Preparation method of aluminum-containing oxalate piezoelectric crystal

Abstract

The invention provides a preparation method of an aluminum-containing oxalate piezo-electric crystal. The chemical formula of the aluminum-containing oxalate piezo-electric crystal is NaBaAl (CO). 5HO, the molecular weight is 541.45, the aluminum-containing oxalate piezo-electric crystal is crystallized in an orthorhombic system polar space group Pca2 (No.29), and the cell parameters are as follows: a = 19.0829 (4), b = 10.8321 (3), c = 16.4577 (4), alpha = beta = gamma = 90 degrees, V = 3401.94 (14) 3 and Z = 8. The aluminum-containing oxalate crystal realizes a non-centrosymmetric structure so as to have piezoelectric response, d33 of the aluminum-containing oxalate crystal is obviously higher than that of quartz, meanwhile, the problem of environmental pollution of a Pb-based material is avoided, and a feasible solution is provided for developing a novel green and high-performance piezoelectric material.

Description

Technical Field

[0001] This invention pertains to the synthesis of inorganic-organic hybrid crystalline materials, and relates to the preparation of an aluminum oxalate piezoelectric crystal. Background Technology

[0002] Piezoelectric materials, as important functional materials capable of bidirectional conversion between mechanical and electrical energy, are widely used in sensors, actuators, energy harvesting, and acoustic devices. Due to limitations of traditional power sources in miniaturization, wearable applications, and special environments, there is an urgent need to rely on piezoelectric crystals to convert mechanical energy in the environment into electrical energy to meet the needs of different application scenarios. Piezoelectric crystals can generate electric polarization effects under external forces, thereby achieving force-to-electricity conversion. This effect is widely used in ultrasonic detection, precision actuation, and vibration energy harvesting, providing crucial support for the development of defense equipment, information technology, and medical devices. Examples include high-sensitivity detection elements in underwater sonar systems, self-powered sensors in aerospace structures, and miniature energy generating units in wearable health monitoring devices. Therefore, designing and synthesizing novel piezoelectric crystals with independent intellectual property rights and excellent comprehensive performance is of great significance for promoting the development of sensing technology, energy harvesting, and related industries in my country.

[0003] The development of existing piezoelectric materials has roughly gone through the following stages: 1. Natural crystal piezoelectric materials: Represented by natural mineral crystals such as quartz (SiO2) and tourmaline, these materials possess good chemical and thermal stability, but their piezoelectric constants are relatively low (e.g., quartz is approximately 2.3 pC / N), making it difficult to meet the requirements of high-sensitivity applications. 2. Piezoelectric ceramic materials: Ferroelectric ceramics, represented by lead zirconate titanate (PZT), possess high piezoelectric properties (d... 33 Lead-containing piezoelectric materials, with a lead content of up to 300–600 pC / N, have long been the mainstream in applications. However, due to the serious environmental pollution caused by their lead content, lead-free materials have become the core direction of piezoelectric material development in recent years. Lead-free piezoelectric ceramics such as sodium potassium niobate (KNN) and bismuth layered compounds have potential, but their preparation is complex and their performance is not uniform. 3. Artificial crystal piezoelectric materials: Compared with ceramic piezoelectric materials, artificial crystal piezoelectric materials have uniform and controllable performance, good high-frequency and high-temperature stability, low loss, high sensitivity, and directional polarization can achieve the best piezoelectric response, making them particularly suitable for high-precision, miniaturized, and high-reliability applications. Since the 20th century, artificially synthesized crystals such as BaTiO3 (high piezoelectric constant, about 190-300 pC / N), LiNbO3 (6–20 pC / N), and potassium dihydrogen phosphate (7–10 pC / N) have been widely studied and applied in optical devices, ultrasonic transducers, and frequency control devices. The piezoelectric properties of these materials are superior to those of natural crystals, but some have problems with harsh preparation conditions or insufficient temperature stability.

[0004] To meet the demands for lead-free, environmentally friendly, and high-temperature stability, researchers have begun exploring aluminum-containing piezoelectric materials. Representative advances include: 1. Aluminum nitride (AlN): With a wurtzite structure (space group P63mc, point group 6mm), it is a typical lead-free piezoelectric material. Its piezoelectric constant d0... 33 Thin films, typically in the range of 5–7 pC / N, are widely used in microelectromechanical systems (MEMS). Doping (e.g., Sc, Cr) can significantly enhance their piezoelectric response. 2. Calcium aluminum silicate (Ca2Al2SiO7, CAS): As a typical aluminosilicate crystal, it has been reported to exhibit a significant piezoelectric effect. Its d 14 ≈6.04pC / N, d 36 It has a strength of approximately 4.04 pC / N and maintains good stability under high temperature conditions, showing potential for high-temperature piezoelectric applications.

[0005] Currently used piezoelectric materials have certain limitations. For example, although quartz is widely used in sensing and frequency control, its longitudinal piezoelectric coefficient d... 11 With a pC / N ratio of only about 2.3, far lower than the 65 pC / N exhibited by the crystal in this application, it is difficult to meet the requirements for high-sensitivity and high-response piezoelectric devices. On the other hand, although traditional ceramic piezoelectric materials (such as the PZT system) have high piezoelectric performance, they contain a large amount of Pb, posing serious environmental and health risks and failing to meet the requirements of green and sustainable development. Therefore, how to ensure high piezoelectric performance while avoiding the introduction of harmful elements is a crucial problem that urgently needs to be solved in the field of piezoelectric materials. Summary of the Invention

[0006] Currently, research on aluminum-containing piezoelectric materials mainly focuses on inorganic salt systems. While these materials possess certain piezoelectric properties, they suffer from limitations such as single crystal types, limited functional tunability, and the lack of organic-inorganic hybrid structures. In particular, organic ligand systems have not been systematically explored in aluminum-containing piezoelectric crystals, representing a significant technological gap. Oxalate (C₂O₄²⁻), as a typical polydentate ligand, can form a stable coordination framework with aluminum ions. Its structural characteristics provide feasibility for the design of novel piezoelectric crystals: 1. Oxalate, through polydentate coordination, can guide aluminum ions to form one-dimensional, two-dimensional, or three-dimensional framework structures. By rationally designing crystal growth conditions, non-centrosymmetric crystals can be obtained, which is a structural prerequisite for realizing the piezoelectric effect. 2. In the oxalate coordination network, units such as Al–O–C–C–O–Al can undergo slight distortions under external forces, achieving reversible changes in the internal dipole moment of the crystal, thereby generating a piezoelectric response. Furthermore, introducing cations or adjusting the crystal arrangement can further optimize the polarization direction, which is beneficial for improving the piezoelectric constant. Therefore, this invention systematically carried out the preparation and synthesis of novel aluminum-containing artificial piezoelectric crystals with oxalate ligands, and successfully obtained NaBaAl(C2O4)3·5H2O crystals. These crystals crystallize in the polar space group Pca21, exhibiting strong piezoelectric activity, and their room temperature d... 33 The value is approximately 68 pC / N, which is more than an order of magnitude higher than that of previously reported aluminum-containing piezoelectric materials such as AlN and Ca2Al2SiO7.

[0007] The purpose of this invention is to provide a method for preparing an aluminum oxalate piezoelectric crystal. This aluminum oxalate crystal not only achieves a non-centrosymmetric structure, thus possessing a piezoelectric response, but also its d... 33 It is significantly superior to quartz, while avoiding the environmental pollution problems of Pb-based materials, providing a feasible solution for developing new green and high-performance piezoelectric materials.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] The first item this proposal seeks to protect is a novel crystal, an aluminum oxalate piezoelectric crystal with the chemical formula NaBaAl(C2O4)3·5H2O, a molecular weight of 541.45, crystallizing in the orthorhombic polar space group Pca21 (No. 29), with unit cell parameters of...

[0010] α=β=γ=90°, Z = 8. The piezoelectricity d of the aluminum oxalate piezoelectric crystal at room temperature. 33 It is approximately 68 pC / N.

[0011] The second aspect of this proposal is to protect the preparation method and steps of the NaBaAl(C2O4)3·5H2O piezoelectric crystal, which is prepared by a hydrothermal method. Specifically, it includes the following steps: sodium oxalate, aluminum nitrate nonahydrate, and barium chloride are uniformly dispersed in deionized water in a polytetrafluoroethylene-lined hydrothermal reactor at a stoichiometric ratio of 3:1:1, and then placed in an oven for reaction at a temperature of 160°C.

[0012] The third aspect of this proposal is to protect the application of the NaBaAl(C2O4)3·5H2O piezoelectric crystal in the fabrication of piezoelectric devices.

[0013] The advantages of this invention are:

[0014] Compared with existing oxalate crystals, which are mostly centrosymmetric and lack piezoelectric effect, and non-centrosymmetric materials such as AlN thin films, which have limited piezoelectric properties, the NaBaAl(C2O4)3·5H2O crystal proposed in this application has the following significant advantages: it crystallizes in the polar space group Pca21, which structurally ensures piezoelectric activity; its room temperature piezoelectric constant d 33 It can reach approximately 68 pC / N, significantly higher than the 3-7 pC / N of AlN films. Meanwhile, compared to typical aluminosilicate crystals such as calcium aluminosilicate (Ca2Al2SiO7), its piezoelectric constant is only d... 14 ≈6.04pC / N, d 36 With a piezoelectricity of approximately 4.04 pC / N, the crystal in this application still represents an order-of-magnitude improvement. Furthermore, as a single-crystal material, it exhibits good anisotropic response and controllability in applications. Therefore, the crystal in this application not only overcomes the lack of piezoelectricity in traditional oxalate systems but also overcomes the limitations of insufficient performance and complex processes in existing thin film or ceramic materials, demonstrating significant technological advantages and application prospects. Attached Figure Description

[0015] Figure 1 It is an asymmetric structural unit of NaBaAl(C2O4)3·5H2O crystal;

[0016] Figure 2 It has a two-dimensional Ba[Al(C2O4)3]- anionic layer structure;

[0017] Figure 3 It has a three-dimensional NaBaAl(C2O4)3·5H2O crystal structure;

[0018] Figure 4 The powder XRD pattern and simulated pattern of NaBaAl(C2O4)3·5H2O are shown.

[0019] Figure 5 The ultraviolet diffuse reflectance spectrum of NaBaAl(C2O4)3·5H2O and its fitting using the Tauc method;

[0020] Figure 6 The room-temperature piezoelectric d-value of NaBaAl(C2O4)3·5H2O crystal was measured at 110 Hz. 33 ;

[0021] Figure 7 The d value of NaBaAl(C2O4)3·5H2O crystal was measured at 110 Hz. 33 Curve showing temperature variation. Detailed Implementation

[0022] To make the above features and advantages of the present invention more apparent and understandable, specific embodiments are provided below for detailed description. Unless otherwise specified, the methods of the present invention are conventional methods in the art.

[0023] Example 1

[0024] A method for preparing an aluminum oxalate piezoelectric crystal includes the following steps:

[0025] NaBaAl(C2O4)3·5H2O single crystals were obtained by a hydrothermal method. Al(NO3)3·9H2O (0.375 g, 1 mmol), Na2C2O4 (0.402 g, 3 mmol), and 5 mL of deionized water were added to a 20 mL polytetrafluoroethylene liner and mixed. The mixture was stirred with a glass rod to ensure complete reaction, and allowed to stand for 30 minutes to produce the precursor oxalate (C2O4). 2- Finally, BaCl2 (0.208, 1 mmol) was added to a stainless steel hydrothermal reactor lined with polytetrafluoroethylene and shaken on a digital shaker for 30 minutes to ensure thorough mixing. The reactor was then placed in a temperature-controlled oven and heated from room temperature to 160°C at a rate of 2°C / min for 200 minutes, maintained at 160°C for 50 hours, and finally cooled to room temperature at a rate of 1.32°C / h. The resulting solid product was collected by filtration and thoroughly washed with deionized water. After vacuum filtration and drying, colorless, transparent needle-like NaBaAl(C2O4)3·5H2O single crystals were obtained.

[0026] The NaBaAl(C2O4)3·5H2O crystal obtained in this invention crystallizes in the orthorhombic polar space group Pca21 (No. 29), and is an example of a two-dimensional anionic layer of Ba[Al(C2O4)3]- and filled with Na. + The new compound formed by cation stacking has the following unit cell parameters:

[0027] α=β=γ=90°, In the crystal structure, the asymmetric unit includes two Na atoms. + Two Ba 2+ Two Al3+ And six oxalate ions, all of which occupy general crystallographic positions. Figure 1 Each aluminum center and its coordinated C2O42- anion combine to form a discrete O-D[Al(C2O4)3]3- complex anion, serving as a secondary building block in the structure. Two Ba... 2+ All ions are in an 11-coordination environment, with eight oxygen atoms originating from the oxalic acid ligand of the [Al(C2O4)3]3- secondary building unit, and the remaining three oxygen atoms originating from coordinated water molecules. These shared oxygen atoms link [Al(C2O4)3]3-SBU with Ba 2+ Ion bridging was used to construct a two-dimensional Ba[Al(C2O4)3]- anion layer in the ab plane, and adjacent Ba[Al(C2O4)3]- anion layers were further stacked along the c-axis. Na + Ions reside in interlayer voids to maintain stability. Na1 + The ion adopts a five-coordinated triangular bipyramidal geometry, with the equatorial plane occupied by three coordinated water molecules, while the two axial positions are determined by oxygen atoms bridging each other. Na2 + The ions are positionally disordered, refined into two parts with occupancy sites, Na₂A and Na₂B, with site occupancy coefficients fixed at 0.52 and 0.48, respectively. Overall, Na₂... + The ion exhibits an octet coordination environment, coordinated by four water molecules and four bridging oxygen atoms. Through these bridging oxygen atoms, Na+... + Ions connect adjacent Ba[Al(C2O4)3]- layers into a three-dimensional framework. Figure 3 ).

[0028] The obtained single crystal was ground into powder and then subjected to powder X-ray diffraction (XRD) tests. The results were compared with simulated spectra obtained based on single crystal structure analysis. Figure 4 As shown, the measured spectrum agrees well with the simulation results. Solid-state UV diffuse reflectance spectroscopy testing and fitting analysis show that its band gap is 4.35 eV, classifying it as a direct band gap semiconductor. Figure 5 ).

[0029] The NaBaAl(C2O4)3·5H2O sample was pressed into a sheet with a thickness of 0.5 mm and then polarized at room temperature with an applied electric field of 2.2 kV for 50 minutes. The variable-temperature piezoelectric response was then measured using a Piezotest PM100 system (Piezotest GmbH, Germany) from room temperature to 200 °C. Measurements were performed under a dynamic load of 0.25 N and a frequency of 110 Hz, with data acquisition and analysis automated using PiezoMeter control software. At each temperature, hundreds of response cycles were recorded using a multi-point accumulation method, and the average value was taken as the piezoelectric charge coefficient d. 33The standard deviation was used to estimate the measurement reliability. The temperature was increased at a controlled rate of 3 °C / min to ensure stable and repeatable results. First, the piezoelectric properties of the NaBaAl(C2O4)3·5H2O sample at room temperature (28.2 °C) were evaluated through four independent measurements, yielding d... 33 The values ​​were 61, 68.8, 64.8, and 76.8 pC / N, respectively. Figure 6 The average value was 67.9 pC / N, with a standard deviation of approximately 6.5 pC / N, confirming the good reproducibility and reliability of the measurements. This result indicates that the material exhibits a stable piezoelectric response at room temperature. Subsequently, variable-temperature measurements were performed at 110 Hz, and the results are as follows. Figure 7 As shown. As the temperature increases from 28.2℃ to 200℃, d 33 The coefficient decreases monotonically, dropping below 10 pC / N at approximately 120 °C and approaching zero above 160 °C. This sharp decay can be attributed to enhanced thermal perturbation at high temperatures, which gradually weakens spontaneous polarization and eventually eliminates the piezoelectric effect. Overall, the continuity of the curves and the consistency of the data validate the reliability of the observed piezoelectric signal while clearly revealing its strong temperature dependence.

[0030] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made within the scope of the claims of the present invention should be included in the scope of the present invention.

Claims

1. An aluminous oxalate piezoelectric crystal characterized by, The chemical formula of the aluminum-containing oxalate piezoelectric crystal is NaBaAl(C2O4)3·5H2O, the molecular weight is 541.45, the crystal is in an orthorhombic polar space group Pca21, the cell parameters are a = 19.0829(4) Å, b = 10.8321(3) Å, c = 16.4577(4) Å, α = β = γ = 90°, V = 3401.94(14) Å 3 , Z = 8.

2. The aluminous oxalate piezoelectric crystal of claim 1 wherein, The piezoelectric d 33 is 68 pC / N.

3. The method of claim 1 or 2, wherein the method is characterized by, It comprises the following steps: adding sodium oxalate, aluminum nitrate nonahydrate and barium chloride in a stoichiometric ratio of 3:1:1 into deionized water, mixing uniformly, and putting into an oven to react at a reaction temperature of 160 DEG C.

4. Use of the aluminiferous oxalate piezoelectric crystal according to claim 1 or 2 for the production of a piezoelectric device.

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