The outside of the high visibility garments are made from Oxford cloth that is washable, soft, and hygroscopic. Hi Vis Jacket come in "lucifer"orange or yellow as is virtually standard for construction, transportation, or sanitation workers for example. Our fashion jackets, though, are just that - fashionable yet highly functional. High visibility in the daytime but reflective at night, they have a soft, smooth exterior. What's more, the polyester wadding on the inside of all our jackets is puffy, elastic, durable, and washable. Hi Vis Jacket,Hi Vis T Shirts,Safety T Shirts,Safety Polo Shirts Yongkang Oti Safety Products Industry Co., LTD , http://www.qbanppe.com
A paper from the State Key Laboratory of Metastable Materials Preparation Technology and Science, Yanshan University, China, "Hardness of Covalent Crystals", proposed a microscopic theory of the hardness of covalent materials, and gave a formula for predicting the hardness of superhard materials. Solved the problem that has plagued material scientists for nearly a century.
According to this formula, you don't have to spend huge sums to synthesize any kind of material, you can know in advance how hard it is. "Hardness of Covalent Crystals" has made humans a big step forward in the study of hardness science theory.
1. "Who is better than who?" The problem of nearly a century
In nature, the brilliant, crystal-clear diamond is the rarest single mineral crystal of the rarest known material.
Diamonds are divided into two categories according to their use: high-quality granules can be used as gem-grade (or diamond-grade) diamonds for decorative objects, and fine-grained granules are used for industrial industrial diamonds. Industrial diamonds are widely used in the fields of electromechanical, optical, construction, transportation, metallurgy, geological exploration, national defense and modern high-tech fields due to their superhardness. Diamond is a rare non-metallic mineral resource. So far, the world's proven reserves are only 1.9 billion carats (equivalent to 380 tons), while China's natural diamond reserves are only 4.18 tons, which is far from satisfying humanity. The demand for diamonds.
Since the end of the 19th century, humans have tried artificial synthetic diamonds. After long-term unremitting efforts by scientists, in 1955, the first time by the American scientist Hall (Hall) to use high temperature and high pressure technology to synthesize diamond with graphite, this is the first milestone in the history of human synthetic superhard materials, which makes humans It is a reality to apply diamonds in large quantities to industry. Today, this method is still the main method for synthesizing industrial diamonds. However, in the practice of diamonds applied to industry, two fatal weaknesses have been discovered: First, the thermal stability is poor. When the temperature reaches 700 °C or higher, the diamond begins to transform into graphite, and the hardness of the latter is different from that of diamond. The second is the difference in chemical inertia. When diamond is processed, it is easy to chemically react with iron in the material, thus losing the superhard characteristics to varying degrees, which limits the application. This prompted scientists to begin an attempt to explore new superhard materials that are comparable in hardness to diamonds and that overcome these weaknesses.
Since the thermal stability and chemical inertness of hexagonal boron nitride are superior to those of graphite, the bond length of boron-carbon bond is very close to the bond length of carbon-carbon bond. Scientists speculate that hexagonal boron nitride may become hardness and A new type of superhard material with superior diamond performance. In 1957, the United States General Electric Company scientist Wentorf successfully synthesized "cubic boron nitride" using hexagonal boron nitride. Its thermal stability and chemical inertness are clearly superior to diamonds, but it has been found to be only 66 GPa, which is equivalent to two-thirds of the diamond hardness (95 GPa), which is completely beyond the expectations of scientists, but "cubic nitrogen." The artificial synthesis of boron has been called the second important milestone of human synthesis of superhard materials by scientists.
In the decades that followed, material scientists around the world never gave up trying to find new superhard materials. After entering the 21st century, in 2001, Ukrainian scientist Solozhanko synthesized the third superhard material cubic BC2N with a hardness of 76GPa. In 2002, Dr. He Ruiwei of a research team in the United States synthesized a superhard material B6O. It is 45GPa. These new achievements have brought new climaxes to the enthusiasm of materials scientists around the world for new superhard materials.
However, in the difficult tracing of finding new superhard materials, scientists have not been deeply aware of the microscopic nature of the commonly used macroscopic physical quantity of hardness. They only know the macroscopic nature of the commonly used physical quantity of hardness, but there is always no microscopic scale. Find the appropriate correspondence, so there is no basic theory that tells the material scientist which atomic arrangement can cause a relatively large hardness. In other words, there has not been a unified theory that predicts the hardness of superhard materials. Therefore, in the specific scientific practice of exploring new superhard materials, scientists can only use a variety of indirect methods to predict new superhard materials. Although some progress has been made, it is impossible to accurately measure the materials before synthesis. Hardness, hardness can only be measured after synthesis. This method requires a lot of manpower, material resources and financial resources. Scientists liken this method to the "cooking method", that is, a dish can only know the color, fragrance, and taste of a dish.
As early as 1934, British scientist O. Neill had reluctantly wrote in one of his monographs: "Hardness is like the storm of the sea, easy to understand, but not easy to measure." American physicist Teter in 1998 A comprehensive review also issued a sigh of "hardness is not only difficult to measure, but difficult to define."
2, "I know who is harder than who" microscopic theory of material hardness
On July 4, 2003, a paper published in the "Physical Review Letter" sponsored by internationally renowned academic journals and the American Physical Society attracted widespread attention from scientists in the global physics and materials science communities.
The article titled "Hardness of Covalent Crystals" is the research result of the Key Laboratory of Metastable Materials Preparation Technology and Science of Yanshan University. Based on the assumption that the hardness of the covalent material is equal to the resistance of the chemical bond to the diamond indenter per unit area, the paper proposes a microscopic theory of the hardness of the covalent material, thus scientifically revealing the microscopic essence of the macroscopic physical quantity of hardness, accurately The hardness of the newly synthesized superhard material, cubic BC2N crystal (B: boron, C: carbon, N: nitrogen), is predicted.
A stone has stirred up thousands of waves, and many eyes have paid attention to China's Yanda. On July 9th, the review journal “Physi-calReviewFocus†sponsored by the American Physical Society gave a special comment and introduction to this achievement. Physical Review Focus
On average, one or two of the best papers published in the journals sponsored by the American Physical Society each week are selected for thematic reviews and introductions. This has made the global physics and materials sciences prepare for the metastable materials of Yanshan University in China. The key laboratory of technology and science is eye-catching.
For this day, researchers at Yanshan University's Key Laboratory of Metastable Materials Preparation Technology and Science have made a lot of efforts. Since 1999, researchers at the laboratory have begun to explore the possibility of finding new superhard materials in boron (B), carbon (C), and nitrogen (N) ternary materials systems. At the beginning of the research work, the researchers faced the old problem that the scientists have not solved for nearly a century, that is, how to design a new superhard material from the boron-carbon-nitrogen ternary system at the atomic level.
Researchers at the Key Laboratory of Yanda Metastable Materials have proposed a microscopic theory describing the hardness of polar covalent solids by assuming that the hardness is the sum of the impedance of each chemical bond on the indenter per unit area. The formula can be expressed as: Hv =556 (Hv: hardness of covalent solids, Na: bond density, fi: Phillips ionicity of the bond, d: bond length of the covalent bond), and generalized to multivariate complex polar covalent solids. The hardness of 29 materials was predicted by this method, including the hardness of the newly synthesized ternary superhard material β-BC2N. Theoretical predictions are in good agreement with experimental values ​​for known material hardness.
This achievement defines and understands the hardness of covalent solids at the electronic level. For the first time in a century, people can predict the hardness based on the atomic arrangement of crystals, laying a theoretical foundation for further design of new superhard materials. basis.
The results were published in the US issue of Phys. Rev. Lett. on July 4, 2003. The first reviewer of the American Journal of Physical Review Express said: "This is a very good article, and the consistency with the experiment (results) is impressive. It is not imposed on people, but from complexities. The physical processing process is revealed." The second reviewer believes: "The connection between the measurable macroscopic properties of the crystal and the microscopic electronic structure is an important topic in modern materials science. Based on J.C. more than 30 years ago. The old concept of ionicity proposed by Phillips, this paper reports an empirical method for predicting the hardness of covalent crystals. For a large number of selected crystals, the results are very consistent with the measured hardness values... When reading this article for the first time The result left a deep impression on me."
After the publication of the article, it has aroused strong repercussions in the international community. J.R.Minkel, a freelance science writer in New York, gave a presentation and commentary on the PhysicalReviewFocus with the theme of “Uncovering the veil of hardness layer by layerâ€. The review begins by writing: "Material-based atomic structures predict the hardness of materials often as difficult as trying to scribe diamonds with chalk. The so-called ionic bond characteristics seem to be related to hardness, based on this property, on July 4th. On the PRL, a research team finally got a clear formula for hardness that successfully predicted the hardness of several materials, including a recently synthesized superhard material. This result helped establish a microscopic model of hardness and Helping to find new superhard compounds.†The American Advanced Ceramics Bulletin published a special review of “Measuring the Hardness (Test and Measurement) of Superhard Materialsâ€, which at the beginning of the review reads: “The hardness of materials based on the atomic structure of the material is predicted. It is generally difficult to recognize." In addition, Germany has made a special review on the topic of "The formula predicts the hardness of covalent crystals" in the high-tech column; Location, professional scientific journal or organization in Iran and other countries have also reached for comment, introduced the achievements made or reproduced.
3, "Understanding it does not require esoteric theory" has made new progress in chemical bond theory
The hardness microscopic theoretical formula established by the State Key Laboratory of Metastable Materials Preparation Technology and Science of Yanshan University uses the bond length of the chemical bond, the bond density and the ionicity of the bond to measure the hardness. In further research on this theory, researchers have found that the true state of ionicity is inconsistent with the expression of traditional classical theory.
Ionicity is an important basic concept common in physics, chemistry, and materials science. It is a measure of the degree of symmetry in charge distribution in space. In the 1930s, American scientist Pauline (L. Pauling, winner of the 1954 Nobel Prize in Chemistry) proposed the first ionic scale based on the difference in electronegativity between the two atoms. In the 1960s, American scientist Phillips (J.C. Phillips) proposed a new and widely used ionic scale based on the chemical theory of chemical bonds. According to these classical chemical bond theories, ionicity exists only in chemical bonds composed of different atoms, and these ionic bondes are accompanied by charge transfer. Within this theoretical framework, the chemical bonds formed by the same species do not exhibit ionicity.
When applying the hardness microscopic theoretical model to calculate the hardness of boron-rich solids containing B12 icosahedron, the researchers found that when they take the ionicity of the BB bond in the B12 icosahedron to 0 as in the classical chemical bond theory, the hardness of the boron-rich solid It is 20 to 44% higher than the experimental value. To this end, they suspect that the BB bond in the B12 icosahedron may be ionic. The first-principles calculations show that the geometric symmetry breaking of the B12 icosahedron causes the asymmetry of the charge distribution of the BB bond in the icosahedron, resulting in the ionicity of the BB bond. To characterize this ionicity, they defined a new ionic scale based on the population of chemical bonds and found the relationship between this scale and the classical Phillips scale. Using this ionicity to calculate the hardness of boron-rich solids, the calculated values ​​are in good agreement with the experimental values.
They also found a new phenomenon in which BB bonds exhibit varying degrees of ionicity in boron-rich solids containing B12 icosahedrons. The chemical bond based population defines a new ionic scale that is consistent with the Phillips scale and is a universal scale. This achievement breaks the traditional concept that "the chemical bond formed by the same atom does not have ionicity". The universal ionic scale proposed is the development and supplement of the classical chemical bond theory. It describes carbon nanotubes, C60 and the same species. The ionic nature of the chemical bonds in the clusters of atoms provides a practical tool.
The results were published in the January 14, 2005 issue of the journal Physics Updates (Phys. Rev. Lett.). The first reviewer believes: “In fact, it is important to be able to generalize ionicity by introducing a new scale, both conceptually and practically.†Second reviewer It is believed that "the 225-covalent bond of the non-polar BB in the B12 icosahedron is 0.37. Such a large ionicity is indeed a far-reaching result, worthy of publication. For the covalent bond in the usual sense, this is an unexpected The nature of this can be obtained through a simple and clear algebraic operation. To understand it does not require a profound theoretical background. Based on these characteristics, many physicists, chemists and materials scientists will be interested in this. â€
Recently, they have used this theory to clarify the debate about the hardness of spinel Si3N4, and clearly give the hardness of the C3N4 isomers. â–¡Reporter Jiang En, reporter Zhang Lihui, Guo Wei [from Qinhuangdao]
background
Traditional hardness test method
Materials are often regarded as milestones in the evolution of human society, because the ability to understand and utilize materials often determines the shape of society and the quality of human life. It is no exaggeration to refer to the history of human civilization as the history of world materials. Among many material families, superhard materials are an important class of functional materials. Generally, materials having a hardness exceeding 40 GPa (GPa is a hardness unit) are referred to as superhard materials.
The local resistance of solids to external objects is an indicator of the softness and hardness of various materials. Since different test methods are specified, there are different hardness standards. The mechanical meanings of various hardness standards are different and cannot be directly converted to each other, but can be compared by experiments.
Traditionally, the hardness is divided into:
1 scratch hardness. It is mainly used to compare the hardness of different minerals. The method is to select a rod with a hard end and a soft end. The material to be tested is drawn along the rod, and the hardness of the material to be tested is determined according to the position of the scratch. Qualitatively speaking, the scratches drawn by hard objects are long, and the scratches drawn by soft objects are short.
2 Press in hardness. It is mainly used for metal materials by pressing a specified indenter into the material to be tested with a certain load, and comparing the softness and hardness of the material to be tested with the local plastic deformation of the surface of the material. Due to the different indenter, load and load duration, there are many kinds of indentation hardness, mainly Brinell hardness, Rockwell hardness, Vickers hardness and microhardness.
3 rebound hardness. It is mainly used for metal materials by making a special hammer drop freely from a certain height to impact the sample of the material to be tested, and to store (and then release) the strain energy during the impact process (via the back of the hammer) Jump height determination) determines the hardness of the material.
link
US Physical Review Focus Website Special Evaluation: They successfully predict material hardness in a beautiful way
Material-based atomic structures predict the hardness of materials often as difficult as scoring diamonds with chalk. The so-called ionic bond characteristic seems to be related to hardness. Based on this property, a research group at PRL (Physical Review Letter) finally got a clear formula for hardness. This formula successfully predicts the hardness of several materials, including a recently synthesized superhard material. This result helped establish a microscopic model of hardness and helped to find new superhard compounds.
Hardness is the ability of one material to resist being scored or pressed by another material. It is difficult to define this property at the atomic scale. And there is no basic theory that tells material scientists how to arrange atoms to obtain a hard structure. Also, some researchers have used different approaches to predict hardness. Although some progress has been made, the problem still exists.
Ionicity is related to the strength of the atomic bond. In a so-called covalent material, such as diamond, ruthenium or silicon, a pair of atoms sharing a pair of electrons equally has greater control over any shared electron. In extreme conditions - an ionic bond material - an atom completely controls the neighboring electrons, and the two atoms are joined together by the newly obtained opposite charges. This electrostatic attraction, known as ionic bonding, is much weaker than the covalent bond of shared electrons. Ionicity is a measure of the degree of electron sharing: covalent bonds have the lowest ionicity and ionic bonds have the highest ionicity.
Tian Yongjun, Gao Inventor and their colleagues at Yanshan University in Qinhuangdao, China, focused their attention on covalent and polar covalent materials. They start by assuming that the hardness is the overall resistance of the chemical bond to the indenter - the more surface bonds, the harder the material. Thus shorter and higher density chemical bonds are advantageous for hardness. The group explained that because the covalent bond is stronger than the ionic bond, the hard material should also have a lower ionicity, which is consistent with other researchers' views.
Combining these principles with the theory of deforming electronically described materials 30 years ago, Tian Yongjun and Gao invention gave hardness formulas based on ionicity, bond length and number of bonded electrons. Using the properties of the known 11 materials, including diamond, Si3N4 and ZrO, they found the best values ​​for the two parameters in the formula.
For 14 hard oxides, semiconductors, and other purely common and polar covalent materials, the final formula predicts their experimental values ​​(with an accuracy of about 10%). The team also calculated the possible atomic structure of the superhard compound BC2N and found that the predicted hardness matched the observed hardness value. The atomic structure of this compound has not been determined experimentally.
JulienHaines of the University of Science in Montpellier, France, said "it seems to be a powerful and useful technique for predicting material hardness." MIT's GerbrandCeder said the authors "unified the views in a rather elegant way," but he wanted to test the model with more materials, especially metals, which had greater challenges. He also said "whenever possible in macro performance with
When establishing a connection between computable properties, it is a step forward."
I know your hardness. Yan University has proposed a microscopic theory for predicting the hardness of materials.
Can you find new superhard materials that are superior to diamonds?