Piezoelectric ceramic applied to clock crystal and preparation method thereof, resonator

Abstract

This invention discloses a piezoelectric ceramic for use in clock crystals, its preparation method, and a resonator. The chemical formula of the piezoelectric ceramic is: [formula omitted for brevity]. Wherein, A is composed of Pb, Sr, and Ba; B is a perovskite structure composed of Ti, Zr, and Hf; D is a dopant element composed of M1, M2, and M3, where M1 is a high-valence rare-earth ion, M2 is an acceptor ion, and M3 is a variable-valence ion. This invention achieves synergistic optimization of core-shell structure and performance for specific vibration modes required for clock applications by using specific A / B site element composition and the chemical formula and proportion range of the "M1 (rare earth) + M2 (acceptor) + M3 (variable-valence)" ternary composite doping system.

Description

Technical Field

[0001] This invention relates to the field of crystal oscillator technology, and in particular to a piezoelectric ceramic used in clock crystals, its preparation method, and a resonator. Background Technology

[0002] Piezoelectric ceramics refer to polycrystalline materials formed by high-temperature sintering of mixed oxides (zirconia, lead oxide, titanium oxide, etc.) and solid-state reaction, and then ferroelectric ceramics that are subjected to DC high-voltage polarization treatment to give them a piezoelectric effect.

[0003] In fields such as 5G communication, the Internet of Things (IoT), wearable devices, and automotive electronics, clock crystal oscillators (such as XO and TCXO) serve as the heart of the system, and miniaturization, high reliability, and low cost have become core requirements. Currently, the mainstream market still uses quartz crystal resonators, which rely on physical cutting (such as AT-cut or tuning fork) to obtain specific frequency and temperature characteristics. Although quartz has the advantages of high quality factor (Q value) and excellent frequency stability, its inherent defects are becoming increasingly prominent: Miniaturization bottleneck: In pursuit of smaller packages (such as 2016, 1610 and even smaller), quartz wafers need to be ground to extremely thin (tens of micrometers), resulting in a sharp drop in mechanical strength, fragility, low processing yield, and high cost.

[0004] Reliability challenges: Quartz is brittle and easily causes instantaneous frequency jumps or permanent damage when subjected to mechanical shocks or vibrations, making it difficult to meet the high reliability requirements of automotive-grade materials.

[0005] Integration difficulties: Quartz crystals are single-crystal materials, and their processing technology (cutting, grinding, frequency modulation) is incompatible with standard semiconductor or ceramic component manufacturing processes, making it difficult to achieve true monolithic integration or embedded packaging.

[0006] Piezoelectric ceramics (such as lead zirconate titanate (PZT)-based materials) possess potential advantages such as high electromechanical coupling coefficients, ease of molding, co-fired integration, and good mechanical strength, and are considered candidate materials to replace quartz. However, conventional piezoelectric ceramics face fundamental challenges when applied to clock crystals: their resonant frequencies are far more sensitive to temperature, time (aging), and drive level than quartz. Specifically: Poor temperature stability: It lacks a zero temperature coefficient cut similar to that of quartz AT cut, and the frequency temperature curve (Δf / fvs. T) is usually parabolic. The inflection point temperature is difficult to control precisely to near room temperature, and the linearity is poor.

[0007] High aging rate: Factors such as unstable internal domain structure and ion migration cause the resonant frequency to drift significantly over time, which cannot meet the stringent requirements for long-term clock stability.

[0008] High frequency loss: In the MHz and above frequency bands, the mechanical quality factor (Qm) of traditional piezoelectric ceramics is often insufficient, resulting in deterioration of phase noise performance.

[0009] Existing technologies (such as CN109231569A and JP2019124263A) mostly focus on improving the piezoelectric constant (d33) or dielectric constant through single or simple composite doping, but few solutions can systematically solve the contradiction between high Q value, near-zero temperature coefficient (Tf), and low aging rate, especially for microstructure design and performance synergistic optimization for specific vibration modes (low-frequency bending or high-frequency shearing) required for clock applications.

[0010] Therefore, developing a special piezoelectric ceramic material that combines the stability of quartz with the advantages of ceramic processing is of great strategic significance for breaking the foreign monopoly on quartz technology and promoting the upgrading of the clock module industry. Summary of the Invention

[0011] The technical problem to be solved by the embodiments of the present invention is to provide a piezoelectric ceramic for use in clock crystals, a method for its preparation, and a resonator, so that it can be applied to clock crystals and improve the performance of clock crystals.

[0012] To address the aforementioned technical problems, this invention proposes a piezoelectric ceramic for use in clock crystals, with the following general chemical formula: ; In this composition, A is composed of Pb, Sr, and Ba; B is a perovskite structure consisting of Ti, Zr, and Hf; D is a dopant element composed of M1, M2, and M3, where M1 is a high-valence rare-earth ion, M2 is an acceptor ion, and M3 is a variable-valence ion; the molar ratio of A, B, and D is (1-α-β-γ):( ):η; The molar ratio of Pb, Sr, and Ba in A is α:α:γ, the molar ratio of Ti, Zr, and Hf in B is δ:ε:ζ, and the molar ratio of M1, M2, and M3 in D is x:y:z; Among them, 0.88≤α≤0.98, 0.01≤β≤0.08, 0.005≤γ≤0.04, and α+β+γ≤1; 0.40≤δ≤0.52, 0.45≤ε≤0.55, 0≤ζ≤0.05, and δ+ε+ζ<1; 0.01≤η≤0.06.

[0013] Furthermore, 1≤x≤3, 0.5≤y≤2, 0.1≤z≤1.

[0014] Furthermore, M1 is La 3+ 、Sm 3+ 、Nd 3+ One or more of them, M2 is Mn 2+ / 3+ Fe 2+ / 3+ Mg 2+ One or more of them, M3 is Ce 3+ / 4+ Bi 3+ / 5+ One or more of them.

[0015] Accordingly, embodiments of the present invention also provide a method for preparing piezoelectric ceramics for use in clock crystals, comprising: Ingredient preparation and ball milling steps: Using Pb3O4, SrCO3, BaCO3, TiO2, ZrO2 and HfO2 as raw materials, mix them according to the general chemical formula ratio, add M1, and use wet ball milling to mix them evenly. Pre-calcination step: Pre-calcination is then performed to synthesize perovskite phase main crystal powder; Secondary doping and grinding steps: Add M2 and M3 according to the chemical formula ratio and perform secondary ball milling; Molding steps: Prepare a green body of the desired shape; Segmented sintering steps: heat to T1 and hold to achieve densification and form crystal nuclei; then rapidly cool to T2 and hold to form a stable core-shell structure; Electrode preparation and polarization steps: Polarization treatment is performed by applying a DC electric field.

[0016] Furthermore, in the pre-firing step, the product is pre-fired at 800℃-950℃ for 2-4 hours.

[0017] Furthermore, in the segmented sintering step, the temperature T1 ranges from 1150℃ to 1250℃, and the temperature is maintained at T1 for 0.5-2 hours.

[0018] Furthermore, in the segmented sintering step, the temperature T2 ranges from 950℃ to 1050℃, and the temperature is maintained at T2 for 5-15 hours.

[0019] Furthermore, in the electrode preparation and polarization steps, polarization is performed under conditions higher than the operating temperature but lower than the Curie temperature.

[0020] Furthermore, in the molding step, a green body of the desired shape is prepared by casting or dry pressing.

[0021] Accordingly, this invention also provides a resonator, which is prepared using the piezoelectric ceramics used in clock crystals as described above.

[0022] The beneficial effects of this invention are as follows: 1. The present invention has excellent frequency temperature stability: Through A / B site composite substitution and core-shell structure design, the present invention effectively smooths the frequency-temperature curve, and optimizes the frequency temperature coefficient (Tf) of the resonator to within ±10 ppm / ℃ in the range of -40℃ to +85℃, which is close to the level of quartz AT cutting.

[0023] 2. This invention features ultra-high Q-value and low aging rate: The "hard" shell layer in the core-shell structure of this invention effectively suppresses the irreversible movement of domain walls, resulting in a material mechanical quality factor Qm > 2500. The loaded Q-value of the resonator can reach over 100,000 at 32.768 kHz and over 80,000 at 10 MHz. Simultaneously, stable grain boundaries and suppressed ion migration result in an annual aging rate of less than ±3 ppm.

[0024] 3. The invention has excellent mechanical reliability: The inherent high strength of the piezoelectric ceramic of the invention gives it a bending strength of over 150 MPa, which is much higher than that of quartz (about 50 MPa). The resonator made from it can withstand mechanical impacts of over 5000g, making it very suitable for automotive and harsh environment applications.

[0025] 4. The present invention has good process compatibility and cost advantages: The materials of the present invention can be prepared using standard ceramic powder processes and co-firing technology, which can easily realize the manufacturing of multi-layer structures, irregular structures (such as tuning forks) and integrated manufacturing with electrodes, providing the possibility for ultra-miniaturization (such as 1008 package) and SiP integration, and has the cost potential for large-scale production. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the core-shell structure of piezoelectric ceramic grains used in clock crystals according to an embodiment of the present invention.

[0027] Figure 2 This is a schematic diagram of a 32.768kHz miniature tuning fork resonator structure fabricated using piezoelectric ceramics for clock crystals according to an embodiment of the present invention.

[0028] Explanation of icon numbers 1. Free-moving electric domains; 2. Domain movement restricted; 3. Core; 4. Outer shell; 5. Cutting head; 6. Metal cover plate; 7. Printed silver electrode; 8. Ceramic tuning fork oscillator; 9. Ceramic encapsulation base. Detailed Implementation

[0029] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0030] In this embodiment of the invention, directional indicators (such as up, down, left, right, front, back, etc.) are only used to explain the relative positional relationship and movement of each component in a specific posture (as shown in the figure). If the specific posture changes, the directional indicator will also change accordingly.

[0031] Furthermore, in this invention, descriptions involving "first," "second," etc., are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features.

[0032] The chemical formula of the piezoelectric ceramic used in this invention embodiment for clock crystals is: ; In this structure, site A is composed of Pb, Sr, and Ba, with Pb as the matrix and Sr and Ba partially substituted to adjust the Curie temperature (Tc) and lower the sintering temperature. Site B is a perovskite structure composed of Ti, Zr, and Hf, the proportion of which determines the phase boundary position and affects the piezoelectric activity. D is a dopant element composed of M1, M2, and M3, where M1 is a high-valence rare earth ion, M2 is an acceptor ion, and M3 is a variable-valence ion; the molar ratio of A, B, and D is (1-α-β-γ):( ):η; The molar ratio of Pb, Sr, and Ba in A is α:α:γ, the molar ratio of Ti, Zr, and Hf in B is δ:ε:ζ, and the molar ratio of M1, M2, and M3 in D is x:y:z.

[0033] Wherein, 0.88≤α≤0.98, 0.01≤β≤0.08, 0.005≤γ≤0.04, and α+β+γ≤1; 0.40≤δ≤0.52, 0.45≤ε≤0.55, 0≤ζ≤0.05, and δ+ε+ζ< 1. The total doping amount η satisfies: 0.01≤η≤0.06.

[0034] 1≤x≤3, 0.5≤y≤2, 0.1≤z≤1. That is, the molar ratio of x:y:z is in the range of (1-3):(0.5-2):(0.1-1).

[0035] In one implementation, M1 is La 3+ 、Sm 3+ 、Nd 3+ One or more of these compounds are used to introduce "soft" properties, improve piezoelectric response, and refine grain size. M2 is Mn. 2+ / 3+ Fe 2+ / 3+ Mg 2+One or more of these compounds function to generate oxygen vacancies, pin domain walls, significantly improve the mechanical quality factor Qm, and reduce dielectric loss. M3 is Ce 3+ / 4+ Bi 3+ / 5+ One or more of these components function to compensate for charge, stabilize the crystal lattice, and suppress lead volatilization during high-temperature sintering, thereby significantly improving aging characteristics.

[0036] This invention achieves synergistic optimization of core-shell structure and performance through specific A / B site element composition and the general chemical formula and ratio range of the "M1 (rare earth) + M2 (acceptor) + M3 (variable valence)" ternary composite doping system. Furthermore, this invention controls the formation of a core-shell structure in ceramic grains through specific sintering processes. Shell: Rich in doped ions (especially M2 and M3), with large lattice distortion, which plays a role in suppressing domain movement, reducing losses, and stabilizing frequency.

[0037] Core layer: relatively pure, highly piezoelectric, responsible for providing the main electromechanical coupling.

[0038] This "hard shell and soft core" microstructure is key to achieving high Q value and good temperature stability.

[0039] The application of this invention in piezoelectric ceramic single grains of clock crystals, such as Figure 1 As shown, the center is the core 3, within which are freely moving electric domains 1; the core 3 has high piezoelectric activity, and the outer shell 4 is a shell rich in acceptors and variable valence ions, which plays a role in suppressing domain movement and stabilizing performance. The outer periphery of the outer shell 4 is a cutting head. The core-shell structure of this invention achieves high Q value, low temperature drift, and low aging.

[0040] The application of this invention in the preparation method of piezoelectric ceramics for clock crystals includes: Ingredient preparation and ball milling steps: Using Pb3O4, SrCO3, BaCO3, TiO2, ZrO2 and HfO2 as raw materials, mix them according to the general chemical formula ratio, add M1, and use wet ball milling to mix them evenly. Pre-calcination step: Pre-calcinate at 800-950℃ for 2-4 hours to synthesize perovskite phase main crystal powder.

[0041] Secondary doping and grinding steps: Add the corresponding mass of M2 and M3 according to the general chemical formula ratio and perform secondary ball milling to ensure that the doping elements mainly agglomerate at the grain boundaries and shell.

[0042] Molding steps: Based on the powder obtained from secondary ball milling, green blanks of the required shape (such as tuning fork plates, rectangular sheets) are prepared by casting or dry pressing.

[0043] Segmented sintering steps: First stage: Heat to 1150-1250℃ (T1) at a relatively rapid rate, hold for a short time (0.5-2 hours) to achieve densification and form crystal nuclei; The second stage: rapidly cool down to 950-1050℃ (T2) and hold at that temperature for a long time (5-15 hours) to promote the diffusion of dopants to the grain boundaries and form a stable core-shell structure; Electrode preparation and polarization steps: The silver-coated or sputtered electrode is polarized by applying a DC electric field at a temperature higher than the device operating temperature (e.g., 150°C) but lower than the Curie temperature.

[0044] The resonator in this embodiment of the invention is made using piezoelectric ceramics used in clock crystals. The resonator made in this embodiment of the invention can achieve a variety of performance threshold combinations, such as: within a specified frequency and temperature range, simultaneously satisfying Q value > a specific value (e.g., 80,000), frequency-temperature deviation < ±20 ppm, and annual aging rate < ±5 ppm.

[0045] Clock crystal resonators (including but not limited to tuning fork type and thickness shear type) made of piezoelectric ceramics according to embodiments of the present invention can be applied to oscillator circuits, wearable devices, automotive electronics, communication modules, etc.

[0046] Example 1: The structure of the 32.768kHz miniature tuning fork resonator of this embodiment of the invention is as follows. Figure 2 As shown, it consists of a metal cover plate 6, printed silver electrodes 7, a ceramic tuning fork oscillator 8, and a ceramic encapsulation base 9. The ceramic tuning fork oscillator 8 uses piezoelectric ceramics with the general chemical formula of which is employed in clock crystals: (Pb 0.94 Sr 0.04 Ba 0.02 (Ti) 0.48 Zr 0.50 Hf 0.02 ) 0.975 (La 0.015 Mn 0.008 Ce 0.002 )O3.

[0047] Process: A 100μm thick sheet is prepared by tape casting and then laser-cut into a standard tuning fork shape. Segmented sintering is then performed: holding at 1250℃ for 1 hour → holding at 1000℃ for 10 hours. This yields a tuning fork resonator.

[0048] Performance: The manufactured tuning fork resonator has a frequency of 32.768 kHz, a Q value >120,000, a frequency deviation Δf / f < ±15 ppm between -40℃ and +85℃, and a frequency drift < ±2 ppm after baking at 85℃ for 1000 hours.

[0049] Example 2: The 26MHz thickness shear mode resonator uses piezoelectric ceramics with the general chemical formula of which are applied in clock crystals: (Pb 0.91 Sr 0.06 Ba 0.03 (Ti) 0.52 Zr 0.46 Hf 0.02 ) 0.96 (Sm 0.02 Fe 0.01 Bi 0.01 )O3.

[0050] Process: Dry pressing to form Φ10mm×0.2mm circular wafers, coated with silver electrodes. Segmented sintering: holding at 1180℃ for 1.5h → holding at 980℃ for 8h. Polarization along the thickness direction to prepare the resonator.

[0051] Performance: Resonant frequency 26.0 MHz, Q value > 85,000, frequency temperature characteristic curve is approximately linear from 0℃ to 70℃ with a slope of about -2 ppm / ℃, and overall deviation ±20 ppm.

[0052] Example 3: The high-frequency, high-stability resonator uses piezoelectric ceramics applied in clock crystals, whose general chemical formula is: (Pb 0.89 Sr 0.08 Ba 0.03 (Ti) 0.45 Zr 0.52 Hf 0.03 ) 0.95 (Nd 0.025 Mg 0.01 Ce 0.005 )O3.

[0053] Process and Performance: 50-100MHz fundamental frequency resonators can be fabricated by adjusting the casting thickness and electrode size. Expected Q value > 50,000, and aging rate excellent ±5 ppm / year.

[0054] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A piezoelectric ceramic used in clock crystals, characterized in that, Its general chemical formula is: ； In this composition, A is composed of Pb, Sr, and Ba; B is a perovskite structure consisting of Ti, Zr, and Hf; D is a dopant element composed of M1, M2, and M3, where M1 is a high-valence rare-earth ion, M2 is an acceptor ion, and M3 is a variable-valence ion; the molar ratio of A, B, and D is (1-α-β-γ):( ):η; The molar ratio of Pb, Sr, and Ba in A is α:α:γ, the molar ratio of Ti, Zr, and Hf in B is δ:ε:ζ, and the molar ratio of M1, M2, and M3 in D is x:y:z; Among them, 0.88≤α≤0.98, 0.01≤β≤0.08, 0.005≤γ≤0.04, and α+β+γ≤1; 0.40≤δ≤0.52, 0.45≤ε≤0.55, 0≤ζ≤0.05, and δ+ε+ζ<1; 0.01≤η≤0.

06.

2. The piezoelectric ceramic used in clock crystals as described in claim 1, characterized in that, 1≤x≤3, 0.5≤y≤2, 0.1≤z≤1.

3. The piezoelectric ceramic used in clock crystals as described in claim 1, characterized in that, M1 is La 3+ 、Sm 3+ 、Nd 3+ One or more of them, M2 is Mn 2+ / 3+ Fe 2+ / 3+ Mg 2+ One or more of them, M3 is Ce 3+ / 4+ Bi 3+ / 5+ One or more of them.

4. The method for preparing piezoelectric ceramics for use in clock crystals as described in any one of claims 1-3, characterized in that, include: Ingredient preparation and ball milling steps: Using Pb3O4, SrCO3, BaCO3, TiO2, ZrO2 and HfO2 as raw materials, mix them according to the general chemical formula ratio, add M1, and use wet ball milling to mix them evenly. Pre-calcination step: Pre-calcination is then performed to synthesize perovskite phase main crystal powder; Secondary doping and grinding steps: Add M2 and M3 according to the chemical formula ratio and perform secondary ball milling; Molding steps: Prepare a green body of the desired shape; Segmented sintering steps: heat to T1 and hold to achieve densification and form crystal nuclei; then rapidly cool to T2 and hold to form a stable core-shell structure; Electrode preparation and polarization steps: Polarization treatment is performed by applying a DC electric field.

5. The method for preparing piezoelectric ceramics for use in clock crystals as described in claim 4, characterized in that, During the pre-firing step, pre-fire at 800℃-950℃ for 2-4 hours.

6. The method for preparing piezoelectric ceramics for use in clock crystals as described in claim 4, characterized in that, In the segmented sintering step, T1 ranges from 1150℃ to 1250℃, and the temperature is held at T1 for 0.5-2 hours.

7. The method for preparing piezoelectric ceramics for use in clock crystals as described in claim 4, characterized in that, In the segmented sintering process, the temperature T2 ranges from 950℃ to 1050℃, and the temperature is maintained at T2 for 5-15 hours.

8. The method for preparing piezoelectric ceramics for use in clock crystals as described in claim 4, characterized in that, In the electrode preparation and polarization steps, polarization is performed under conditions higher than the operating temperature but lower than the Curie temperature.

9. The method for preparing piezoelectric ceramics for use in clock crystals as described in claim 4, characterized in that, In the molding process, the green body of the desired shape is prepared by tape casting or dry pressing.

10. A resonator, characterized in that, It is prepared using the piezoelectric ceramics used in clock crystals as described in any one of claims 1-3.

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