1,600°C). Moreover, it does not suffer from the same limitations as ZrW₂O₈, as it is thermodynamically stable and does not undergo any phase transformations.

The synthesis and NTE behavior of Zr₂WP₂O₁₂ ceramics have been reported previously (Isobe et al., 2008, 2009; Cetinkol and Wilkinson, 2009; Tani et al., 2010). Zr₂WP₂O₁₂ ceramics show stable NTE with an average linear CET of about −5 × 10⁻⁶ K⁻¹. In addition, the Zr₂WP₂O₁₂ ceramics display excellent mechanical properties (Isobe et al., 2008, 2009; Cetinkol and Wilkinson, 2009). ZrO₂ ceramics and fibers has been widely used in optics, electronics and high temperature fields (Lommens et al., 2005; Yang et al., 2007). In some special occasions, ZrO₂ need to keep precision dimensional stability with the change in temperature, because a mismatch in size among different precision devices can cause some problems. The average linear CTE of ZrO₂ is about 10 × 10⁻⁶ K⁻¹ from room temperature to 1,000°C. The absolute values of the CTE of ZrO₂ and Zr₂WP₂O₁₂ are thus similar but have opposite signs, suggesting that these materials are good candidates for the preparation of ceramic composites with tunable CTEs. It is beneficial that ZrO₂ does not react with Zr₂WP₂O₁₂ at high temperatures, as it is a starting material in the solid state synthesis of Zr₂WP₂O₁₂.

A new series of Zr₂WP₂O₁₂/ZrO₂ ceramic composites that are expected to show an adjustable CTE were synthesized by a solid state reaction method. This work is devoted to exploring the effects of mass ratio of Zr₂WP₂O₁₂ and ZrO₂ on the microstructure, density, and CTE values of the Zr₂WP₂O₁₂/ZrO₂ ceramic composites.

All Zr₂WP₂O₁₂, ZrO₂, and Zr₂WP₂O₁₂/ZrO₂ ceramics (mass ratios: 1:1, 2:1, 3:1, 4:1) were synthesized through a conventional solid state route. The raw materials were ZrO₂ (Aladdin, purity ≥99.95%), WO₃ (Aladdin, purity ≥99.95%), and NH₄H₂PO₄ powders (Aladdin, purity ≥99.5%). A summary of samples prepared can be found in Table 1. Reactant mixtures were milled for 6 h to form a homogeneous powder and dried at 80°C, followed by heating at 500°C for 3 h. After this pre-sintering step, the mixtures were uni-axially cold pressed into pellets of 7 mm in diameter and about 2 mm in thickness. Pellets were calcined at 1,200°C in air for 6 h and cooled down in the furnace.

Powder X-ray diffraction experiments were performed on a Shimadzu XRD 7000 using CuKα radiation. Data were collected at 40 kV and 30 mA over the 10° to 60° 2θ range with a scanning speed of 5°/min. The fractured surface morphologies of the samples were observed using a TESCAN VEGA3 scanning electron microscope (SEM). The relative densities of the resulting samples were measured using Archimedes' method. The CTEs of the samples were measured with a Seiko 6300 TMA/SS thermal mechanical analyzer at a heating rate of 5°C/min in air between 25 and 700°C.

Zr₂WP₂O₁₂/ZrO₂ ceramic composites with adjustable thermal expansion coefficients were successfully fabricated by a solid state reaction method. The composites consisted of orthorhombic Zr₂WP₂O₁₂ and monoclinic ZrO₂ with no intermediate phases observed. With increasing amount of Zr₂WP₂O₁₂, the relative densities of the Zr₂WP₂O₁₂/ZrO₂ ceramic composites increased gradually. The CTE of the Zr₂WP₂O₁₂/ZrO₂ composites can be tailored from 4.1 × 10⁻⁶ K⁻¹ to −3.3 × 10⁻⁶ K⁻¹ by changing the weight fraction of Zr₂WP₂O₁₂. For a mass ratio of Zr₂WP₂O₁₂/ZrO₂ of 2:1, the Zr₂WP₂O₁₂/ZrO₂ ceramic composite showed close to zero thermal expansion with an average linear CTE of −0.09 × 10⁻⁶ K⁻¹ between 25 and 700°C.

HL, XC, and ZZ designed experiments; WS and GX carried out experiments; HL, ZZ, and XZ analyzed experimental results and wrote the manuscript.