题目内容
Cu的原子数分数为2%的Al-Cu合金先从520℃快速冷却至27℃，并保温3h后，形成平均间距为1.5×10-6cm的G.P.区。已知在027℃时，Cu在Al中的扩散系数D=2.3×10-25cm2/s，假定过程为扩散控制，试估计该合金的空位形成能及淬火空位浓度。
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The Atmosphere 1 The atmosphere is a mixture of several gases. There are about ten chemical elements which remain permanently in gaseous form in the atmosphere under all natural conditions. Of these permanent gases, oxygen makes up about 21 percent and nitrogen about 78 percent. Several other gases, such as argon, carbon dioxide, hydrogen, neon, krypton and xenon, comprise the remaining one percent of the volume of the dry air. The amount of water vapor and its variations in amount and distribution is of extraordinary importance in weather changes. Atmospheric gases hold in suspension great quantities of dust, pollen, smoke and other impurities which are always present in considerable, but variable amounts. 2 The atmosphere has no definite upper limits but gradually thins until it becomes imperceptible. Until recently it was assumed that the air above the first few miles gradually grew thinner and colder at a constant rate. It was also assumed that upper air had little influence on weather changes. Recently studies of the upper atmosphere, currently being conducted by earth satellites and missile probing, have shown these assumptions to be incorrect. The atmosphere has three well-defined strata. 3 The layer of the air next to the earth, which extends upward for about ten miles, is known as the troposphere. On the whole, it makes up about 75 percent of all the weight of the atmosphere, because most of the solar radiation is absorbed by the earth’s surface which warms the air immediately surrounding it. A steady decrease of temperature with the increasing elevation is a mot striking characteristic. The upper layers are colder because of their greater distance from the earth’s surface and rapid radiation of heat into space. The temperatures within the troposphere decrease about 3.5 degrees per 1000 feet increase in altitude. Within the troposphere, winds and air currents distribute heat and moisture. Strong winds, called jet streams are located at the upper levels of the troposphere. These jet streams are both complex and widespread in occurrence. They normally show a wave shaped pattern and move from west to east at velocities of 150 mph, but velocities as 400 mph have been noted. The influences of changing locations and strengths of jet streams upon weather conditions and patterns are no doubt considerable. Current intensive research may eventually reveal their true significance. 4 Above the troposphere to a height of about 50 miles is a zone called the stratosphere. The stratosphere is separated from the troposphere by a zone of uniform temperatures called the tropopause. Within the lower portions of the stratosphere is a layer of ozone gases which filter out most of the ultraviolet rays from the sun. The ozone layer varies with air pressure. If this zone were not there, the full blast of the sun’s ultraviolet light would burn our skins, blind our eyes and eventually result in our destruction. Within the stratosphere, the temperature and atmospheric composition are relatively uniform. 5 The layer upward of about 50 miles is the most fascinating but the least known of the three strata. It is called the ionosphere because it consists of electrically charges particles called ions, thrown from the sun. The northern lights ( aurora borealis ) originate within this highly charged portion of the atmosphere. Its effect upon weather conditions, if any, is as yet unknown.
A careless person is apt to make mistakes
A new era is upon us A new era is upon us. Call it what you will: the service economy, the information age, the knowledge society. It all translates to a fundamental change in the way we work. Already we’re partly there. The percentage of people who earn their living by making things has fallen dramatically in the Western World. Today the majority of jobs in America, Europe and Japan ( two thirds or more in many of these countries) are in the service industry, and the number is on the rise. More part-time jobs, more people are self-employed. But the breadth of the economic transformation can’t be measured by numbers alone, because it also is giving rise to a radical new way of thinking about the nature of work itself. Long-held notions about jobs and careers, the skills needed to succeed, even the relation between individuals and employers---all these are being challenged. We have only to look behind us to get some sense of what may lie ahead. No one looking ahead 20 years possibly could have foreseen the ways in which a single invention, the chip (集成块), would transform our world to its applications in personal computers, digital communications and factory robots. Tomorrow’s achievements in biotechnology, artificial intelligence or even some still unimagined technology could produce a similar wave of dramatic changes. But one thing is certain: information and knowledge will become even more vital, and the people who possess it, whether they work in manufacturing or services, will have the advantage and produce the wealth. Computer knowledge will become as basic requirement as the ability to read and write. The ability to solve problems by applying information instead of performing routine tasks will be valued above all else, if you cast your mind ahead 10 years, information services will be predominant. It will be the way you do your job.
Man is fated to suffer many disappointments in his quest for truth.
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