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南京理工大學(xué)泰州科技學(xué)院
畢業(yè)設(shè)計(jì)(論文)外文資料翻譯
系 部： 機(jī)械工程系
專(zhuān) 業(yè)： 機(jī)械工程及自動(dòng)化
姓 名： 李聲超
學(xué) 號(hào)： 05010216
外文出處： A. Safari and D. J. Waller, "Fine Scale PZT
Fiber/Polymer Composites”
附 件： 1.外文資料翻譯譯文；2.外文原文。
指導(dǎo)教師評(píng)語(yǔ)：
譯文準(zhǔn)確，條理比較清楚，語(yǔ)句較通暢，基本能符合漢語(yǔ)的習(xí)慣。專(zhuān)業(yè)用語(yǔ)翻譯較為準(zhǔn)確，整體翻譯質(zhì)量較好。
簽名：
年 月 日
附件1：外文資料翻譯譯文
通過(guò)注射成型制造壓電陶瓷/聚合物復(fù)合材料
Leslie J. Bowen 和 Kenneth W. French
原料系統(tǒng)(有限)公司
摩洛哥康考德希爾克雷斯特大道53號(hào), 郵編01742
摘要
賓夕法尼亞州立大學(xué)材料研究室的研究已經(jīng)證明通過(guò)使用壓電陶瓷/聚合物復(fù)合材料可以改進(jìn)檢漏器(水診器)潛能。作為美國(guó)海軍研究局的資助計(jì)劃的一部分，旨在開(kāi)發(fā)針對(duì)這些合成物且具有成本效益制造技術(shù)，材料系統(tǒng)正在尋求一種陶瓷制造方法的注射成型。本文簡(jiǎn)要概覽了陶瓷注射成型過(guò)程的關(guān)鍵細(xì)節(jié)，并且記敘了制造壓電陶瓷/聚合物復(fù)合材料的步驟及方法論。注射成型壓電陶瓷的設(shè)備和應(yīng)用程序都是區(qū)別于傳統(tǒng)的材料的加工。
緒論
壓電陶瓷/聚合物復(fù)合材料提供了設(shè)計(jì)的多功能性和性能優(yōu)勢(shì)，在遙感和驅(qū)動(dòng)應(yīng)用方面都超越單獨(dú)的陶瓷與聚合物的壓電材料。這些合成物已經(jīng)被開(kāi)始用于高解析度超聲醫(yī)學(xué)以及海軍的發(fā)展應(yīng)用。在過(guò)去的十三年里，許多復(fù)合的配置已經(jīng)按照一個(gè)實(shí)驗(yàn)室的規(guī)模被構(gòu)造且評(píng)估。其中最成功的組合之一，被指定復(fù)合物的紐納姆號(hào)，有一個(gè)三維連接陶瓷階段(壓電纖維)內(nèi)含三維連接有機(jī)聚合物的階段。檢漏器的性能系數(shù)可使得這個(gè)復(fù)合物超過(guò)那些通過(guò)適當(dāng)選擇階段特征和復(fù)合結(jié)構(gòu)的固體材料10000倍。
賓州州立大學(xué)復(fù)合物的制備是通過(guò)在一個(gè)跳汰機(jī)和封裝環(huán)氧樹(shù)脂中手調(diào)擠壓壓電陶瓷棒，之后限制適當(dāng)?shù)暮穸炔O化陶瓷。除了這種材料所展示出的性能優(yōu)勢(shì)，賓州州立大學(xué)的工作所凸顯的問(wèn)題涉及合成物的大規(guī)模制造或者甚至以原型為目的。這些是：
(1)在通過(guò)聚合物封裝時(shí)大量的壓電陶瓷光纖的庫(kù)存和供給需求。
(2)在極化過(guò)程中發(fā)生率高的介電擊穿是起于在一個(gè)典型的大型陣列遇到一個(gè)或多個(gè)有缺陷的纖維的顯著概率。
在過(guò)去的五年里，為了提高制造行業(yè)的生存能力并降低材料成本已經(jīng)多次嘗試簡(jiǎn)化傳感器的組裝工藝。早期的嘗試包括將壓電陶瓷的固體塊切割至理想的配置和聚合體階段的空缺回填。這項(xiàng)技術(shù)已經(jīng)被超聲醫(yī)學(xué)工業(yè)接受并用于制造高頻傳感器。最近，纖維材料公司已經(jīng)證明了其用于纖維增強(qiáng)復(fù)合材料的編織技術(shù)在裝配壓電材料方面的適應(yīng)性。另外的一項(xiàng)探索技術(shù)涉及復(fù)制多孔織物已經(jīng)有適當(dāng)?shù)倪B通性。
對(duì)于極其精密尺度的復(fù)合材料，纖維的直徑大約為20至100微米，長(zhǎng)寬比大于5以滿足裝置性能需要的目標(biāo)。因此，這些困難再加上額外的成型與處理龐大數(shù)量且無(wú)缺陷的極其精細(xì)的纖維的挑戰(zhàn)。最近，西門(mén)子公司的研究人員表明非常精密尺度的復(fù)合材料可以通過(guò)一種不定的模具技術(shù)來(lái)制造。然而，這種方法需要為每一個(gè)部分制造一個(gè)新的模具。
本文介紹一種壓電復(fù)合加工的新方法，即:陶瓷注射成型。陶瓷注射成型無(wú)論對(duì)海軍的壓電陶瓷/聚合物復(fù)合材料或是對(duì)于極其加工規(guī)模的壓電復(fù)合材料(如那些所需的高頻超聲醫(yī)療及無(wú)損評(píng)估)都是一種具有成本效益的制造方法。注塑成型過(guò)程克服了通過(guò)網(wǎng)型預(yù)成型陶瓷纖維整列使裝配導(dǎo)向陶瓷纖維進(jìn)入復(fù)合材料傳感器的困難。除了這個(gè)優(yōu)勢(shì)，該方法使得比那些以前的設(shè)想具有更復(fù)雜陶瓷元素幾何的復(fù)合傳感器成為可能，以致產(chǎn)生了為提高聲阻抗匹配性的更高的設(shè)計(jì)柔性以及橫向模式的取消。
過(guò)程描述
注塑成型被廣泛應(yīng)用于塑料行業(yè)作為一種較低成本、形狀復(fù)雜的迅速大規(guī)模生產(chǎn)。此種方法最適合應(yīng)用于陶瓷小截面形狀，例如線程導(dǎo)向，以及無(wú)需燒結(jié)至很高密度的大而復(fù)雜的形狀，如渦輪機(jī)的葉片鑄造插入。最近，這種方法已被研究用作生產(chǎn)熱發(fā)動(dòng)機(jī)渦輪部件的技術(shù)。
如圖1所示，注塑成型方法已被用于壓電陶瓷的成型。通過(guò)將熱塑性塑料與陶瓷粉末的混合物有機(jī)結(jié)合并注入一個(gè)冷卻模具，復(fù)雜的形狀就能方便且快速的正常與塑料結(jié)合成型。預(yù)防例如像金屬接觸硬化的表面，盡量減少金屬?gòu)幕旌团c成型器械受到的污染。對(duì)于陶瓷，型腔必須無(wú)損拆除，迫使高的固體載荷，嚴(yán)格控制型腔移除的過(guò)程，以及適當(dāng)?shù)膴A具。一旦型腔移除，隨后點(diǎn)火，極化并且環(huán)氧樹(shù)脂的封裝過(guò)程是和那些常規(guī)壓電陶瓷/復(fù)合材料類(lèi)似。因此，此方法在替代制造路線上提供了很大優(yōu)勢(shì)：復(fù)雜，能夠同時(shí)處理許多纖維的近似網(wǎng)狀；快速的生產(chǎn)能力(通常是一部分幾秒)；統(tǒng)計(jì)過(guò)程控制的兼容性；材料的低浪費(fèi)；有關(guān)傳感器設(shè)計(jì)的柔性(允許PZT中元素空間和形狀的變化)；以及在中量至大量之間的低成本。一般來(lái)說(shuō)，由于最初加工的高成本，陶瓷注射成型的方法是最適用于復(fù)雜形狀的構(gòu)成，需要低成本大批量。
圖1 注射成型過(guò)程流程 圖2 制作合成物的預(yù)成型方法
合成物的制造及評(píng)價(jià)
制造1-3壓電復(fù)合材料的方法如圖2a所示，這闡述了使用一個(gè)完整的陶瓷
胚型到纖維定位作用的壓電陶瓷預(yù)先成型的概念。在聚合物封裝后采用磨削去除陶瓷胚。除了簡(jiǎn)化許多纖維的處理，這種預(yù)先成型的方法允許廣泛地選擇壓電陶瓷元素幾何元素范圍，以使其性能最優(yōu)化。工具的設(shè)計(jì)是取得注塑壓電復(fù)合材料成功的重要因素。如圖2b所示的方法使用了無(wú)需導(dǎo)致額外重組成本的嵌入式的并允許局部變化的設(shè)計(jì)。圖2c所示如何配置個(gè)別的預(yù)加工的成品以形成大批生產(chǎn)
在實(shí)踐中，材料和成型參數(shù)必須最優(yōu)化并成型工具的設(shè)計(jì)相結(jié)合以實(shí)現(xiàn)在成型后完整的脫模。關(guān)鍵的參數(shù)包括：壓電陶瓷/裝夾工具之比，壓電元件的直徑和錐度，壓電陶瓷基本軸向厚度，工具表面的磨光，以及成型零件的脫模機(jī)構(gòu)的設(shè)計(jì)。為了評(píng)估這些工藝參數(shù)而不承擔(dān)過(guò)多的工藝成本，一種工具的設(shè)計(jì)根據(jù)實(shí)驗(yàn)?zāi)康牟捎弥挥袃膳诺母髯?9個(gè)壓電陶瓷要素。每一行的要素都包括三個(gè)錐角(0，1和2度)以及兩個(gè)直徑(0.5mm和1mm)。為了容許成型收縮，預(yù)加工的工件尺寸維持在50mmX50mm，以盡量減少在制模周期中的冷卻部分折斷外層纖維的可能性。
圖3所示的綠色陶瓷瓶坯的制造使用這種工具配置。請(qǐng)注意，所有壓電陶瓷在成型后的完整的脫模，包括那些沒(méi)有縱向尖端不方便的脫模?？諝庵械木徛訜嵋呀?jīng)被發(fā)現(xiàn)是一個(gè)適合去除有機(jī)粘合劑的方法。最后，燒壞的粘合劑被燒結(jié)在一個(gè)理論值在97-98%的富含氧化鉛的氣體中。在燒結(jié)這些合成物型坯時(shí)沒(méi)有遇到任何控制重量減輕的問(wèn)題，甚至是那些用于高頻超聲的高尺寸精度，高表面質(zhì)量的型坯。
圖3 注射成型1-3預(yù)成型合成物 圖4 電子顯微鏡掃描PZT表面
圖4說(shuō)明了表面為壓模和作為燒結(jié)的纖維，顯示出大約10um寬的存在的淺的折線，這是在注射成型過(guò)程中特有的。那個(gè)沿其長(zhǎng)度方向顯現(xiàn)出微小孔型設(shè)計(jì)的纖維取決于從工具中的脫模過(guò)程。圖5所示近似網(wǎng)狀的成型方式用于制造非常精細(xì)尺度的型坯的能力；所示壓電元件的尺寸只有30um。由作為這些燒結(jié)的表面指出，壓電陶瓷的顯微結(jié)構(gòu)是密集且均勻的，由直徑為2-3um的細(xì)碎的等軸晶體構(gòu)成。
圖5 由近似網(wǎng)狀的成型的精密尺度的合成物
為了示范上述合成物制造的方法，注射成型和燒結(jié)的纖維行在用于成型合成物型坯的壓電陶瓷被磨光之后，大約總體10%的5H*壓電陶瓷合成物以及環(huán)氧樹(shù)脂Spurrs在制造時(shí)通過(guò)環(huán)氧成對(duì)封裝。圖6所示復(fù)合材料樣品使用剛才復(fù)合的壓電陶瓷/粘結(jié)劑混合物以及再生材料制造?；厥諒?fù)合物和成型的材料似乎是完全可行的，并且結(jié)果大大提高材料的利用率。
表1比較了使用粉末制造商準(zhǔn)備好的那些被報(bào)道的用于模壓的5H壓電陶瓷樣品注射成型壓電陶瓷樣品的壓電和介電的性能。當(dāng)燒結(jié)條件最優(yōu)于壓電陶瓷5H的條件，壓電和介電的性能都較所有材料有可比性。當(dāng)壓電陶瓷5H的原料物質(zhì)被考慮到受注射成型設(shè)備污染鐵的敏感性，這些有關(guān)的測(cè)量方法對(duì)于這種注射成型的壓電陶瓷材料可以忽略這類(lèi)污染。
*粉末的提供方是俄亥俄州貝德福德的摩根士丹利公司，105A街區(qū)。
表1 壓電陶瓷注塑成型的參數(shù) 圖6 上述方法精制壓電陶瓷/樹(shù)脂合成物的注塑成型
總結(jié)
陶瓷注射成型已被證明是一種可行的制造壓電陶瓷和壓電陶瓷/聚合物傳感器的方法。注射成型壓電陶瓷的電相關(guān)特性區(qū)別于那些通過(guò)傳統(tǒng)的準(zhǔn)備好的粉末壓模，沒(méi)有證據(jù)證明在混合物以及成型設(shè)備中產(chǎn)生的金屬雜質(zhì)會(huì)產(chǎn)生污染影響。通過(guò)陶瓷的注射成型來(lái)制造合成物型坯，之后使用型坯來(lái)形成大批生產(chǎn)，此種方法已經(jīng)證明用于網(wǎng)狀大量制造壓電復(fù)合物傳感器。
致謝
這項(xiàng)工作由海軍研究事務(wù)所的Stephen E.Newfield先生贊助指導(dǎo)。作者要感謝Hong Pham女士提供的技術(shù)援助，以及材料研究實(shí)驗(yàn)所的Tomas Shrout博士，賓州州立大學(xué)所做的電器測(cè)量工作。
參考文獻(xiàn)
[1] R. E. Newnham等著，《復(fù)合壓電式傳感器》，材料工程，第二卷，93-106頁(yè)，1980年12月出版
[2] C. Nakaya等著，IEEE超音波專(zhuān)業(yè)座談會(huì)，1985年十月16-18日。P634
[3] S. D. Darrah等著，《大面積壓電復(fù)合材料》關(guān)于活性物質(zhì)和構(gòu)造的ADPA會(huì)議，亞歷山德里亞，十一月4-8日，1991年，埃德。灣諾爾斯，物理研究所出版，頁(yè)139-142 。
[4] A. Safari and D. J. Waller著，《精密尺度的煙點(diǎn)陶瓷纖維/聚合物復(fù)合材料》，在關(guān)于活性物質(zhì)和構(gòu)造的ADPA會(huì)議上提交，亞里山德里亞，危吉利亞，十一月4-8號(hào)，1991年。
[5] U. Bast, D. Cramer and A. Wolff著，《一種用來(lái)制造1-3連通形壓電復(fù)合材料的新方法》，第七屆CIMTEC ， 意大利蒙特卡蒂尼， 6月24至30號(hào)， 1990年，Ed.P. Vincenzini, Elsevier，2005-2015頁(yè)
[6] G. Bandyopadhyay and K. W. French著，《網(wǎng)狀的硅的氮化物應(yīng)用于發(fā)動(dòng)機(jī)的制造》，對(duì)渦輪增壓器轉(zhuǎn)自及動(dòng)力，108，536-539頁(yè)，1986年出版
[7] J. Greim等著，《燒結(jié)注塑渦輪增壓轉(zhuǎn)子》，第三屆關(guān)于熱動(dòng)力的陶瓷材料及構(gòu)造國(guó)際研討，內(nèi)華達(dá)州拉斯維加斯，1365-1375頁(yè)，Amer. Cer. Soc，1989年
附件2：外文原文
FABRICATION OF PIEZOELECTRIC CERAMlClPOLYMER COMPOSITES BY INJECTION MOLDING.
Leslie J. Bowen and Kenneth W. French,
Materials Systems Inc.
53 Hillcrest Road, Concord, MA 01742
Abstract
Research at the Materials Research Laboratory, Pennsylvania State University has demonstrated the potential for improving hydrophone performance using piezoelectric ceramic/polymer composites. As part of an ONR-funded initiative to develop cost-effective manufacturing technology for these composites, Materials Systems is pursuing an injection molding ceramic fabrication approach. This paper briefly overviews key features of the ceramic injection molding process, then describes the approach and methodology being used to fabricate PZT ceramic/polymer composites. Properties and applications of injection molded PZT ceramics are compared with conventionally processed material.
Introduction
Piezoelectric ceramic/polymer composites offer design versatility and performance advantages over both single phase ceramic and polymer piezoelectric materials in both sensing and actuating applications. These composites have found use in high resolution medical ultrasound as well as developmental Navy applications. Many composite configurations have been constructed and evaluated on a laboratory scale over the past thirteen years. One of the most successful combinations, designated 1-3 composite in Newnham’s notation [l 1, has a one-dimensionally connected ceramic phase (PZT fibers) contained within a three-dimensionally connected organic polymer phase. Hydrophone figures of merit for this composite can be made over 10,000 times greater than those of solid PZT ceramic by appropriately selecting the phase characteristics and composite structure.
The Penn State composites were fabricated [ l ] by hand-aligning extruded PZT ceramic rods in a jig and encapsulating in epoxy resin, followed by slicing to the appropriate thickness and poling the ceramic. Aside
from demonstrating the performance advantages of this material, the Penn State work highlighted the
difficulties involved in fabricating 1-3 composites on a large scale, or even for prototype purposes. These are:
1) The requirement to align and support large numbers of PZT fibers during encapsulation by the polymer.
2) The high incidence of dielectric breakdown during poling arising from the significant probability of encountering one or more defective fibers in a typical large array.
Over the past five years several attempts have been made to simplify the assembly process for 1-3 transducers with the intention of improving manufacturing viability and lowering the material cost. Early attempts involved dicing solid blocks of PZT ceramic into the desired configuration and back-filling the spaces with a polymer phase. This technique has industry for manufacturing high frequency transducers [2]. More recently, Fiber Materials Corp. has demonstrated the applicability of its weaving technology for fiber-reinforced composites to the assembly of piezoelectric composites [31. Another exploratory technique involves replicating porous fabrics having the appropriate connectivity [4].
For extremely fine scale composites, fibers having diameters in the order of 25 to 100 pn and aspect ratios in excess of five are required to meet device performance objectives. As a result, these difficulties are compounded by the additional challenge of forming and handling extremely fine fibers in large quantities without defects. Recently, researchers at Siemens Corp. have shown that very fine scale composites can be produced by a fugitive mold technique. However, this method requires fabricating a new mold for every part [5].
This paper describes a new approach to piezoelectric composite fabrication, viz: Ceramic injection molding. Ceramic injection molding is a costeffective fabrication approach for both Navy piezoelectric ceramic/polymer composites and for the fabrication of ultrafine scale piezoelectric composites, such as those required for high frequency medical ultrasound and nondestructive evaluation. The injection molding process overcomes the difficulty of assembling oriented ceramic fibers into composite transducers by net-shape preforming ceramic fiber arrays. Aside from this advantage, the process makes possible the construction of composite transducers having more complex ceramic element geometries than those previously envisioned, leading to greater design flexibility for improved acoustic impedance matching and lateral mode cancellation.
Process Description
Injection molding is widely used in the plastics industry as a means for rapid mass production of complex shapes at low cost. Its application to ceramics has been most successful for small crosssection shapes, e.g. thread guides, and large, complex shapes which do not require sintering to high density, such as turbine blade casting inserts. More recently, the process has been investigated as a production technology for heat-engine turbine components [6,7].
The injection molding process used for PZT molding is shown schematically in Figure 1.
By injecting a hot thermoplastic mixture of ceramic powder and organic binder into a cooled mold, complex shapes can be formed with the ease and rapidity normally associated with plastics molding. Precautions, such as hard-facing the metal contact surfaces, are important to minimize metallic contamination from the compounding and molding machinery. For ceramics, the binder must be removed nondestructively, necessitating high solids loading, careful control of the binder removal process, and proper fixturing. Once the binder is removed, the subsequent firing, poling and epoxy encapsulation processes are similar to those used for conventional PZT/polymer composites [1]. Thus, the process offers the following advantages over alternative fabrication routes: Complex, near net-shape capability
for handling many fibers simultaneously; rapid throughput (typically seconds per part); compatibility with statistical process control; low material waste; flexibility with respect to transducer design (allows variation in PZT element spacing and shape); and low cost in moderate to high volumes. In general, because of the high initial tooling cost, the ceramics injection molding process is best applied to complex-shaped components which require low cost in high volumes.
Figure 1 : Injection Molding Process Route. Figure 2: Preform Approach to Composite Fabrication.
Composite Fabrication and Evaluation
The approach taken to fabricate 1-3 piezoelectric composites is shown in Figure 2a, which illustrates a PZT ceramic preform concept in which fiber positioning is achieved using a co-molded integral ceramic base. After polymer encapsulation the ceramic base is removed by grinding. Aside from easlng the handling of many fibers, this preform approach allows broad latitude in the selection of piezoelectric ceramic element geometry for composite performance optimization. Tool design is important for successful injection molding of piezoelectric composites. The approach shown in Figure 2b uses shaped tool inserts to allow changes in part design without incurring excessive retooling costs. Figure 2c shows how individual preforms are configured to form larger arrays
In practice, material and molding parameters must be optimized and integrated with injection molding tool design to realize intact preform ejection after molding. Key parameters include: PZT/binder ratio, PZT element diameter and taper, PZT base thickness, tool surface finish, and the molded part ejection mechanism design. In order to evaluate these process parameters without incurring excessive tool cost, a tool design having only two rows of 19 PZT elements each has been adopted for experimental purposes. Each row contains elements having three taper angles (0, 1 and 2 degrees) and two diameters (0.5 and l mm). To accommodate molding shrinkage, the size of the preform is maintained at 5Ox50mm to minimize the
possibility of shearing off the outermost fibers during the cooling portion of the molding cycle.
Figure 3 shows green ceramic preforms fabricated using this tool configuration. Note that all of the PZT elements ejected intact after molding, including those having no longitudinal tapering to facilitate ejection. Slow heating in air has been found to be a suitable method for organic binder removal. Finally, the burned-out preforms are sintered in a PbOrich atmosphere to 97-98% of the theoretical density. No problems have been encountered with controlling the weight loss during sintering of these composite preforms, even for those fine-scale, high-surface area preforms which are intended for high frequency ultrasound.
Figure 4 illustrates the surfaces of as-molded and as-sintered fibers, showing the presence of shallow fold lines approximately 10pm wide, which are characteristic of the injection molding process. The fibers exhibit minor grooving along their length due to ejection from the tool. Figure 5 shows the capability of near net-shape molding for fabricating very fine scale preforms; PZT element dimensions only 30pm wide have been demonstrated. The as-sintered surface of these elements indicates that the PZT ceramic microstructure is dense and uniform,n consisting of equiaxed grains 2-3pm in diameter.
Figure 3: Injection Molded 1-3 Composite Preforms. Figure 4: Scanning Electron Micrographs of As-molded
(Upper) and As-sintered (Lower) Surfaces of PZT Fibers
Figure 5: Fine-scale 2-2 Composite formed by Near Netshape olding (Upper Micrograph). As-sintered Surface
(Lower Micrograph).
In order to demonstrate the lay-up approach for composite fabrication, composites of approximately 10 volume percent PZT-5H" fibers and Spurrs epoxy resin were fabricated by epoxy encapsulating laid-up pairs of injection molded and sintered fiber rows followed by grinding away the PZT ceramic stock used to mold the composite preform. Figure 6 shows composite samples made from freshly-compounded PZT/binder mixture and from reused material. Recycling of the compounded and molded material appears to be entirely feasible and results in greatly enhanced material utilization. Table 1 compares the piezoelectric and dielectric properties of injection molded PZT ceramic specimens with those reported for pressed PZT-5H samples prepared by the powder manufacturer. When the sintering conditions are optimized for the PZT-5H formulation, the piezoelectric and dielectric properties are comparable for both materials. Since the donordoped PZT-5H formulation is expected to be particularlysensitive to iron contamination from the injection molding equipment, the implication of these measurements is that such contamination is negligible in this injection molded PZT material.
Summary
Ceramic injection molding has been shown to be a viable process for fabricating both PZT ceramics and piezoelectric ceramic/polymer transducers. The electrical properties of injection molded PZT ceramics are comparable with those prepared by conventional powder pressing, with no evidence of deleterious effects from metallic contamination arising from contact with the compounding and molding equipment. By using ceramic injection molding to fabricate composite preforms, and then laying up the preforms to form larger composite arrays, an approach has been demonstrated for net-shape manufacturing of piezoelectric composite transducers in large quantities. Acknowledgements This work was funded by the Office of Naval
Research under the direction of Mr. Stephen E. Newfield. The authors wish to thank Ms. Hong Pham for technical assistance, and Dr. Thomas Shrout of the Materials Research Laboratory, Penn. State University
for electrical measurements
References
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