摘要
羟基磷灰石(Hydroxyapatite,HA或HAP)涂层材料以其良好的生物相容性和强度,在人工义肢植入、骨骼修复等领域有着广泛的应用。然而传统热喷涂制备方法易产生相变产物,在人体环境中易溶解,造成涂层脱落；化学、电化学法操作复杂,反应条件要求精确,难以推广应用。本课题提出在常温下利用高速粉体气射流喷射制备HA涂层的方法,具有常温喷涂无相变产物、方法简单等特点。本文针对冷喷涂法制备HA涂层对送粉系统微量、均匀送粉的要求,利用超声行波送粉原理开发了一种新型送粉系统。该系统对HA粉体的最小送粉速率约为0.002g/s,最大送粉速率约为0.25g/s。
1.该送粉系统主要由压电陶瓷片、有机玻璃管、驱动电源及混料装置组成。送粉原理为：轴向极化的环形压电陶瓷片受到高频正弦交流电激励时产生超声频段的径向收缩振动。振动沿有机玻璃管壁传播时受到管壁及粉体的吸振作用,能量衰减,从而在管壁上形成衰减行波。此时,有机玻璃管内壁上的质点微观下做椭圆运动,方向与波传递方向相反,从而驱动粉体由料杯向压电片方向移动。从出料口流出的粉体经由混料装置混入高压气流,进入laval喷嘴进行喷射。
2.通过对超声行波送粉过程的理论分析,得出：系统送粉能力主要与驱动电源的输出电压和频率,压电陶瓷片的性能和结构参数,以及有机玻璃管的长度和阻尼系数等因素有关。
3.由环形压电陶瓷片的仿真分析结果得知,径向伸缩模态的谐振频率随内径或外径尺寸的增大而减小,而与厚度的关系较小；当驱动电压一定时,内环质点径向振动的振幅随驱动频率趋近其谐振频率而逐渐增大,并在谐振频率处达到最大值；当驱动频率与谐振频率相等时,内环质点径向振动振幅随驱动电压的升高而增大,且与驱动电压呈线性关系。
4.有机玻璃管的有限元仿真结果表明,在压电陶瓷片谐振频率附近,有机玻璃管有多阶径向收缩振动模态；有机玻璃管的阻尼系数越大,其径向振动振幅沿管壁的衰减越明显；管内壁上各点径向振动的振幅随着与振源距离的增加而逐渐减小
5.利用白行搭建的不同结构参数的送粉系统对羟基磷灰石粉体进行了送粉试验。由单因素对比试验结果得知,送粉速率随着驱动电压的升高而增大,应用时可以通过改变驱动电压调节送粉速率；驱动频率对送粉速率的影响规律曲线与压电陶瓷片的谐响应曲线相似,在压电陶瓷片的谐振频率处系统达到最大送粉速率,且谐振频率右侧频段的曲线线性度好,应用时可以在此频段内通过改变驱动频率调节送粉速率；送粉速率随着输送管送粉段长度的增大而减小；料杯内粉体的高度对送粉速率影响较小；在可输送范围内,HAP粉体的粒度对送粉速率的影响较小。
6.在相同实验条件下,利用该系统对不同种类的粉体进行了送粉试验。试验结果表明,该送粉系统对流动性好,密度大,颗粒表面较粗糙的粉体具有良好的输送能力。
Because of good biocompatibility and strength, HAP (Hydroxyapatite, HA or HAP) is widely applied to some fields such as prosthetic implants and bone repair etc. However, the traditional preparation methods for thermal spraying are easy to produce phase transition matter, which is easy to dissolve in the human body, resulting in coatings flaking. This paper proposed a method for making HA coatings using high-speed gas jet. This method is simple and with no phase transition matter produced. Because powder feeding system needs to meet the requirement of feeding powder with uniform speed and micro quantity when producing HA coatings by cold spray, the paper developed a new powder feeding system based on the theory of feeding powder using ultrasonic wave. In the system, the smallest feeding rate for HA powder is about 0.002g/s, and the maximum rate is about 0.25g/s.
1. The powder feeding system is composed of piezoelectric ceramics, PMMA tube, drive power and the mixing device. The principles for the powder feeding are as follows:the annular piezoelectric ceramics polarized axially generates radial contraction vibration with the frequency of supersonic range when inspired by the high-frequency sinusoidal alternating current. During propagating, vibration wave becomes attenuation traveling wave, because the wall of the PMMA tube and the powder can absorb vibration leading to the energy loss. In this case, the particles on the wall of the PMMA tube moves with oval shape. The moving direction is opposite to the one of wave propagation. So the powder is droved along the direction from the feeding cup to the piezoelectric. The powder fluids out of the discharge hole is mixed high-pressure air in the mixing device and injected through Laval nozzle.
2. Based on the theoretical analysis of the powder process using ultrasonic, some conclusion are obtained. The feeding capacity of the system is mainly related to the voltage and frequency of the drive power, the performance and structural parameters of the piezoelectric ceramics, the length and damping coefficients of the PMMA tube.
3. The simulation results of the annular piezoelectric ceramics show that the resonant frequency of the radial telescopic mode decreases with the increase of the size of the inner or the outer diameter and has a little relationship with the thickness of the annular piezoelectric ceramics. When the driving voltage is constant, the more the drive frequency approaches the resonant frequency, the bigger the amplitude of the radial vibration of the particle on the inner ring becomes. The amplitude reaches the maximum at the resonant frequency. When the driving frequency is equal to the resonant frequency, the amplitude of the radial vibration increases with the rise of the drive voltage.
4. The finite element simulation results of the PMMA tube indicate that the PMMA tube has multiple-order radial contraction vibration modes near the resonant frequency of the piezoelectric ceramic. The attenuation of the amplitude of the radial vibration along the wall of the PMMA tube becomes more obvious with the increase of the damping coefficient of the PMMA tube. The amplitude of the radial vibration of each point on the wall decreases with the increase of the distance from the point the vibration source.
5. Some trials about feeding HAP powder using the feeding powder system with different structure parameters have been done. The contrast tests of the single factor show that the feeding rate increases with the rise of the drive voltage. The feeding rate can be adjusted by changing the driving voltage. The curve of the relationship between the drive frequency and the feeding rate is similar with the one of the relationship between the piezoelectric and the feeding rate. The feeding rate gets the maximum at the resonant frequency of the piezoelectric ceramic. The curve has better linearity at the right side of the resonant frequency, in which the feeding rate can be adjusted by changing drive frequency. The feeding rate decreases with the increase of the length of the feeding pipe and has little relationship with the height of the powder in the cup. The particle size of the HAP powder has little effect on the feeding rate in the available transport.
6. Under the same experimental conditions, some feeding tests using different types of powder has been done. The test results show that the feeding powder system has good feeding capacity when the powder has good mobility, rough surface and large density.
引文
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