by Josef Zens
二氧化矽(SiO2)是工業以及地質學中最重要的物質之一。由GFZ德國地質科學研究中心的Sergey
Lobanov主持的研究發展出一套新方法，可以測量在高達110
GPa(gigapascal，十億帕斯卡)，也就是平常大氣壓力的110萬倍下二氧化矽玻璃的密度。他們並非在同步光子設施中利用高度聚焦的X光進行研究，而是運用白光雷射光束與鑽石砧。研究人員將他們創新且簡單的方法發表在當期的《物理評論通訊》(Physical Review Letters)。
鑽石砧可以創造出比大氣壓力高出數百萬倍的極端壓力。(圖片來源：Sergey Lobanov)
在極端條件下測量密度的難處
對於地球科學來說，礦物、岩石以及熔融物質在數百萬大氣壓力以及數千度的環境當中，密度會變成多少是非常重要的問題，因為這主導了行星的長期演化過程與火山作用。但要怎麼測量如此極端的環境當中物質的密度？對於結晶質的礦物或岩石來說，科學家回答這項問題的方法是透過X光繞射，測量週期性排列的原子彼此之間的距離。然而，當物質的結構沒有規則
(即「非晶質」)，像是玻璃或者熔岩，問題就會變得困難許多。在此情況下，科學家得直接測量樣本的體積，接著除以質量來得到密度。不過要進行這種測量卻極為困難，因為樣品在高壓之下體積會變得相當小。過往在測量的時候需要用到大型的X光設施以及高度專門化的設備，使得價錢相當昂貴。由德國地球科學研究中心的Sergey
Lobanov主持的科學團隊最近提出了一種新方法，他們利用鞋盒大小的設備製造出來的雷射，就能測量在壓力約莫為地球2000多公里深的地方，樣品的體積會變成多少。
在地球內部，岩石所承受的壓力高到難以想像，比平常的大氣壓力還要高出數百萬倍。然而跟許多人認為的不同，地函其實是固體而非液體。這些岩石展現出「黏塑性」(viscoplastic)的行為模式：它們雖然能以每年幾公分的速度流動，但是被槌子用力敲打卻會破裂。儘管它們動得相當緩慢，但地殼的板塊構造運動卻是由此產生，並連帶形成了火山作用。地表的板塊隱沒之後會擠出水分，諸如此類的化學變化可以改變岩石的熔點，使得融化的岩漿迅速產生。當岩漿找到途徑進入地殼甚至來到地表，便會發生火山爆發。
無序物質的密度
世上沒有一種儀器可以穿透地函來詳細研究這些作用，因此人類必須仰賴計算、地震訊號以及實驗室試驗來深入了解地球內部，而鑽石砧這種儀器便能產生地球內部常見的高溫高壓環境。放置在鑽石砧內用來研究的樣品比針尖還小，已經是次奈升的範圍(至少小於一毫升的一千萬分之一)。當物質被如此高的壓力擠壓，內部的結構會出現變化。為了精確分析這些變化，科學家運用X光照射晶體來產生它們的繞射模式，藉此推導晶格的體積，進而得出物質的密度。然而玻璃與熔岩這類非晶質物質對於它們的密度目前來說仍守口如瓶。原因是無序物質的X光繞射並沒有辦法就它們的體積與密度提供直接資訊。
小技巧：用雷射取代X光來測量
二氧化矽是工業與地質學當中最重要的物質之一。Sergey
Lobanov主持的研究團段最近利用一種簡單的技巧，成功測量在高達110
GPa的壓力下二氧化矽玻璃的折射率以及密度。此壓力比平常的大氣壓力高出110萬倍，通常出現於地球2000多公里深的地方。研究人員利用多色雷射測量樣品加壓後的反射光亮度，透過其中蘊含的資訊可以得出折射率。折射率是一種物質的基本特性，它不只描述了光通過物質的時候減慢與彎折的程度，還有雷射行經樣品內部時的路徑長短。具有高折射率與密度的物質，像是鑽石與金屬，通常看起來較為明亮且閃閃發光。不過Lobanov和同僚並非用肉眼觀察那些微小的樣品，而是以強力的光譜儀記錄樣品在高壓下出現的亮度變化。這些測量結果可以得出二氧化矽玻璃的折射率，並提供定量密度所需的關鍵資訊。
科學家測量散射的雷射光束便能得知二氧化矽玻璃的折射率，以及定量密度時所需的關鍵資訊。(圖片來源：Sergey Lobanov)
測量玻璃密度對於地球科學的重要性
「地球在45億年前是一顆熔岩組成的巨大球體。若要了解地球如何冷卻並產生固態的地殼與地函，就得知道熔岩在極端高壓下的物理性質。然而，高壓下的熔岩研究起來極為困難，為了迴避某些困難之處，地質學家選擇玻璃而非熔岩來進行研究。玻璃是把高溫且黏稠的熔岩迅速冷卻所製成，因此玻璃的結構通常也可以代表身為材料的熔岩的結構。先前測量高壓下的玻璃密度需要用到大型且昂貴的同步光子設施，因為它們才能產生緊密聚焦的X光束，觀測鑽石砧裡的微小樣品。這些實驗非常困難，在一百萬大氣壓力下的玻璃密度迄今只有少數幾筆測量結果。但現在我們證明只要是透明的玻璃，至少在壓力到達110
GPa之前都可以利用這種光學技術，精確測量樣品體積和密度的演變過程，」Lobanov表示。「這種方法可以在同步光子設施以外進行，因此更方便也更便宜。將來的研究如果要用接近地球目前的熔岩成分，或是消逝已久的熔岩成分來製成玻璃，我們的成果可以當作它們的基礎。這類研究能提供新的定量化資訊，讓我們解開早期地球的演化過程以及造成火山噴發的機制。」
創造新的可能性來探討本來為不透明的非晶質固體
由於實驗所需的樣品尺寸非常小因此厚度也很薄，就算一大塊看起來像是岩石的物質在製成樣品後也會變透明。對於在體積大的狀態下看起來是不透明的非晶質固體來說，研究人員表示這些進展開啟了新的可能性來研究它們的力學與電學性質。論文作者表示他們的發現在材料科學和地球物理具有深遠的影響。此外，運用電腦研究玻璃和熔岩在極端條件下的輸送性質的時候，這道資訊也可以作為基準。
Lobanov是亥姆霍茲青年研究團隊「CLEAR」化學、物理與地質材料分部的主持人，他強調研究可以成功得歸功於GFZ同事間互信合作的氛圍。「我們有辦法進行實驗來探測高壓下的樣本只是其中一個原因，」Lobanov說。「但是與其他分部的同僚之間進行的討論，其重要性完全不亞於此，這幫助我構思我的想法並加以實行。」
How to look
thousands of kilometers deep into the Earth?
Researchers led by Sergey Lobanov from
the GFZ German Research Centre for Geosciences have developed a new method to
measure the density of silicon dioxide (SiO2) glass, one of the most
important materials in industry and geology, at pressures of up to 110
gigapascals, 1.1 million times higher than normal atmospheric pressure. Instead
of employing highly focused X-rays at a synchrotron facility, they used a white
laser beam and a diamond anvil cell. The researchers report on their new and
simple method in the current issue of Physical
Review Letters.
The problem of
density measurement under extreme conditions
In geosciences, the density of minerals, rocks, and
melts at pressures up to several million atmospheres and temperatures of
several thousand degrees is of critical importance because it governs the
long-term planetary evolution as well as volcanic processes. But how can the
density of a material be measured at such extreme conditions? To answer this
question for a crystalline mineral or a rock, scientists use X-ray diffraction
with which one measures the spacing between the periodically arranged atoms. There
is, however, a problem if the material has a disordered structure, i.e. is
non-crystalline, like glasses or molten rocks. In this case, the volume of the
sample has to be measured directly – the density of a material equals its mass
divided by volume. However, such measurements are extremely difficult because
of the tiny volume of the sample brought to high pressure. Previously, these
measurements required large scale X-ray facilities and highly specialized
equipment, thus being very expensive. Now, a team led by scientist Sergey
Lobanov of the GFZ German Research Centre for Geosciences is introducing a new
method in which a laser the size of a shoebox allows them to measure the volume
of samples brought to pressures similar to that at the depth of more than 2000
km in the Earth.
Inside the Earth, the rock is under unimaginably high
pressure, up to several million times higher than normal atmospheric pressure.
However, contrary to widespread belief, the Earth's mantle is not liquid, but
solid. The rock behaves in a viscoplastic fashion: It moves centimeter by
centimeter per year, but it would burst under a hammer blow. Nevertheless, the
slow movements drive the Earth's crustal plates and tectonics, which in turn
trigger volcanism. Chemical changes, for example, caused by water squeezed out
of subducted crustal plates, can change the melting point of the rock in such a
way that suddenly molten magma is formed. When this magma makes its way to the
Earth's crust and to the surface, volcanic eruptions occur.
Density of
disordered materials
No instrument in the world can penetrate the Earth's
mantle to study such processes in detail. Therefore, one must rely on
calculations, seismic signals and laboratory experiments to learn more about
the Earth's interior. A diamond anvil cell can be used to generate the
extremely high pressures and temperatures that prevail there. The samples
explored in it are smaller than the tip of a pin. Their volume is in the sub
nanoliter range (e.g. at least 10 million times smaller than 1 milliliter).
When material is compressed under such high pressures, the internal structure
changes. To analyze this precisely, X-rays are used on crystals to generate
diffraction patterns. This allows conclusions to be drawn about the volume of
the crystal lattice and thus also the density of the material. Non-crystalline
materials, such as glasses or molten rocks, have so far kept their innermost
secrets to themselves. This is because for disordered materials X-ray
diffraction does not provide direct information on their volume and density.
Simple trick:
measurement with laser instead of X-ray beam
Using a simple trick, researchers led by Sergey
Lobanov have now succeeded in measuring the refractive index and density of
silicon dioxide (SiO2) glass, one of the most important materials in
industry and geology, at pressures of up to 110 gigapascals. This is a pressure
that prevails at a depth of more than 2,000 kilometers in the Earth's interior
and is 1.1 million times higher than normal atmospheric pressure. The
researchers used a multicolor laser to measure the brightness of its reflection
from the pressurized sample. The brightness of the laser reflection contained
information on the refractive index, a fundamental material property that
describes how light slows down and bends as it travels through the material,
but also the path length of the laser inside the sample. Materials with a high
refractive index and density, such as diamonds and metals, typically appear
bright and shiny to our eye. Instead of looking at the tiny samples with a
naked eye, Lobanov and his colleagues used a powerful spectrometer to record
changes in brightness at high pressure. These measurements yielded the
refractive index of SiO2 glass and provided key information to
quantify its density.
Significance of
the density measurement of glasses for the geosciences
“Earth was a giant ball of molten rock 4.5 billion
years ago. To understand how Earth has cooled and produced a solid mantle and
crust, we need to know the physical properties of molten rocks at extreme
pressure. However, studying melts at high pressure is extremely challenging and
to circumvent some of these challenges geologists choose to study glasses
instead of melts. Glasses are produced by quickly cooling hot but viscous
melts. As a result, the structure of glasses often represents the structure of
melts they were formed from. Previous measurements of glass density at high
pressure required large and expensive synchrotron facilities that produce a
tightly focused beam of X-rays that can be used to view the tiny sample in a
diamond anvil cell. These were challenging experiments and only the densities
of very few glasses have been measured to a pressure of 1 million atmospheres.
We have now shown that the evolution of the sample volume and density of any
transparent glass can be accurately measured up to pressures of at least 110
GPa using optical techniques," Lobanov says. "This can be done
outside of synchrotron facilities and is therefore much easier and less costly.
Our work thus paves the way to future studies of glasses that approximate
Earth’s present-day and long-gone melts. These future studies will provide new
quantitative answers about the evolution of the early Earth as well as the
driving forces behind volcanic eruptions."
New possibilities
for the investigation of non-crystalline, initially non-transparent solids
Because the samples are extremely small and therefore
ultra-thin, even materials that look like a lump of rock in large pieces become
translucent. According to the researchers, these developments open up new
possibilities for studying the mechanical and electronic properties of
non-crystalline solids that appear nontransparent in larger volumes. According
to the authors of the study, their findings have far-reaching implications for
materials science and geophysics. In addition, this information could serve as
a benchmark for computational studies of the transport properties of glasses
and melts under extreme conditions.
Lobanov emphasizes that this kind of study was only
made possible by the collegial environment at the GFZ. He heads a Helmholtz Young
Investigator Group called CLEAR in the "Chemistry and Physics of
Geomaterials" section. "Our experimental capabilities to probe
samples at high pressure is only one thing," says Lobanov, "at least
as important were the discussions with colleagues in other sections, which
helped me develop the ideas and implement them."
原始論文：Sergey S. Lobanov, Sergio Speziale, Björn Winkler, Victor
Milman, Keith Refson, Lukas Schifferle. Electronic, Structural, and Mechanical Properties of
SiO2 Glass at High Pressure Inferred from its Refractive Index. Physical Review Letters,
2022; 128 (7) DOI: 10.1103/PhysRevLett.128.077403
引用自：GFZ GeoForschungsZentrum Potsdam, Helmholtz
Centre. "How to look thousands of kilometers deep into the Earth.”
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