Laser Heated Gradient NMR Studies of Ceramic Liquids

陶瓷的技术重要性日益增长，突显了对陶瓷熔体基本物理和化学性质进行新研究的必要性，这些熔体的温度非常高(高达2,000摄氏度以上)，而且进行了大量的工业加工。我们已经建立了用于测量绝缘材料在超高温下的性质的光谱和时间域核磁共振测量所需的设备，并且迄今为止已经在1700到2600℃之间测量了几种熔融陶瓷中27Al的化学位移和核自旋弛豫时间。该设备将被用于扩展这些测量在众所周知的耐火系统，例如硅-氧化铝，钙-氧化铝和Y-氧化铝，以及在研究中的新系统。此外，我们计划通过在我们的核磁共振探针中加入磁场梯度来进一步开发超高温核磁共振测量技术。这有两个目的，一是提高我们对仪器中样品温度梯度的理解，二是实现扩散系数的直接测量。随着这些创新被添加到众所周知的基本测量体系中，核磁共振的应用有可能显著增加我们对广泛陶瓷体系中的液态的理解。然而，在熔融陶瓷中，测量尤其具有挑战性。典型陶瓷液体成分的高温和强烈的反应性排除了使用样品容器的可能性。在我们的实验中，一个直径约3 mm的小球体被悬浮在向上流动的Ar气体中，并由CO2激光器加热到~2000℃以上。对于核磁共振测量，样品还必须位于超导磁体中心的大磁场中。样品的温度和组成可能不同，核磁共振数据提供了关于这些变化对液体中不同核的平均化学环境的影响的直接信息。扩散速率也可以得到。从这些信息中，人们对熔融陶瓷的原子级结构和动力学性质有了更清晰的理解。有了这个，我们可以更好地了解熔体的宏观性质，例如它的粘度，并更好地控制所产生的陶瓷材料的性质，用于许多应用。该项目是与AFOSR(6.1)共同资助的。

The growing technological importance of ceramics has highlighted the need for new studies of the fundamental physical and chemical properties of ceramic melts, where temperatures are very high (up to and beyond 2,000 Celsius) and where much industrial processing is performed. We have built the apparatus required for both spectroscopic and time-domain NMR measurements of the properties of insulating materials at ultra-high temperatures, and have to date determined both chemical shifts and nuclear spin relaxation times of 27Al in several molten ceramics between 1700 and 2600 C. This apparatus will be used to extend these measurements in well-known refractory systems, e.g. silica-alumina, calcium-alumina and yttrium-alumina, as well as in new systems under investigation. In addition, we plan to further develop the technique of ultra high-temperature NMR measurements by incorporation of a magnetic field gradient into our NMR probe. This has the twin purposes of improving our understanding of sample temperature gradients in the apparatus, and of implementing the direct measurement of diffusivities. With the addition of these innovations to the well-known repertory of basic measurements, the application of NMR has the potential for significantly increasing our understanding of liquid phases over a broad range of ceramic systems. In molten ceramics, however, measurements are particularly challenging. The high temperatures and strong reactivities of a typical ceramic liquid's constituents preclude the use of sample containers. In our experiments a small sphere of material, about three mm in diameter, is levitated in an upward flow of argon gas and heated above ~2000 C by a CO2 laser. For NMR measurements the sample must also be located in a large magnetic field, at the center of a superconducting magnet. Sample temperature and composition may be varied, and the NMR data provide direct information about the effect of these variations upon the average chemical environments of different nuclei in the liquid. Diffusion rates can also be obtained. From this information a clearer understanding emerges, of the atomic scale structural and dynamic properties of molten ceramics. With this, we can better understand the macroscopic properties of a melt, for example its viscosity, and better control the properties of the resulting ceramic material for many applications.This project is co-funded with AFOSR (6.1).